I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

I can't imagine these leds to be anything special, the relay including
DIN rail socket was only £3

So how come it is working?

In article ,
Bob Minchin writes:
I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

I can't imagine these leds to be anything special, the relay including
DIN rail socket was only £3

So how come it is working?

It probably has a standard diode in reverse across the LED.

That indicator circuit is costing you just over half a watt.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

(Andrew Gabriel) writes:

In article ,
Bob Minchin writes:
I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

I can't imagine these leds to be anything special, the relay including
DIN rail socket was only £3

So how come it is working?

It probably has a standard diode in reverse across the LED.

Some LEDs actually consist of two LED diodes, of either the same or of
different colours, in inverse parallel (sometimes ambiguously called
'back-to-back' IIRC).
For something made in volume, putting the two diodes on the same die
would probably be the cheapest approach if you wanted the result to
work on AC.

That indicator circuit is costing you just over half a watt.

They should have used an RC snubber, something like a 0.1uF capacitor
with a 100 ohm resistor in series, as a current limiter. But that
probably costs more to manufacture.

--
Windmill, Use t m i l l
J.R.R. Tolkien:- @ S c o t s h o m e . c o m
All that is gold does not glister / Not all who wander are lost

In article ,
lid (Windmill) writes:
(Andrew Gabriel) writes:

In article ,
Bob Minchin writes:
I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

I can't imagine these leds to be anything special, the relay including
DIN rail socket was only £3

So how come it is working?

It probably has a standard diode in reverse across the LED.

Some LEDs actually consist of two LED diodes, of either the same or of
different colours, in inverse parallel (sometimes ambiguously called
'back-to-back' IIRC).
For something made in volume, putting the two diodes on the same die
would probably be the cheapest approach if you wanted the result to
work on AC.

That indicator circuit is costing you just over half a watt.

They should have used an RC snubber, something like a 0.1uF capacitor
with a 100 ohm resistor in series, as a current limiter. But that
probably costs more to manufacture.

You can work out an equivalent RC circuit like this...

Need to know the peak current LED can handle - most are rated 20mA
continuous, and can handle non-repetitive peaks of more, say 40mA.
Peak current happens when circuit is energised at peak mains voltage
with the capacitor discharged (capacitor looks momentarily like a
short circuit). [1]

Peak mains is 340V
340V / 40mA = 8500 ohms.

Now we need a capacitor with an impedance at 50Hz to make up the
difference bwteen the original 100kohms and this 8500 ohms.
100kohms - 8500 ohms = 91500 ohms.

C = 1 / (2 * pi * f * Z), so

1 / (2 * pi * 50 * 91500) = 0.0347µF

So, adjusting to standard component values, you want an 8k2 resistor
in series with a 0.033µF capacitor, which gives an LED current of
2.3mA (instead of the original 2.4mA).

Power consumption drops to 0.046W, less than 1/10th of the original.

Note that this circuit requires that the LED conducts both directions,
either back-to-back LEDs or LED back to back with a standard diode.


[1] Actually, there's a worse case if circuit is energised very shortly
after it was de-energised and the capacitor still holds some charge in
the opposite sense to the mains voltage, in which case 340*2 is the max
voltage, but this is an unlikely scenario you might choose to ignore.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

(Andrew Gabriel) writes:

In article ,
lid (Windmill) writes:
(Andrew Gabriel) writes:

In article ,
Bob Minchin writes:
I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

I can't imagine these leds to be anything special, the relay including
DIN rail socket was only £3

So how come it is working?

It probably has a standard diode in reverse across the LED.

Some LEDs actually consist of two LED diodes, of either the same or of
different colours, in inverse parallel (sometimes ambiguously called
'back-to-back' IIRC).
For something made in volume, putting the two diodes on the same die
would probably be the cheapest approach if you wanted the result to
work on AC.

That indicator circuit is costing you just over half a watt.

They should have used an RC snubber, something like a 0.1uF capacitor
with a 100 ohm resistor in series, as a current limiter. But that
probably costs more to manufacture.

You can work out an equivalent RC circuit like this...

Need to know the peak current LED can handle - most are rated 20mA
continuous, and can handle non-repetitive peaks of more, say 40mA.
Peak current happens when circuit is energised at peak mains voltage
with the capacitor discharged (capacitor looks momentarily like a
short circuit). [1]

I was surprised to learn that the trick to getting more light from LEDs
was to pulse them with very high peak currents (for, of course, very
short periods of time so as to keep the average dissipation within
reasonable limits).

