HI All
As part of my glass fusing kiln project, I've a need for my Raspberry Pi
to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks
Adrian

On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:
So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

https://www.broadcom.com/products/op...ctor/hcpl-3700

http://www.onsemi.com/PowerSolutions...do?id=HCPL3700

or

http://www.ebay.co.uk/itm/272619019157

6 - 25 quid depending on number of channels required

See also

https://www.raspberrypi.org/forums/v...=9617&start=25

Owain

On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:
HI All
As part of my glass fusing kiln project, I've a need for my Raspberry Pi
to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks
Adrian

easiest simplest cheapest is a wallwart. But doubling contactor workload will halve its life, very roughly, causing failure.


NT

On 19/07/2017 15:12, wrote:
On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:
So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

https://www.broadcom.com/products/op...ctor/hcpl-3700

http://www.onsemi.com/PowerSolutions...do?id=HCPL3700

or

http://www.ebay.co.uk/itm/272619019157

6 - 25 quid depending on number of channels required

See also

https://www.raspberrypi.org/forums/v...=9617&start=25

Owain

Thanks - that's useful....
I'd thought of a good old mains-coil relay - which would provide me with
clean contacts - but the ebay link is interesting, and looks to be
nicely-built... for 7 quid plus postage. Sounds good, thanks.
Adrian

On 19/07/17 14:46, Adrian Brentnall wrote:
HI All
As part of my glass fusing kiln project, I've a need for my Raspberry Pi
to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Optocoupler I'd say.


mains-cap-bridge rect-LED

5vresistor-pi input AND phototransistor, phototransistor-gnd.




Thanks
Adrian


--
"Anyone who believes that the laws of physics are mere social
conventions is invited to try transgressing those conventions from the
windows of my apartment. (I live on the twenty-first floor.) "

Alan Sokal

On 19/07/2017 16:04, wrote:
On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:
HI All
As part of my glass fusing kiln project, I've a need for my Raspberry Pi
to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks
Adrian

easiest simplest cheapest is a wallwart. But doubling contactor workload will halve its life, very roughly, causing failure.


NT

Yes - a wallwart's an idea.... space is a bit limited, but I guess I
could extract a wallwart from its 'shell' and mount the gubbins inside
my enclosure....

Not sure quite what you mean about the 'doubling contactor workload'..?

There's two contactors - in the revised design the 'safety' contactor
will be pulled in for the duration of each fusing schedule (about 7
hours - and probably no more than 5 schedules per week.
The other one, the 'heat' contactor is a bit more stressed - a single
schedule can see it cycling on/off 900 times....
so I was planning on making that one a 'replace every 2 years' item -
they're only ‚¬20 each, so it's not the end of the world, cost-wise.

What would be embarrassing would be if the 'heat' contactor were to
stick 'on' - which is where the monitoring RPi comes in (has its own
thermocouple and will drop the 'safety' contactor if it doesn't like
what it's seeing.
Hence the (slightly paranoid, I'll admit g!) aim of checking that the
monitoring RPi can turn off the 'safety' contactor, and that the mains
actually disappears from the 'safety' contactor's output when it's
de-energised...

Anyway - thanks for the idea - that's the background!
Adrian

On 19/07/2017 16:44, The Natural Philosopher wrote:
On 19/07/17 14:46, Adrian Brentnall wrote:
HI All
As part of my glass fusing kiln project, I've a need for my Raspberry
Pi to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition
being detected. The safety contactor is in series with the main
switching contactor - which, because it's switched quite frequently,
is probably more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Optocoupler I'd say.


mains-cap-bridge rect-LED

5vresistor-pi input AND phototransistor, phototransistor-gnd.


That's certainly a possibility... but I might just have a spare 5v
wallwart kicking around somewhere here..... g
- which would have a similar effect.

Thanks
Adrian

On 19/07/2017 16:51, Adrian Brentnall wrote:
On 19/07/2017 16:44, The Natural Philosopher wrote:
On 19/07/17 14:46, Adrian Brentnall wrote:
HI All
As part of my glass fusing kiln project, I've a need for my Raspberry
Pi to 'know' when one of the big, 240v-operated contactors has pulled
in.

(It's a 'safety' sp/no contactor, with the ability to drop mains
power from the 7.5kw heating elements in the event of a fault
condition being detected. The safety contactor is in series with the
main switching contactor - which, because it's switched quite
frequently, is probably more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Optocoupler I'd say.


mains-cap-bridge rect-LED

5vresistor-pi input AND phototransistor, phototransistor-gnd.


That's certainly a possibility... but I might just have a spare 5v
wallwart kicking around somewhere here..... g
- which would have a similar effect.

Thanks
Adrian

Be aware that the output voltage from your wallwart may take several
seconds to decay. Solved with a load resistor.

Cheers
--
Clive

On 19/07/2017 17:13, Clive Arthur wrote:
On 19/07/2017 16:51, Adrian Brentnall wrote:
On 19/07/2017 16:44, The Natural Philosopher wrote:
On 19/07/17 14:46, Adrian Brentnall wrote:
HI All
As part of my glass fusing kiln project, I've a need for my
Raspberry Pi to 'know' when one of the big, 240v-operated contactors
has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains
power from the 7.5kw heating elements in the event of a fault
condition being detected. The safety contactor is in series with the
main switching contactor - which, because it's switched quite
frequently, is probably more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi
was able, at the start of a fusing session, to pulse the safety
contactor on/off - just so's it knows that, if it wants to, it _can_
drop the contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v
on the switched contacts of the contactor into somethaing that the
RPi likes - either clean-contacts or a 5v logic signal?

Optocoupler I'd say.


mains-cap-bridge rect-LED

5vresistor-pi input AND phototransistor, phototransistor-gnd.


That's certainly a possibility... but I might just have a spare 5v
wallwart kicking around somewhere here..... g
- which would have a similar effect.

Thanks
Adrian

Be aware that the output voltage from your wallwart may take several
seconds to decay. Solved with a load resistor.

Cheers

Thanks for reminding me, Clive!
As you say - solved with a load resistor - or even a load resistor and a
green 'safety' LED. (You can never have too many indicator leds! g)
Thanks
Adrian

Adrian Brentnall wrote:

I'd thought of a good old mains-coil relay - which would provide me with
clean contacts

No volt-free AUX contacts on the contactor I presume?

