Bill Noble wrote:
On 11/6/2010 9:00 PM, wrote:
On Sat, 06 Nov 2010 13:44:23 -0700, Rich Grise
wrote:

Winston wrote:


Aw, gawrsh, 'tweren't nothin'. ;-)

Thanks!
Rich
Variacs are MUCH more to be preferred (reccomended) for the job.

at the risk of being hostile, "nonsense" - variacs are NOT to be
preferred, unless you have no clue about electronics and are unwilling
to purchase a suitable thyrsitor or SCR controller.

Bill, help me understand please.

I have an autotransformer in good condition; sized
to easily drive my simple unregulated power supply.
The contact brush is nearly new and I have yet to
detect any contact noise or from it.

My Powerstat *is* much larger than a triac controller
and I agree that it does not self-regulate, but in this
application and with the stability of power that I
have available, I am not concerned about line voltage
variations or load current variations because I can
keep an eye on the performance of the hot knife and
tweak power into it as necessary.

I agree that it is a Lexus solution to a Corolla
problem, but the Powerstat is paid for, it is installed
and it works fine. Let me emphasize that it's lack
of isolation does not concern me because my downstream
power supply has a fully isolating transformer.

Would you agree that it is perfectly reasonable,
thrifty, safe, correct and honorable to use it to power
that transformer power supply, especially if it means
that I can work on my project instead of wait for a
triac controller to arrive in the mail?

Is there one reason that it would be either
unreasonable, wasteful, unsafe, incorrect,
dishonorable or somehow wrong for me to do so?

I ask the literal question without a trace of sarcasm
or snark.

What say you?

--Winston

In article ,
Winston wrote:

Rich Grise wrote:
Wild_Bill wrote:

In the DIY article I saw, there was a step-down transformer utilized, and
numerous comments about how some dimmers won't last long when attempting
to drive (the primary winding of) a transformer, instead of driving a
resistive load of incandescent lamp.

I'm an electronics guy by experience and training, and I'd be terribly
reluctant to use SCR control on the primary of a transformer. I'm "only" a
tech, so I can't quote numbers; it's just sort of a gut feeling that the
inductive reactance could cause a phase shift and upset the firing sequence
of the SCRs.

You are right, Rich. (And thanks for your post because
it caused me to solve a problem I didn't even know I was
about to encounter!)

http://www.st.com/stonline/books/pdf/docs/3566.pdf

See the first five pages for an accessible analysis of the
hazards of driving inductive loads with 'SCR' type devices.

See pages '6/16' and '7/16' for a clever triac circuit ST
developed to drive inductive loads without the flaws of
the earlier circuits.

Quoting them:
"This circuit has been developed by the STMicroelectronics
applications laboratory and used with success for a wide
range of equipment."


See the text at the bottom of page
'6/16' for a circuit explanation. See Figure 9 on page
'8/16' for component values.

But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).

The presented load is in fact largely resistive, and I think the point
made by another poster about DC unbalance saturating the transformer
core is a key issue. If one is in fact driving an inductor, one can
have both problems at once; the problems are independent.

In any event, Lutron et al make semiconductor dimmers intended to drive
resistive loads through step-down transformers.

Joe Gwinn

On 11/7/2010 5:54 AM, Winston wrote:
Bill Noble wrote:
On 11/6/2010 9:00 PM, wrote:
On Sat, 06 Nov 2010 13:44:23 -0700, Rich Grise
wrote:

Winston wrote:


Aw, gawrsh, 'tweren't nothin'. ;-)

Thanks!
Rich
Variacs are MUCH more to be preferred (reccomended) for the job.

at the risk of being hostile, "nonsense" - variacs are NOT to be
preferred, unless you have no clue about electronics and are unwilling
to purchase a suitable thyrsitor or SCR controller.

Bill, help me understand please.

I have an autotransformer in good condition; sized
to easily drive my simple unregulated power supply.
The contact brush is nearly new and I have yet to
detect any contact noise or from it.