I would guess that they might tolerate a pulse of a few amps if it
was only for a few microseconds.
Only the manufacturer would know details about the thermal inertia of
the LED plus its mounting, but next time, instead of being like you very
conservative, I'm going to just try a standard RC snubber as provided
ready made in one not-too-expensive package, and see what happens.


Peak mains is 340V
340V / 40mA = 8500 ohms.

Now we need a capacitor with an impedance at 50Hz to make up the
difference bwteen the original 100kohms and this 8500 ohms.
100kohms - 8500 ohms = 91500 ohms.

C = 1 / (2 * pi * f * Z), so

1 / (2 * pi * 50 * 91500) = 0.0347µF

So, adjusting to standard component values, you want an 8k2 resistor
in series with a 0.033µF capacitor, which gives an LED current of
2.3mA (instead of the original 2.4mA).

Power consumption drops to 0.046W, less than 1/10th of the original.

It would be even less if you could get away with a 100 ohm series
resistor.

Note that this circuit requires that the LED conducts both directions,
either back-to-back LEDs or LED back to back with a standard diode.


[1] Actually, there's a worse case if circuit is energised very shortly
after it was de-energised and the capacitor still holds some charge in
the opposite sense to the mains voltage, in which case 340*2 is the max
voltage, but this is an unlikely scenario you might choose to ignore.

I worried about that sort of thing (and lightning-induced transients)
around 20 years ago, but the LEDs are still functioning in a circuit
similar to yours. They have, I think, dimmed a little, but it was a cheap
surplus three-wire red+green LED pair, which I intended to appear
yellowish when both were on and power was applied to the freezer, and a
warning red when the power had been turned off at the switch.

--
Windmill, Use t m i l l
J.R.R. Tolkien:- @ S c o t s h o m e . c o m
All that is gold does not glister / Not all who wander are lost

On Fri, 13 Sep 2013 21:35:32 +0100, Bob Minchin
wrote:

I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

I can't imagine these leds to be anything special, the relay including
DIN rail socket was only £3

So how come it is working?


To say nothing of the not inconsiderable back-EMF of the coil.

Perhaps they are special, with at least a series diode.

As a mass produced item, effectively a solid-state "neon", it needn't
be expensive.



--
Graham.

%Profound_observation%

On Fri, 13 Sep 2013 21:35:32 +0100, Bob Minchin wrote:

I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

Yeah but that 100k resistor limits the current to 2mA.

I've some Omron MY2IN mains relays with LED indicators. The diagram
of the coil part printed on them is:

+----------+
13 -----+ +----- 14
| |
+----+ +----+
| +----------+ |
| |
+-----|--/\/\/\-----+

Mains is applied across 13 and 14.

What do you think they are doing? It's neat and I've taken the cover
off and checked.

--
Cheers
Dave.

Dave Liquorice wrote:
On Fri, 13 Sep 2013 21:35:32 +0100, Bob Minchin wrote:

I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

Yeah but that 100k resistor limits the current to 2mA.

I've some Omron MY2IN mains relays with LED indicators. The diagram
of the coil part printed on them is:

+----------+
13 -----+ +----- 14
| |
+----+ +----+
| +----------+ |
| |
+-----|--/\/\/\-----+

Mains is applied across 13 and 14.

What do you think they are doing? It's neat and I've taken the cover
off and checked.

These are very similar Omron MY2N-J - They came from an Ebay discounter
with the makers name obscured by some paint but a little solvent
revealed the information.


I will take one apart to investigate further when I get an idle moment
and also try a standard LED in a similar configuration.

I can see no sign of series or shunt diode.

In article o.uk,
"Dave Liquorice" writes:
On Fri, 13 Sep 2013 21:35:32 +0100, Bob Minchin wrote:

I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

Yeah but that 100k resistor limits the current to 2mA.

I've some Omron MY2IN mains relays with LED indicators. The diagram
of the coil part printed on them is:

+----------+
13 -----+ +----- 14
| |
+----+ +----+
| +----------+ |
| |
+-----|--/\/\/\-----+

Mains is applied across 13 and 14.

What do you think they are doing? It's neat and I've taken the cover
off and checked.