Adrian Brentnall wrote:

Not sure quite what you mean about the 'doubling contactor workload'..?

I think if it's running a kiln, the extra load of a 5V wall-wart isn't
going to trouble the contactor :-)

On 19/07/2017 18:12, Andy Burns wrote:
Adrian Brentnall wrote:

I'd thought of a good old mains-coil relay - which would provide me with
clean contacts

No volt-free AUX contacts on the contactor I presume?

Sadly not!
FWIW, the contactor is being driven by a 12v relay, that _does_ have
spare clean contracts.... but knowing that the 12v relay has
picked/dropped isn't quite the same as knowing that the contactor has
done the same.

I'm trying to get to the point where the 'safety' circuit knows that, if
it wants to, it's capable of shutting off the main power to the elements.

Thanks
Adrian

Adrian Brentnall wrote:

I'm trying to get to the point where the 'safety' circuit knows that, if
it wants to, it's capable of shutting off the main power to the elements.

And if it knows it has turned off the relay, but detects the contactor
is still on?

On 19/07/2017 18:42, Andy Burns wrote:
Adrian Brentnall wrote:

I'm trying to get to the point where the 'safety' circuit knows that, if
it wants to, it's capable of shutting off the main power to the elements.

And if it knows it has turned off the relay, but detects the contactor
is still on?

As in 'Never check for an error condition you can't handle...."? g

Well - the plan is that _before_ a firing schedule starts, the
monitoring RPi will pulse the 'safety' relay - and reassure itself that
the mains went 'on' and then 'off'.

No guarantee (of course) that this means it'll actually work in the
event of an error, but it does prove that the safety wasn't welded 'on'
at the start of the exercise.... which is probably better than nothing...?

Adrian

On Wed, 19 Jul 2017 14:46:56 +0100, Adrian Brentnall wrote:

HI All As part of my glass fusing kiln project, I've a need for my
Raspberry Pi to 'know' when one of the big, 240v-operated contactors has
pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks Adrian


Easiest of the lot is a simple 240vAC relay. Check the contacts though -
don't go for one with simple heavy duty contacts as they will eventually
oxidise up on 5v inputs to the Pi - there isn't enough voltage or current
to keep them clean. You need at least 24v and 50-100mA really, unless you
have serrated or (preferably) silver/gold plated contacts.

Also, remember that you must NEVER depend on software to open that
protection contactor if there is a problem. There should be some sort of
crude manual or non-resettable over-temp switch on the kiln to look after
that. Possibly even something like the melting link drop-wire switches
that they install over diesel generators to shut the fuel off in case of
a fire. Software can fail to open it as well as fail to close it. You
have to assume that all control has died and the kiln has caught fire.

On 19/07/2017 19:54, mick wrote:
On Wed, 19 Jul 2017 14:46:56 +0100, Adrian Brentnall wrote:

HI All As part of my glass fusing kiln project, I've a need for my
Raspberry Pi to 'know' when one of the big, 240v-operated contactors has
pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks Adrian


Easiest of the lot is a simple 240vAC relay. Check the contacts though -
don't go for one with simple heavy duty contacts as they will eventually
oxidise up on 5v inputs to the Pi - there isn't enough voltage or current
to keep them clean. You need at least 24v and 50-100mA really, unless you
have serrated or (preferably) silver/gold plated contacts.

Thanks - 'wetting current' I seem to recall....
Just wondering - perhaps a reed relay might do the job.... I've got some
of those somewhere - simple capacitive dropper to reduce the mains down
to whatever the coil wants - and (unless I've remembered it wrong) the
reed is in an evacuated glass envelope so no worries with oxidation?
Think that they used reeds in early computers...?


Also, remember that you must NEVER depend on software to open that
protection contactor if there is a problem. There should be some sort of
crude manual or non-resettable over-temp switch on the kiln to look after
that. Possibly even something like the melting link drop-wire switches
that they install over diesel generators to shut the fuel off in case of
a fire. Software can fail to open it as well as fail to close it. You
have to assume that all control has died and the kiln has caught fire.


Hmm - well - I'm only trying to produce a system that's as safe as the
usual kit that's sold to glass fusers.
Example - the commercial controller I'm using has a 'safety' output from
the controller board - designed to do the same thing as my safety system.
In the commercial kiln it came out of - this output was simply not wired
up - so when there was an odd fault on the thermocouple, the kiln
continued to heat, and was somewhere around 950c when I caught it!
(Normal operating range would be up to 800c)

I've looked at ultra-simple over-temperature safety facilities - and
then it dawned on me that I'm already planning to have an RPi doing some
other monitoring / logging activities, so it might as well do this
function as well.
It'll probably involve a very simple fail-safe hardware watchdog as well
(including a set of contacts in series with the control to the safety
relay) - just to make sure that the RPi hasn't gone walkabout..

If we get to the point of there being actual fire outside the kiln then
I've lost the game anyway - as it's installed in a timber shed g
I've not managed (so far) to find something mechanical that would drop
the power at (say) 950c - and the RPi system I'm proposing will be a
great improvement over most commercial kilns...

Thanks
Adrian

How big is the switch movement. Opto device actually in line with the moving
bit so its blocked at either on or off?
Brian

--
----- -
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Adrian Brentnall" wrote in message
...
HI All
As part of my glass fusing kiln project, I've a need for my Raspberry Pi
to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on the
switched contacts of the contactor into somethaing that the RPi likes -
either clean-contacts or a 5v logic signal?

Thanks
Adrian

HI Brian

Good idea... but it's a din-rail-mounted contactor - fully enclosed...
I think that either the 'gutted' wallwart idea or the 'proper'
opto-isolated interface board (from ebay) are probably the best ways to
go...

Thanks
Adrian

On 20/07/2017 08:16, Brian Gaff wrote:
How big is the switch movement. Opto device actually in line with the moving
bit so its blocked at either on or off?
Brian

On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:
(It's a 'safety' sp/no contactor,

Could you use a changeover contactor, and detect the changeover from L1 to L2? That gives you a safeguard against a broken connection to the sensor, in that either L1 or L2 must be live, and not both.