My Powerstat *is* much larger than a triac controller
and I agree that it does not self-regulate, but in this
application and with the stability of power that I
have available, I am not concerned about line voltage
variations or load current variations because I can
keep an eye on the performance of the hot knife and
tweak power into it as necessary.

I agree that it is a Lexus solution to a Corolla
problem, but the Powerstat is paid for, it is installed
and it works fine. Let me emphasize that it's lack
of isolation does not concern me because my downstream
power supply has a fully isolating transformer.

Would you agree that it is perfectly reasonable,
thrifty, safe, correct and honorable to use it to power
that transformer power supply, especially if it means
that I can work on my project instead of wait for a
triac controller to arrive in the mail?

Is there one reason that it would be either
unreasonable, wasteful, unsafe, incorrect,
dishonorable or somehow wrong for me to do so?

I ask the literal question without a trace of sarcasm
or snark.

What say you?

--Winston

There is absolutely nothing wrong with using what you have. I was
objecting to the blanket statement about a solution being better - from
a technology point of view, the statement was false. From a practical
point of view, if you have all the pieces to make a slightly less
wonderful solution, but it is an adequate solution, then of course you
can do it.

Here is the pro/con of your solution, whcih I assume is a variac driving
an unregulated DC power supply:

pro
you have the parts
it is easy to assemble
it will be reliable enough for your (non production line) use

con
heavier, more bulky
more likely to get damaged if power supply gets knocked off the table
if you had to buy parts, it would be more costly (but you don't)
no ability to regulate current/temperature of the wire automatically

so, for YOU, given that you have the parts, use what you have.

On 11/7/2010 8:56 AM, Joseph Gwinn wrote:
In ,
wrote:

Rich Grise wrote:
Wild_Bill wrote:

In the DIY article I saw, there was a step-down transformer utilized, and
numerous comments about how some dimmers won't last long when attempting
to drive (the primary winding of) a transformer, instead of driving a
resistive load of incandescent lamp.

I'm an electronics guy by experience and training, and I'd be terribly
reluctant to use SCR control on the primary of a transformer. I'm "only" a
tech, so I can't quote numbers; it's just sort of a gut feeling that the
inductive reactance could cause a phase shift and upset the firing sequence
of the SCRs.

You are right, Rich. (And thanks for your post because
it caused me to solve a problem I didn't even know I was
about to encounter!)

http://www.st.com/stonline/books/pdf/docs/3566.pdf

See the first five pages for an accessible analysis of the
hazards of driving inductive loads with 'SCR' type devices.

See pages '6/16' and '7/16' for a clever triac circuit ST
developed to drive inductive loads without the flaws of
the earlier circuits.

Quoting them:
"This circuit has been developed by the STMicroelectronics
applications laboratory and used with success for a wide
range of equipment."


See the text at the bottom of page
'6/16' for a circuit explanation. See Figure 9 on page
'8/16' for component values.

But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).

The presented load is in fact largely resistive, and I think the point
made by another poster about DC unbalance saturating the transformer
core is a key issue. If one is in fact driving an inductor, one can
have both problems at once; the problems are independent.

In any event, Lutron et al make semiconductor dimmers intended to drive
resistive loads through step-down transformers.

Joe Gwinn


Joe - not correct because of the way triacs work - if driven by a pure
sine wave, it is true (or nearly so). But if you turn on say 90 deg
after zero crossing you have a huge current spike - that is due to the
inductive nature of the transformer and the fact that there is no flux
in the core at turn on. Work out the math if you don't believe me, or
try it with a transformer and a battery and a scope - measure the
current pulse at turn on with say 12V applied to the 12V winding of a
transformer - you are looking for the first 1 ms of current

Joseph Gwinn wrote:
(...)

But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).

This is a puzzlement for me, Joe.

Just now I modeled a 1:1 transformer in SPICE
and loaded the output with a selection of
different resistances.