It could be that the coil is also a transformer for the LED, although
I haven't seen that done.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Andrew Gabriel wrote:
In article o.uk,
"Dave Liquorice" writes:
On Fri, 13 Sep 2013 21:35:32 +0100, Bob Minchin wrote:

I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

Yeah but that 100k resistor limits the current to 2mA.

I've some Omron MY2IN mains relays with LED indicators. The diagram
of the coil part printed on them is:

+----------+
13 -----+ +----- 14
| |
+----+ +----+
| +----------+ |
| |
+-----|--/\/\/\-----+

Mains is applied across 13 and 14.

What do you think they are doing? It's neat and I've taken the cover
off and checked.

It could be that the coil is also a transformer for the LED, although
I haven't seen that done.

Yes that was an idea that occurred to me too but the transparent cover
does not look as it that is the case.

On 14/09/13 12:20, Bob Minchin wrote:
Andrew Gabriel wrote:
In article o.uk,
"Dave Liquorice" writes:
On Fri, 13 Sep 2013 21:35:32 +0100, Bob Minchin wrote:

I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse
biassed
would let all the light out in big rush!

Yeah but that 100k resistor limits the current to 2mA.

I've some Omron MY2IN mains relays with LED indicators. The diagram
of the coil part printed on them is:

+----------+
13 -----+ +----- 14
| |
+----+ +----+
| +----------+ |
| |
+-----|--/\/\/\-----+

Mains is applied across 13 and 14.

What do you think they are doing? It's neat and I've taken the cover
off and checked.

It could be that the coil is also a transformer for the LED, although
I haven't seen that done.

Yes that was an idea that occurred to me too but the transparent cover
does not look as it that is the case.
once you have a coil, making it an (auto) transformer is trivial..

once you have an LED. packaging it with a reverse diode, or indeed a
full wave bridge, is also trivial.

Too little information, but as an ex electronic designer, that's the
sorts of solutions I would go for.


--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers.

On Sat, 14 Sep 2013 12:20:28 +0100, Bob Minchin wrote:

I've some Omron MY2IN mains relays with LED indicators. The
diagram
of the coil part printed on them is:

+----------+
13 -----+ +----- 14
| |
+----+ +----+
| +----------+ |
| |
+-----|--/\/\/\-----+

Mains is applied across 13 and 14.

It could be that the coil is also a transformer for the LED,
although
I haven't seen that done.

Yes that was an idea that occurred to me too but the transparent cover
does not look as it that is the case.

A transformer is what it appears to be. On my relay there is no
connection from either 13 or 14 to any of the LED and resistor
wiring. I'll admit at first casual inspection I did think the metal
bits all joined together but real close examination showed that not
to be the case and there are separate fine wires disappearing into
the coil from the bits that the LED/R connect to.

--
Cheers
Dave.

On Fri, 13 Sep 2013 21:35:32 +0100, Bob Minchin
wrote:

I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

I can't imagine these leds to be anything special, the relay including
DIN rail socket was only £3

So how come it is working?

There are dual red/green LED's comprised of a red and a green in
parallel back to back, so you can light either one by suitable DC
polarity, or both to make yellow on AC. Maybe this is what you've got?
--
Dave W


--- news://freenews.netfront.net/ - complaints: ---

Bob Minchin wrote:
I've just bought some mains coil relays from eBay and they are fitted
with a led indicator.
It looks like the LED has a series 100k resistor connected in parallel
with the coil.
I always thought that leds had relatively low PIV and if reverse biassed
would let all the light out in big rush!

I can't imagine these leds to be anything special, the relay including
DIN rail socket was only £3

So how come it is working?
Some experimental results:

A cooking grade red led out of my stock shows a reverse voltage
breakdown of about 15v.
This diode in series with 100k resistor taken up in ac volts on a variac
seems happy (for several minutes at least) at 280v ac (the maximum I can
go to.
The red Led in the relay shows no such reverse breakdown up to 100v
(again the maximum my bench supply will do)

So whilst a standard led would appear to survive AC with a suitable
current limiting resistor, at first sight the ones fitted in the relays
would appear to have a series diode inside the package.
However the forward voltage on the relay led and my standard led
measured at identical currents are the same within 10mV which would
suggest that the relay diode cannot have an extra series diode.

Until proven otherwise, I can only assume that the relay led is a
special device with a very high 100v reverse breakdown voltage.

I'm off to lash up a higher voltage variable supply now.....