Owain

On 20/07/2017 09:30, wrote:
On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:
(It's a 'safety' sp/no contactor,

Could you use a changeover contactor, and detect the changeover from L1 to L2? That gives you a safeguard against a broken connection to the sensor, in that either L1 or L2 must be live, and not both.

Owain

Probably could.... but I've got the sp/no ones (and a pair of spares!)
at the moment...

My thought was along the lines of
....
RPi waits for 'start' button to be pressed

RPi checks that 240v isn't present on contactor contacts
RPi commands'safety' contactor to close
RPi waits a while & then checks that 240v appears on contactor contacts
RPi commands 'safety' contactor to open
RPi waits a while & then checks that 240v appears on contactor contacts
RPi waits a while & then checks that 240v is no longer present on
contactor contacts

RPi then commands 'safety' contactor to close - lights an 'all OK' led,
and operator presses the 'go' button on the separate kiln controller -
secure in the knowledge that the safety monitor RPi is capable of
dropping the safety relay if it detects a fault condition.
Most likely/hazardous fault condition is main contactor stuck 'on' -
which would otherwise lead to kiln overheating, possible damage to kiln
and fire!

Adrian

On 19 Jul 2017 18:54:19 GMT, mick wrote:

Also, remember that you must NEVER depend on software to open that
protection contactor if there is a problem.

Some of that can be mitigated by power loss to the Pi dropping every
thing to off. ie you need to force things on.

A hardware timer (555 or WHY) that has a high output to enable the
control lines that has to be regulary reset by the Pi. If it doesn't
get reset and times out everything is dropped off. The reset needs
to see a transition (lo to hi) or a short high pulse of no longer
than a given duration and both edges detetected.

Mains detection. There is a chip Allegro Microsystems ACS712ELCTR,
2.1 kV isolation, produces a voltage output proportional to the
current flow. Available in 5, 20 and 30 A versions.

You have to assume that all control has died and the kiln has caught
fire.

True enough but "control failure" ought to include "fail safe", ie
things have to be forced on. Yes the Pi could crash and keep the
GPIOs on, that's where the hardware timer comes in.

--
Cheers
Dave.

On Wed, 19 Jul 2017 16:40:59 +0100, Adrian Brentnall wrote:

... the ebay link is interesting, and looks to be nicely-built... for 7
quid plus postage.

Don't you need more than one, output of safety contactor and feed to
kiln?

Also that is only an on/off indication. The Allegro chip would enable
you to monitor the currentdrawn and thus power consumed and maybe
detect overload with more sensitivity than normal protective devices.

--
Cheers
Dave.

HI Dave
On 20/07/2017 12:47, Dave Liquorice wrote:
On 19 Jul 2017 18:54:19 GMT, mick wrote:

Also, remember that you must NEVER depend on software to open that
protection contactor if there is a problem.

Some of that can be mitigated by power loss to the Pi dropping every
thing to off. ie you need to force things on.

As I said somewhere up this thread - I'm aiming for an improved level of
safety, but not quite up to nuclear power station (or railway
signalling) standards.. g

The kiln runs unattended most of the time - I'm just looking to trap the
most likely failure mode - where the commercial controller gets
'confused' and calls for heat over & above what's required, or where the
main switching relay to the heating elements fails 'short'.


A hardware timer (555 or WHY) that has a high output to enable the
control lines that has to be regulary reset by the Pi. If it doesn't
get reset and times out everything is dropped off. The reset needs
to see a transition (lo to hi) or a short high pulse of no longer
than a given duration and both edges detetected.

I did this (mutter) years ago with three dil relays - two of which had
big electrolytics across them, and their n/o contacts wired in series.
The third c/o relay was pulsed on/off by the controller (back in those
days a completely-inappropriately-specced Texas 9900 computer!) - and
put 12v on each of the relays in turn.
If the pulsing relay stopped, or stuck 'on' or 'off' then one of the
relays would eventually drop out (time-delay provided by the big
capacitor) and the 'safety' contacts would break - providing a 'the
computer's fallen over' signal...

555 would do the same thing - but not as mechanically satisfying g


Mains detection. There is a chip Allegro Microsystems ACS712ELCTR,
2.1 kV isolation, produces a voltage output proportional to the
current flow. Available in 5, 20 and 30 A versions.

Clever!
Actually, I'm less concerned with the 'failure to heat' (i.e. element
open-circuit) situation, as I am with the 'failure to stop heating' (=
relay stuck on or thermocouple failure).
'Not heating' is a fail-safe situation - and isn't a big worry for me


You have to assume that all control has died and the kiln has caught
fire.

True enough but "control failure" ought to include "fail safe", ie
things have to be forced on. Yes the Pi could crash and keep the
GPIOs on, that's where the hardware timer comes in.


Precisely g.
Even more years ago, I was involved with animal feed mills -
auto-formulation & mixing / routeing - that sort of thing.
Most of that problems we encountered in those applications were down to
the heavy-duty hardware doing 'silly things' - and the code eventually
became full of 'idiot-checks' - based on real-life scenarios.

Same with the automated fork-truck systems. Things like the truck
knowing which aisle it was in (in the high-bay warehouse) by measuring
the length of a reflective strip stuck to the racking. Worked well until
the driving wheel skidded on some fool's banana skin (really!) and the
truck tried to turn right into the next aisle - except it was _already_
in the last aisle.

Anyway - as I say, this isn't NASA - just trying to make the kit
reasonably safe, and know that it can fail 'gracefully' if it decides it
doesn't like the information it's seeing.

Thanks
Adrian

HI Dave
On 20/07/2017 13:06, Dave Liquorice wrote:
On Wed, 19 Jul 2017 16:40:59 +0100, Adrian Brentnall wrote:

... the ebay link is interesting, and looks to be nicely-built... for 7
quid plus postage.

Don't you need more than one, output of safety contactor and feed to
kiln?

I'm not over-bothered about the actual feed to the kiln.
If there's volts there then the kiln will heat up and all is good.
If there isn't volts then it won't heat, and I could spot that in the
'safety' software, or wait until the kiln controller notices and
complains. Either way - it's safe.


Also that is only an on/off indication. The Allegro chip would enable
you to monitor the currentdrawn and thus power consumed and maybe
detect overload with more sensitivity than normal protective devices.