While driving the input with 60 Hz, I noticed
that the primary voltage peaked before the
primary current did at about ~89 degrees
separation with the secondary loaded at 1
ohm through 100K ohms.

That all looks inductively reactive to me.
What did I do wrong? Are you talking about
saturated core mode?

Thanks!

--Winston

Bill Noble wrote:

(...)

so, for YOU, given that you have the parts, use what you have.


Thanks for the clarification.

My Powerstat is bolted to my workbench. If it falls
onto the floor, I have much more pressing issues on my
hands than just damage to that one tool.

--Winston

In article ,
Winston wrote:

Joseph Gwinn wrote:
(...)

But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).

This is a puzzlement for me, Joe.

Just now I modeled a 1:1 transformer in SPICE
and loaded the output with a selection of
different resistances.

While driving the input with 60 Hz, I noticed
that the primary voltage peaked before the
primary current did at about ~89 degrees
separation with the secondary loaded at 1
ohm through 100K ohms.

That all looks inductively reactive to me.

Then there is problem with the spice model. More below.


What did I do wrong? Are you talking about
saturated core mode?

In saturated core mode, the primary inductance of the drive winding
drops to a very low value, because saturated iron has roughly the
permeability of air, and one will see huge current surges when the core
saturates and the inductive reactance drops to near zero. This is not
what I think you are seeing.


Never mind computer modeling systems like spice. Let's talk physics.

As seen from the terminals, the voltages of an ideal single-phase
one-path transformer are all in phase (or inverted in sign). The
relation between voltage and current will depend on what is connected to
the transformer. If all loads are resistive, then currents will be in
phase with voltages. If loads are in aggregate reactive, then voltage
and current will get out of phase.

Another way to think about this is to note that no matter how many
windings there are, there is exactly one core, and the net flux through
this core links all windings equally.

Practical transformers very well approximate ideal transformers so long
as:

The inductive reactance of the windings exceeds the impedance of what's
connected to those windings by a factor of at least five or ten.

The stray inductance is insignificant.

The core remains out of saturation.

Eddy currents are reduced to insignificance by lamination of the core or
by use of a ferrite core.

The frequency isn't so high that stray capacitance has a significant
effect.

This is the short form. Transformer design can be a career. But for
all their imperfections, transformers work pretty well.


So, the spice model seems to be giving non-physical results, but
computer models are known for such things, and one must always validate
such models.

War story: Many years ago, I was interested in the physics of xenon
flash lamps. The physical model is pretty simple, a charged capacitor
discharging into an arc, which arc heated the xenon to incandescence,
the resulting light and heat radiation carrying the energy away. This
yields a set of coupled ordinary differential equations that one solves
numerically, time step by time step. I got it all working, and all was
well. Then I changed the duration of the time steps, and more energy
came out as light and heat than was stored in the capacitor. Oops.
Violated the conservation of energy.

Turns out I had made two mathematical mistakes, which mistakes cancelled
one another only for the original step duration.


Joe Gwinn

In article ,
Bill Noble wrote:

On 11/7/2010 8:56 AM, Joseph Gwinn wrote:
In ,
[snip]

But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).

The presented load is in fact largely resistive, and I think the point
made by another poster about DC unbalance saturating the transformer
core is a key issue. If one is in fact driving an inductor, one can
have both problems at once; the problems are independent.

In any event, Lutron et al make semiconductor dimmers intended to drive
resistive loads through step-down transformers.

Joe Gwinn


Joe - not correct because of the way triacs work - if driven by a pure
sine wave, it is true (or nearly so). But if you turn on say 90 deg
after zero crossing you have a huge current spike - that is due to the
inductive nature of the transformer and the fact that there is no flux
in the core at turn on.

It's true that the triac turns on abruptly, in a microsecond or two. At
90 degrees, this imposes a full voltage step on the transformer input.
But, wouldn't inductance slow the spike down, whatever the state of the
core? This is a classic homework problem. The current rises linearly
from zero, and the higher the inductance the slower the rate of rise.