True. But it's not really a big deal at the moment..
The most likely / critical failure of the kiln control is the 'heat'
relay being stuck 'on'...
as that can lead to unfortunate things happening (and, in the worst
case, my workshop becoming a big bonfire!)
That's why I'd like the safety monitoring circuit to know that it
_could_ drop out the big relay if it felt so inclined...

Thanks for the perspectives - it's all useful
Adrian

On Wednesday, 19 July 2017 16:50:10 UTC+1, Adrian Brentnall wrote:
On 19/07/2017 16:04, tabbypurr wrote:
On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:

HI All
As part of my glass fusing kiln project, I've a need for my Raspberry Pi
to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks
Adrian

easiest simplest cheapest is a wallwart. But doubling contactor workload will halve its life, very roughly, causing failure.


NT

Yes - a wallwart's an idea.... space is a bit limited, but I guess I
could extract a wallwart from its 'shell' and mount the gubbins inside
my enclosure....

the optoisolator would be far smaller, but drive it with series R&C not just C. Pick R to limit peak i, then C to control mean i.

Not sure quite what you mean about the 'doubling contactor workload'..?

If your plan is to switch it twice instead of once, it will last something like half as long. Ie your scheme, if that's how it works, would cause failure.

There's two contactors - in the revised design the 'safety' contactor
will be pulled in for the duration of each fusing schedule (about 7
hours - and probably no more than 5 schedules per week.
The other one, the 'heat' contactor is a bit more stressed - a single
schedule can see it cycling on/off 900 times....
so I was planning on making that one a 'replace every 2 years' item -
they're only ‚¬20 each, so it's not the end of the world, cost-wise.

900 times is a lot. An SSR wouldn't care. Another way to boost reliability could be to split the heating load so the rate of temp climb is much slower when it's thermostatically cycling. It also means less current being switched.


What would be embarrassing would be if the 'heat' contactor were to
stick 'on' - which is where the monitoring RPi comes in (has its own
thermocouple and will drop the 'safety' contactor if it doesn't like
what it's seeing.
Hence the (slightly paranoid, I'll admit g!) aim of checking that the
monitoring RPi can turn off the 'safety' contactor, and that the mains
actually disappears from the 'safety' contactor's output when it's
de-energised...

I would not be happy using software & active electronics as basic safety protection. I don't know why you don't just connect the power through a bit of metal inside the kiln that melts if it goes above working temp. Then if it's running a reducing atmosphere your objects could come out brass plated

Anyway - thanks for the idea - that's the background!
Adrian


NT

On Wednesday, 19 July 2017 17:13:31 UTC+1, Clive Arthur wrote:

Be aware that the output voltage from your wallwart may take several
seconds to decay. Solved with a load resistor.

Cheers

or by removing the reservoir cap.


NT

On Wednesday, 19 July 2017 21:32:32 UTC+1, Adrian Brentnall wrote:
On 19/07/2017 19:54, mick wrote:
On Wed, 19 Jul 2017 14:46:56 +0100, Adrian Brentnall wrote:

HI All As part of my glass fusing kiln project, I've a need for my
Raspberry Pi to 'know' when one of the big, 240v-operated contactors has
pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks Adrian


Easiest of the lot is a simple 240vAC relay. Check the contacts though -
don't go for one with simple heavy duty contacts as they will eventually
oxidise up on 5v inputs to the Pi - there isn't enough voltage or current
to keep them clean. You need at least 24v and 50-100mA really, unless you
have serrated or (preferably) silver/gold plated contacts.

Thanks - 'wetting current' I seem to recall....
Just wondering - perhaps a reed relay might do the job.... I've got some
of those somewhere - simple capacitive dropper to reduce the mains down
to whatever the coil wants - and (unless I've remembered it wrong) the
reed is in an evacuated glass envelope so no worries with oxidation?
Think that they used reeds in early computers...?

the world's most unreliable type of relay

Also, remember that you must NEVER depend on software to open that
protection contactor if there is a problem. There should be some sort of
crude manual or non-resettable over-temp switch on the kiln to look after
that. Possibly even something like the melting link drop-wire switches
that they install over diesel generators to shut the fuel off in case of
a fire. Software can fail to open it as well as fail to close it. You
have to assume that all control has died and the kiln has caught fire.


Hmm - well - I'm only trying to produce a system that's as safe as the
usual kit that's sold to glass fusers.
Example - the commercial controller I'm using has a 'safety' output from
the controller board - designed to do the same thing as my safety system.
In the commercial kiln it came out of - this output was simply not wired
up - so when there was an odd fault on the thermocouple, the kiln
continued to heat, and was somewhere around 950c when I caught it!
(Normal operating range would be up to 800c)

Are you going to use this in or attached to or anywhere near your house? If so you're mad to build a fire waiting to happen. Other people's madness is no defence.

I've looked at ultra-simple over-temperature safety facilities - and
then it dawned on me that I'm already planning to have an RPi doing some
other monitoring / logging activities, so it might as well do this
function as well.
It'll probably involve a very simple fail-safe hardware watchdog as well
(including a set of contacts in series with the control to the safety
relay) - just to make sure that the RPi hasn't gone walkabout..

If we get to the point of there being actual fire outside the kiln then
I've lost the game anyway - as it's installed in a timber shed g
I've not managed (so far) to find something mechanical that would drop
the power at (say) 950c - and the RPi system I'm proposing will be a
great improvement over most commercial kilns...

Thanks
Adrian

Just stick your brass strip at the right height in the kiln so its mp is reached when the main body of the kiln is at 950.


NT

On Thursday, 20 July 2017 13:06:15 UTC+1, Dave Liquorice wrote:
On Wed, 19 Jul 2017 16:40:59 +0100, Adrian Brentnall wrote:

... the ebay link is interesting, and looks to be nicely-built... for 7
quid plus postage.

Don't you need more than one, output of safety contactor and feed to
kiln?

Also that is only an on/off indication. The Allegro chip would enable
you to monitor the currentdrawn and thus power consumed and maybe
detect overload with more sensitivity than normal protective devices.

£7 for an optoisolator? You jest.
https://www.rapidonline.com/catalogu...y=optoisolator


NT

On Thursday, 20 July 2017 13:52:33 UTC+1, Adrian Brentnall wrote:
HI Dave
On 20/07/2017 12:47, Dave Liquorice wrote:
On 19 Jul 2017 18:54:19 GMT, mick wrote:

Also, remember that you must NEVER depend on software to open that
protection contactor if there is a problem.

Some of that can be mitigated by power loss to the Pi dropping every
thing to off. ie you need to force things on.

As I said somewhere up this thread - I'm aiming for an improved level of
safety, but not quite up to nuclear power station (or railway
signalling) standards.. g

The kiln runs unattended most of the time - I'm just looking to trap the
most likely failure mode - where the commercial controller gets
'confused' and calls for heat over & above what's required, or where the
main switching relay to the heating elements fails 'short'.


A hardware timer (555 or WHY) that has a high output to enable the
control lines that has to be regulary reset by the Pi. If it doesn't
get reset and times out everything is dropped off. The reset needs
to see a transition (lo to hi) or a short high pulse of no longer
than a given duration and both edges detetected.

I did this (mutter) years ago with three dil relays - two of which had
big electrolytics across them, and their n/o contacts wired in series.
The third c/o relay was pulsed on/off by the controller (back in those
days a completely-inappropriately-specced Texas 9900 computer!) - and
put 12v on each of the relays in turn.
If the pulsing relay stopped, or stuck 'on' or 'off' then one of the
relays would eventually drop out (time-delay provided by the big
capacitor) and the 'safety' contacts would break - providing a 'the
computer's fallen over' signal...

555 would do the same thing - but not as mechanically satisfying g


Mains detection. There is a chip Allegro Microsystems ACS712ELCTR,
2.1 kV isolation, produces a voltage output proportional to the
current flow. Available in 5, 20 and 30 A versions.

Clever!
Actually, I'm less concerned with the 'failure to heat' (i.e. element
open-circuit) situation, as I am with the 'failure to stop heating' (=
relay stuck on or thermocouple failure).
'Not heating' is a fail-safe situation - and isn't a big worry for me


You have to assume that all control has died and the kiln has caught
fire.

True enough but "control failure" ought to include "fail safe", ie
things have to be forced on. Yes the Pi could crash and keep the
GPIOs on, that's where the hardware timer comes in.


Precisely g.
Even more years ago, I was involved with animal feed mills -
auto-formulation & mixing / routeing - that sort of thing.
Most of that problems we encountered in those applications were down to
the heavy-duty hardware doing 'silly things' - and the code eventually
became full of 'idiot-checks' - based on real-life scenarios.

Same with the automated fork-truck systems. Things like the truck
knowing which aisle it was in (in the high-bay warehouse) by measuring
the length of a reflective strip stuck to the racking. Worked well until
the driving wheel skidded on some fool's banana skin (really!) and the
truck tried to turn right into the next aisle - except it was _already_
in the last aisle.

Anyway - as I say, this isn't NASA - just trying to make the kit
reasonably safe, and know that it can fail 'gracefully' if it decides it
doesn't like the information it's seeing.

Thanks
Adrian

So instead of installing somehing elementary and failsafe you're going for something unnecesarily complicated, more expensive and dangerous.

On Thu, 20 Jul 2017 06:27:39 -0700 (PDT), wrote:

Also that is only an on/off indication. The Allegro chip would
enable
you to monitor the currentdrawn and thus power consumed and maybe
detect overload with more sensitivity than normal protective devices..

£7 for an optoisolator? You jest.
https://www.rapidonline.com/catalogu...y=optoisolator

The Allgero chip is £3 something plus VAT from Farnell. It's not an
optoisolator but a hall effect device and the load current passes
through it. Its output voltage is proportional to the current. Not
easy to do with an opto-isolator.

Or are you refering to the £6.95 single channel ready to play
optoisolated mains to 3.3 / 5 V on/off interface board on ebay?

--
Cheers
Dave.

On Thursday, 20 July 2017 19:42:59 UTC+1, Dave Liquorice wrote:
On Thu, 20 Jul 2017 06:27:39 -0700 (PDT), tabbypurr wrote:

Also that is only an on/off indication. The Allegro chip would
enable
you to monitor the currentdrawn and thus power consumed and maybe
detect overload with more sensitivity than normal protective devices.

£7 for an optoisolator? You jest.
https://www.rapidonline.com/catalogu...y=optoisolator

The Allgero chip is £3 something plus VAT from Farnell. It's not an
optoisolator but a hall effect device and the load current passes
through it. Its output voltage is proportional to the current. Not
easy to do with an opto-isolator.

Or are you refering to the £6.95 single channel ready to play
optoisolated mains to 3.3 / 5 V on/off interface board on ebay?

Either something has passed me by or Adrian just wanted to know if the elements were powered up. My link shows what I was referring to.


NT

On 20/07/2017 14:19, wrote:
On Wednesday, 19 July 2017 16:50:10 UTC+1, Adrian Brentnall wrote:
On 19/07/2017 16:04, tabbypurr wrote:
On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:

HI All
As part of my glass fusing kiln project, I've a need for my Raspberry Pi
to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks
Adrian

easiest simplest cheapest is a wallwart. But doubling contactor workload will halve its life, very roughly, causing failure.


NT

Yes - a wallwart's an idea.... space is a bit limited, but I guess I
could extract a wallwart from its 'shell' and mount the gubbins inside
my enclosure....

the optoisolator would be far smaller, but drive it with series R&C not just C. Pick R to limit peak i, then C to control mean i.

Not sure quite what you mean about the 'doubling contactor workload'..?

If your plan is to switch it twice instead of once, it will last something like half as long. Ie your scheme, if that's how it works, would cause failure.

Sort of.
Under my scheme the safety contactor would be switched on/off in a
'self-test' mode once per firing schedule, and then once on and once off
during the actual run.

Given that theoretically this doubles the wear on the contactor, but
that it only gets switched twice per firing (which lasts about 7 hours,
and takes place maybe 5 times per week) I can live with it.


There's two contactors - in the revised design the 'safety' contactor
will be pulled in for the duration of each fusing schedule (about 7
hours - and probably no more than 5 schedules per week.
The other one, the 'heat' contactor is a bit more stressed - a single
schedule can see it cycling on/off 900 times....
so I was planning on making that one a 'replace every 2 years' item -
they're only ‚¬20 each, so it's not the end of the world, cost-wise.