Work out the math if you don't believe me, or
try it with a transformer and a battery and a scope - measure the
current pulse at turn on with say 12V applied to the 12V winding of a
transformer - you are looking for the first 1 ms of current

This sounds like we would have significant DC current through the
transformer winding, which could saturate the core. A saturated core
will certainly cause a current spike. Maybe I don't understand the
proposed test circuit.


I recall that Lutron had patents in this area, and these patents
provided a summary of the issues to be solved, so I did some searching.

A good discussion appears in US patent 4,876,498. A later version of
the same patent is 4,954,768. Leviton reacted with their own solution,
in patent 7,482,758. And 5,477,111 speaks directly to control of
inductive loads like motors. (Go to http://www.pat2pdf.org to get
copies.)

Anyway, the big issue in these patents is DC causing saturation, and (in
two-wire circuits) inductance-caused errors in knowing when to trigger
the triacs.


Joe Gwinn

On Sat, 06 Nov 2010 22:19:02 -0700, Bill Noble
wrote:

On 11/6/2010 9:00 PM, wrote:
On Sat, 06 Nov 2010 13:44:23 -0700, Rich Grise
wrote:

Winston wrote:


Aw, gawrsh, 'tweren't nothin'. ;-)

Thanks!
Rich
Variacs are MUCH more to be preferred (reccomended) for the job.

at the risk of being hostile, "nonsense" - variacs are NOT to be
preferred, unless you have no clue about electronics and are unwilling
to purchase a suitable thyrsitor or SCR controller. The variac is
larger, more prone to physical dammage, they wear out, and they are not
a regulating supply, a simple triac controller can hold constant
voltage at the output and avoid the inductive transients by switching at
zero crossing, or by using higher rated parts. Your statements are
wrong, the controllers that do this job are available for a few dollars
(go to harbor freight and buy a "router speed control" when it's on
sale) and have been common for a few decades.

A variac does have its uses, particularly if distortions of the AC
waveform are undesireable, but this is not one of them
I was not talking about specialized thyrister controllers - I was
talking about "lamp dimmers" - and I am not electronically
illiterate. The one advantage I have found with variacs is they are
linear in response, and rcontrol smoothly from virtually ZERO to full
(or even higher than full) voltage.
MOST thyrister controllers - even specialized ones for motor control,
have a somewhat limitted control range. Many will not control to below
approxemately 5%.
That is not an issue in this application since with a stepdown
isolating transformer in the mix the control range would be quite
narrow anyway.

Yes, specialized controllers will do the job, and do it well - but
last time I priced a 600 watt magnetically rated Lutron it was over
$69, and I've picked up numerous used and surplus Variacs, from 0.5 to
2.5 kva for under $20 each.

Most of the amateur built (in the USA they call then experimental)
plane builders I know who have used hot wire cutters have used
Variacs. Myself, I'm building with ChroMo steel and 6061T6 Al.

On Sun, 07 Nov 2010 03:20:13 -0500, "Michael A. Terrell"
wrote:


wrote:

On Sat, 06 Nov 2010 11:45:19 -0700, Rich Grise
? wrote:

?Wild_Bill wrote:
??
?? In the DIY article I saw, there was a step-down transformer utilized, and
?? numerous comments about how some dimmers won't last long when attempting
?? to drive (the primary winding of) a transformer, instead of driving a
?? resistive load of incandescent lamp.
??
?I'm an electronics guy by experience and training, and I'd be terribly
?reluctant to use SCR control on the primary of a transformer. I'm "only" a
?tech, so I can't quote numbers; it's just sort of a gut feeling that the
?inductive reactance could cause a phase shift and upset the firing sequence
?of the SCRs.
?
?Of course, I'd be happy to hear of anyone else's experience with this kind
?of setup.
?
?Thanks,
?Rich
I have used lamp dimmers on transformers in the past with mixed
results. Generally a 600 watt or better dimmer handles a 50-100 watt
transformer load reasonably well. The control at the low end, IIRC, is
far from linear and predictable.