900 times is a lot. An SSR wouldn't care.

Understood - but, as I said, I happen to like the click-clack of
contactors - and I'm less confident with ssr's.

Another way to boost reliability could be to split the heating load so
the rate of temp climb is much slower when it's thermostatically
cycling. It also means less current being switched.

All good in theory - but irrelevant in this application, where it's
important that the firing schedule consists of certain predetermined
ramp/temperature/hold combinations...


What would be embarrassing would be if the 'heat' contactor were to
stick 'on' - which is where the monitoring RPi comes in (has its own
thermocouple and will drop the 'safety' contactor if it doesn't like
what it's seeing.
Hence the (slightly paranoid, I'll admit g!) aim of checking that the
monitoring RPi can turn off the 'safety' contactor, and that the mains
actually disappears from the 'safety' contactor's output when it's
de-energised...

I would not be happy using software & active electronics as basic safety protection. I don't know why you don't just connect the power through a bit of metal inside the kiln that melts if it goes above working temp. Then if it's running a reducing atmosphere your objects could come out brass plated

Fair enough - you're welcome to your opinion.
I've looked at the possibility of having a 'hardware only' safety
facility - and I'd rather do it the way I describe.
Thanks
Adrian


Anyway - thanks for the idea - that's the background!
Adrian


NT

On 20/07/2017 14:20, wrote:
On Wednesday, 19 July 2017 17:13:31 UTC+1, Clive Arthur wrote:

Be aware that the output voltage from your wallwart may take several
seconds to decay. Solved with a load resistor.

Cheers

or by removing the reservoir cap.


NT

Indeed
Adrian

On 20/07/2017 14:24, wrote:
On Wednesday, 19 July 2017 21:32:32 UTC+1, Adrian Brentnall wrote:
On 19/07/2017 19:54, mick wrote:
On Wed, 19 Jul 2017 14:46:56 +0100, Adrian Brentnall wrote:

HI All As part of my glass fusing kiln project, I've a need for my
Raspberry Pi to 'know' when one of the big, 240v-operated contactors has
pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks Adrian


Easiest of the lot is a simple 240vAC relay. Check the contacts though -
don't go for one with simple heavy duty contacts as they will eventually
oxidise up on 5v inputs to the Pi - there isn't enough voltage or current
to keep them clean. You need at least 24v and 50-100mA really, unless you
have serrated or (preferably) silver/gold plated contacts.

Thanks - 'wetting current' I seem to recall....
Just wondering - perhaps a reed relay might do the job.... I've got some
of those somewhere - simple capacitive dropper to reduce the mains down
to whatever the coil wants - and (unless I've remembered it wrong) the
reed is in an evacuated glass envelope so no worries with oxidation?
Think that they used reeds in early computers...?

the world's most unreliable type of relay

Opinions on the web seem to vary - but I don't think I'm going the relay
route anyway.


Also, remember that you must NEVER depend on software to open that
protection contactor if there is a problem. There should be some sort of
crude manual or non-resettable over-temp switch on the kiln to look after
that. Possibly even something like the melting link drop-wire switches
that they install over diesel generators to shut the fuel off in case of
a fire. Software can fail to open it as well as fail to close it. You
have to assume that all control has died and the kiln has caught fire.


Hmm - well - I'm only trying to produce a system that's as safe as the
usual kit that's sold to glass fusers.
Example - the commercial controller I'm using has a 'safety' output from
the controller board - designed to do the same thing as my safety system.
In the commercial kiln it came out of - this output was simply not wired
up - so when there was an odd fault on the thermocouple, the kiln
continued to heat, and was somewhere around 950c when I caught it!
(Normal operating range would be up to 800c)

Are you going to use this in or attached to or anywhere near your house? If so you're mad to build a fire waiting to happen. Other people's madness is no defence.

Thank you for that opinion.



I've looked at ultra-simple over-temperature safety facilities - and
then it dawned on me that I'm already planning to have an RPi doing some
other monitoring / logging activities, so it might as well do this
function as well.
It'll probably involve a very simple fail-safe hardware watchdog as well
(including a set of contacts in series with the control to the safety
relay) - just to make sure that the RPi hasn't gone walkabout..

If we get to the point of there being actual fire outside the kiln then
I've lost the game anyway - as it's installed in a timber shed g
I've not managed (so far) to find something mechanical that would drop
the power at (say) 950c - and the RPi system I'm proposing will be a
great improvement over most commercial kilns...

Thanks
Adrian

Just stick your brass strip at the right height in the kiln so its mp is reached when the main body of the kiln is at 950.

Yes - looked into that but I think I prefer a less brute-force approach.
Adrian

On 20/07/2017 14:29, wrote:
On Thursday, 20 July 2017 13:52:33 UTC+1, Adrian Brentnall wrote:
HI Dave
On 20/07/2017 12:47, Dave Liquorice wrote:
On 19 Jul 2017 18:54:19 GMT, mick wrote:

Also, remember that you must NEVER depend on software to open that
protection contactor if there is a problem.

Some of that can be mitigated by power loss to the Pi dropping every
thing to off. ie you need to force things on.

As I said somewhere up this thread - I'm aiming for an improved level of
safety, but not quite up to nuclear power station (or railway
signalling) standards.. g

The kiln runs unattended most of the time - I'm just looking to trap the
most likely failure mode - where the commercial controller gets
'confused' and calls for heat over & above what's required, or where the
main switching relay to the heating elements fails 'short'.


A hardware timer (555 or WHY) that has a high output to enable the
control lines that has to be regulary reset by the Pi. If it doesn't
get reset and times out everything is dropped off. The reset needs
to see a transition (lo to hi) or a short high pulse of no longer
than a given duration and both edges detetected.

I did this (mutter) years ago with three dil relays - two of which had
big electrolytics across them, and their n/o contacts wired in series.
The third c/o relay was pulsed on/off by the controller (back in those
days a completely-inappropriately-specced Texas 9900 computer!) - and
put 12v on each of the relays in turn.
If the pulsing relay stopped, or stuck 'on' or 'off' then one of the
relays would eventually drop out (time-delay provided by the big
capacitor) and the 'safety' contacts would break - providing a 'the
computer's fallen over' signal...

555 would do the same thing - but not as mechanically satisfying g


Mains detection. There is a chip Allegro Microsystems ACS712ELCTR,
2.1 kV isolation, produces a voltage output proportional to the
current flow. Available in 5, 20 and 30 A versions.

Clever!
Actually, I'm less concerned with the 'failure to heat' (i.e. element
open-circuit) situation, as I am with the 'failure to stop heating' (=
relay stuck on or thermocouple failure).
'Not heating' is a fail-safe situation - and isn't a big worry for me


You have to assume that all control has died and the kiln has caught
fire.

True enough but "control failure" ought to include "fail safe", ie
things have to be forced on. Yes the Pi could crash and keep the
GPIOs on, that's where the hardware timer comes in.


Precisely g.
Even more years ago, I was involved with animal feed mills -
auto-formulation & mixing / routeing - that sort of thing.
Most of that problems we encountered in those applications were down to
the heavy-duty hardware doing 'silly things' - and the code eventually
became full of 'idiot-checks' - based on real-life scenarios.

Same with the automated fork-truck systems. Things like the truck
knowing which aisle it was in (in the high-bay warehouse) by measuring
the length of a reflective strip stuck to the racking. Worked well until
the driving wheel skidded on some fool's banana skin (really!) and the
truck tried to turn right into the next aisle - except it was _already_
in the last aisle.

Anyway - as I say, this isn't NASA - just trying to make the kit
reasonably safe, and know that it can fail 'gracefully' if it decides it
doesn't like the information it's seeing.

Thanks
Adrian

So instead of installing somehing elementary and failsafe you're going for something unnecesarily complicated, more expensive and dangerous.


Yup!
Adrian

On 19/07/2017 16:40, Adrian Brentnall wrote:
On 19/07/2017 15:12, wrote:
On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:
So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

https://www.broadcom.com/products/op...ctor/hcpl-3700


http://www.onsemi.com/PowerSolutions...do?id=HCPL3700

or

http://www.ebay.co.uk/itm/272619019157

6 - 25 quid depending on number of channels required

See also

https://www.raspberrypi.org/forums/v...=9617&start=25

Owain

Thanks - that's useful....
I'd thought of a good old mains-coil relay - which would provide me with
clean contacts - but the ebay link is interesting, and looks to be
nicely-built... for 7 quid plus postage. Sounds good, thanks.
Adrian

What make and model of contactor is it? Many din-rail mounted contactors
are designed to allow one or more auxiliary contacts to be added. They
simply clip on at the top and are automatically operated by the plunger
mechanism of the contactor.

SteveW

On 20/07/2017 14:19, wrote:
On Wednesday, 19 July 2017 16:50:10 UTC+1, Adrian Brentnall wrote:
On 19/07/2017 16:04, tabbypurr wrote:
On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:

HI All
As part of my glass fusing kiln project, I've a need for my Raspberry Pi
to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks
Adrian

easiest simplest cheapest is a wallwart. But doubling contactor workload will halve its life, very roughly, causing failure.


NT

Yes - a wallwart's an idea.... space is a bit limited, but I guess I
could extract a wallwart from its 'shell' and mount the gubbins inside
my enclosure....

the optoisolator would be far smaller, but drive it with series R&C not just C. Pick R to limit peak i, then C to control mean i.

Not sure quite what you mean about the 'doubling contactor workload'..?

If your plan is to switch it twice instead of once, it will last something like half as long. Ie your scheme, if that's how it works, would cause failure.

There's two contactors - in the revised design the 'safety' contactor
will be pulled in for the duration of each fusing schedule (about 7
hours - and probably no more than 5 schedules per week.
The other one, the 'heat' contactor is a bit more stressed - a single
schedule can see it cycling on/off 900 times....
so I was planning on making that one a 'replace every 2 years' item -
they're only ‚¬20 each, so it's not the end of the world, cost-wise.

900 times is a lot. An SSR wouldn't care. Another way to boost reliability could be to split the heating load so the rate of temp climb is much slower when it's thermostatically cycling. It also means less current being switched.


What would be embarrassing would be if the 'heat' contactor were to
stick 'on' - which is where the monitoring RPi comes in (has its own
thermocouple and will drop the 'safety' contactor if it doesn't like
what it's seeing.
Hence the (slightly paranoid, I'll admit g!) aim of checking that the
monitoring RPi can turn off the 'safety' contactor, and that the mains
actually disappears from the 'safety' contactor's output when it's
de-energised...

I would not be happy using software & active electronics as basic safety protection. I don't know why you don't just connect the power through a bit of metal inside the kiln that melts if it goes above working temp. Then if it's running a reducing atmosphere your objects could come out brass plated

Nothing wrong with software and active electronics - lots of industrial
stuff does that. However a RPi is not the way to go for that. For an
amateur, it'd need to be something like a PIC microcontroller, coded in
assembler so that the code can be substantiated - simple code, with no
interrupts, no parallel threads, completely deterministic, with watchdog
and brown-out detection and fail-safe output.

SteveW

On Thursday, 20 July 2017 20:54:49 UTC+1, Adrian Brentnall wrote:
On 20/07/2017 14:19, tabbypurr wrote:
On Wednesday, 19 July 2017 16:50:10 UTC+1, Adrian Brentnall wrote:
On 19/07/2017 16:04, tabbypurr wrote:
On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:

Not sure quite what you mean about the 'doubling contactor workload'..?

If your plan is to switch it twice instead of once, it will last something like half as long. Ie your scheme, if that's how it works, would cause failure.

Sort of.
Under my scheme the safety contactor would be switched on/off in a
'self-test' mode once per firing schedule, and then once on and once off
during the actual run.

Given that theoretically this doubles the wear on the contactor, but
that it only gets switched twice per firing (which lasts about 7 hours,
and takes place maybe 5 times per week) I can live with it.

You'd be better off having no protection than what you describe, it would last twice as long before it fails.


There's two contactors - in the revised design the 'safety' contactor
will be pulled in for the duration of each fusing schedule (about 7
hours - and probably no more than 5 schedules per week.
The other one, the 'heat' contactor is a bit more stressed - a single
schedule can see it cycling on/off 900 times....
so I was planning on making that one a 'replace every 2 years' item -
they're only ‚¬20 each, so it's not the end of the world, cost-wise.