The transformer core can saturate from the DC bias caused by cheap
dimmers.
Like I said - far from linear and predictable control.
I favour Variacs and have about 5 different sizes - as well as a
couple good-sized isolation transformers- very usefull for
troubleshooting old radio equipment etc - prevents getting shocks from
"live" chassis, and combined allows ramping up power to see what
happens instead of just plugging it in and letting all the "magic
smoke" escape at once.
Have a "buck-boost" transformer as well that can add or subtact 6 or
12 volts from line voltafe - but I like the variac better for that
too. (4 terminal, not 3)

wrote:

On Sun, 07 Nov 2010 03:20:13 -0500, "Michael A. Terrell"
wrote:


wrote:

On Sat, 06 Nov 2010 11:45:19 -0700, Rich Grise
? wrote:

?Wild_Bill wrote:
??
?? In the DIY article I saw, there was a step-down transformer utilized, and
?? numerous comments about how some dimmers won't last long when attempting
?? to drive (the primary winding of) a transformer, instead of driving a
?? resistive load of incandescent lamp.
??
?I'm an electronics guy by experience and training, and I'd be terribly
?reluctant to use SCR control on the primary of a transformer. I'm "only" a
?tech, so I can't quote numbers; it's just sort of a gut feeling that the
?inductive reactance could cause a phase shift and upset the firing sequence
?of the SCRs.
?
?Of course, I'd be happy to hear of anyone else's experience with this kind
?of setup.
?
?Thanks,
?Rich
I have used lamp dimmers on transformers in the past with mixed
results. Generally a 600 watt or better dimmer handles a 50-100 watt
transformer load reasonably well. The control at the low end, IIRC, is
far from linear and predictable.


The transformer core can saturate from the DC bias caused by cheap
dimmers.
Like I said - far from linear and predictable control.
I favour Variacs and have about 5 different sizes - as well as a
couple good-sized isolation transformers- very usefull for
troubleshooting old radio equipment etc - prevents getting shocks from
"live" chassis, and combined allows ramping up power to see what
happens instead of just plugging it in and letting all the "magic
smoke" escape at once.
Have a "buck-boost" transformer as well that can add or subtact 6 or
12 volts from line voltafe - but I like the variac better for that
too. (4 terminal, not 3)


You can use a handful of old filament transformers & switches to get
a lot more choices from a Buck/Boost configuration. 240V variacs are
usually cheap, but have to be derated becasue you are using them at half
voltage.


--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

Joseph Gwinn wrote:
In ,
wrote:

(...)

That all looks inductively reactive to me.

Then there is problem with the spice model. More below.

Joe, I did the experiment in the real world just now.

You are right. With a resistive load on the secondary
I saw undetectable phase shift between voltage and
current on the primary. With an inductively reactive
load on the secondary, I saw the expected significant
lag in the current peak in relation to the voltage peak
on the primary.

That will be what I learned today and I thank you.

(...)

War story: Many years ago, I was interested in the physics of xenon
flash lamps. The physical model is pretty simple, a charged capacitor
discharging into an arc, which arc heated the xenon to incandescence,
the resulting light and heat radiation carrying the energy away. This
yields a set of coupled ordinary differential equations that one solves
numerically, time step by time step. I got it all working, and all was
well. Then I changed the duration of the time steps, and more energy
came out as light and heat than was stored in the capacitor. Oops.
Violated the conservation of energy.

Turns out I had made two mathematical mistakes, which mistakes cancelled
one another only for the original step duration.

Wow!
That one must have had you scratching your head for a while!

Thanks again

--Winston

In article ,
Winston wrote:

Joseph Gwinn wrote:
In ,
wrote:

(...)

That all looks inductively reactive to me.