900 times is a lot. An SSR wouldn't care.

Understood - but, as I said, I happen to like the click-clack of
contactors - and I'm less confident with ssr's.

Another way to boost reliability could be to split the heating load so
the rate of temp climb is much slower when it's thermostatically
cycling. It also means less current being switched.

All good in theory - but irrelevant in this application, where it's
important that the firing schedule consists of certain predetermined
ramp/temperature/hold combinations...

I think you've misunderstood. Splitting the heating power makes no difference to the firing temperature profile. What it affects is how fast the heating cycles when the kiln is set to any fixed temperature. So by splittig the load your hardware can last far longer.

It's also safer since one contactor failure only applies part load, keeping teh kiln at a safe temp.


NT

On 21/07/2017 00:07, Steve Walker wrote:
On 19/07/2017 16:40, Adrian Brentnall wrote:
On 19/07/2017 15:12, wrote:
On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:
So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

https://www.broadcom.com/products/op...ctor/hcpl-3700


http://www.onsemi.com/PowerSolutions...do?id=HCPL3700

or

http://www.ebay.co.uk/itm/272619019157

6 - 25 quid depending on number of channels required

See also

https://www.raspberrypi.org/forums/v...=9617&start=25

Owain

Thanks - that's useful....
I'd thought of a good old mains-coil relay - which would provide me
with clean contacts - but the ebay link is interesting, and looks to
be nicely-built... for 7 quid plus postage. Sounds good, thanks.
Adrian


Hi Steve

What make and model of contactor is it?

It's this one (links to ebay page) https://tinyurl.com/ycn6wmjd

Many din-rail mounted contactors
are designed to allow one or more auxiliary contacts to be added. They
simply clip on at the top and are automatically operated by the plunger
mechanism of the contactor.

Hmm - don't see any signs of that on this device - but worth knowing for
the future...
Thanks
Adrian

On 21/07/2017 00:36, Steve Walker wrote:
On 20/07/2017 14:19, wrote:
On Wednesday, 19 July 2017 16:50:10 UTC+1, Adrian Brentnall wrote:
On 19/07/2017 16:04, tabbypurr wrote:
On Wednesday, 19 July 2017 14:47:03 UTC+1, Adrian Brentnall wrote:

HI All
As part of my glass fusing kiln project, I've a need for my
Raspberry Pi
to 'know' when one of the big, 240v-operated contactors has pulled in.

(It's a 'safety' sp/no contactor, with the ability to drop mains power
from the 7.5kw heating elements in the event of a fault condition
being
detected. The safety contactor is in series with the main switching
contactor - which, because it's switched quite frequently, is probably
more likely to fail on 'in service').

In the interests of safety, it'd be helpful if the monitoring RPi was
able, at the start of a fusing session, to pulse the safety contactor
on/off - just so's it knows that, if it wants to, it _can_ drop the
contactor out.

So - what's the easiest / simplest / cheapest way to turn the 240v on
the switched contacts of the contactor into somethaing that the RPi
likes - either clean-contacts or a 5v logic signal?

Thanks
Adrian

easiest simplest cheapest is a wallwart. But doubling contactor
workload will halve its life, very roughly, causing failure.


NT

Yes - a wallwart's an idea.... space is a bit limited, but I guess I
could extract a wallwart from its 'shell' and mount the gubbins inside
my enclosure....

the optoisolator would be far smaller, but drive it with series R&C
not just C. Pick R to limit peak i, then C to control mean i.

Not sure quite what you mean about the 'doubling contactor workload'..?

If your plan is to switch it twice instead of once, it will last
something like half as long. Ie your scheme, if that's how it works,
would cause failure.

There's two contactors - in the revised design the 'safety' contactor
will be pulled in for the duration of each fusing schedule (about 7
hours - and probably no more than 5 schedules per week.
The other one, the 'heat' contactor is a bit more stressed - a single
schedule can see it cycling on/off 900 times....
so I was planning on making that one a 'replace every 2 years' item -
they're only ‚¬20 each, so it's not the end of the world, cost-wise.

900 times is a lot. An SSR wouldn't care. Another way to boost
reliability could be to split the heating load so the rate of temp
climb is much slower when it's thermostatically cycling. It also means
less current being switched.


What would be embarrassing would be if the 'heat' contactor were to
stick 'on' - which is where the monitoring RPi comes in (has its own
thermocouple and will drop the 'safety' contactor if it doesn't like
what it's seeing.
Hence the (slightly paranoid, I'll admit g!) aim of checking that the
monitoring RPi can turn off the 'safety' contactor, and that the mains
actually disappears from the 'safety' contactor's output when it's
de-energised...

I would not be happy using software & active electronics as basic
safety protection. I don't know why you don't just connect the power
through a bit of metal inside the kiln that melts if it goes above
working temp. Then if it's running a reducing atmosphere your objects
could come out brass plated

Nothing wrong with software and active electronics - lots of industrial
stuff does that. However a RPi is not the way to go for that. For an
amateur, it'd need to be something like a PIC microcontroller, coded in
assembler so that the code can be substantiated - simple code, with no
interrupts, no parallel threads, completely deterministic, with watchdog
and brown-out detection and fail-safe output.

SteveW


Understand where you're coming from - and the bit about 'keeping it
simple'. If I was doing this as part of a commercial product I'd
probably go that way (I was writing control systems in Texas 9900 /
99000 assembler back in the '90's)
However - in this case, the kiln is currently running quite happily
under it's own commercial controller, and the RPi is there, currently
providing a monitoring function (logging time vs temperature, drawing a
graph of same, counting/timing relay cycles, estimating firing costs....
and I was planning on adding the safety-monitoring to the overall program.

I do understand that an RPi will be doing 'other things' at the same
time (this is part of the attraction as it allows remote monitoring via
Remote Desktop and (eventually) web - which is why I was planning on
using a hardware watchdog to keep an eye on the RPi.

Agreed that, in safety, the simpler the better - and I did look at using
a completely hardware solution using a dedicated thermocouple amp ic
switching a relay - but, in this case, the RPi's there and has more
capabilities...

Appreciate your input - but, believe me, I've had all these discussions
(with myself!) already g
Adrian