Then there is problem with the spice model. More below.

Joe, I did the experiment in the real world just now.

You are right. With a resistive load on the secondary
I saw undetectable phase shift between voltage and
current on the primary. With an inductively reactive
load on the secondary, I saw the expected significant
lag in the current peak in relation to the voltage peak
on the primary.

That will be what I learned today and I thank you.

(...)

War story: Many years ago, I was interested in the physics of xenon
flash lamps. The physical model is pretty simple, a charged capacitor
discharging into an arc, which arc heated the xenon to incandescence,
the resulting light and heat radiation carrying the energy away. This
yields a set of coupled ordinary differential equations that one solves
numerically, time step by time step. I got it all working, and all was
well. Then I changed the duration of the time steps, and more energy
came out as light and heat than was stored in the capacitor. Oops.
Violated the conservation of energy.

Turns out I had made two mathematical mistakes, which mistakes cancelled
one another only for the original step duration.

Wow!
That one must have had you scratching your head for a while!

It did. If only I could patent it, I could be rich and famous.
Boundless power source. I would have settled for rich.

More seriousy, models can be dangerous.


Joe Gwinn

On Sun, 07 Nov 2010 16:26:14 -0500, Joseph Gwinn
wrote:

In article ,
Bill Noble wrote:

On 11/7/2010 8:56 AM, Joseph Gwinn wrote:
In ,
[snip]

But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).

The presented load is in fact largely resistive, and I think the point
made by another poster about DC unbalance saturating the transformer
core is a key issue. If one is in fact driving an inductor, one can
have both problems at once; the problems are independent.

In any event, Lutron et al make semiconductor dimmers intended to drive
resistive loads through step-down transformers.

Joe Gwinn


Joe - not correct because of the way triacs work - if driven by a pure
sine wave, it is true (or nearly so). But if you turn on say 90 deg
after zero crossing you have a huge current spike - that is due to the
inductive nature of the transformer and the fact that there is no flux
in the core at turn on.

It's true that the triac turns on abruptly, in a microsecond or two. At
90 degrees, this imposes a full voltage step on the transformer input.
But, wouldn't inductance slow the spike down, whatever the state of the
core? This is a classic homework problem. The current rises linearly
from zero, and the higher the inductance the slower the rate of rise.

You would, ideally, use a reverse phase dimmer/controller. They turn
the current OFF at a particular phase angle, instead of on. Some of
the high end dimmers for magnetic transformer low voltage lighting are
reverse phase dimmers. They are NOT cheap. They do, however, reduce
the EMI/RFI emissions significantly and almost totally eliminate the
filament hum common to standard Thyrister dimmers (which Variacs also
totally eliminate , which is why they are still used in many studio
lighting systems) Darn electronic theatre lighting systems are pretty
noisy - I work next to one several hours a day.


Work out the math if you don't believe me, or
try it with a transformer and a battery and a scope - measure the
current pulse at turn on with say 12V applied to the 12V winding of a
transformer - you are looking for the first 1 ms of current

This sounds like we would have significant DC current through the
transformer winding, which could saturate the core. A saturated core
will certainly cause a current spike. Maybe I don't understand the
proposed test circuit.


I recall that Lutron had patents in this area, and these patents
provided a summary of the issues to be solved, so I did some searching.

A good discussion appears in US patent 4,876,498. A later version of
the same patent is 4,954,768. Leviton reacted with their own solution,
in patent 7,482,758. And 5,477,111 speaks directly to control of
inductive loads like motors. (Go to http://www.pat2pdf.org to get
copies.)

Anyway, the big issue in these patents is DC causing saturation, and (in
two-wire circuits) inductance-caused errors in knowing when to trigger
the triacs.


Joe Gwinn

On Sun, 07 Nov 2010 21:28:49 -0500, "Michael A. Terrell"
wrote:


wrote:

On Sun, 07 Nov 2010 03:20:13 -0500, "Michael A. Terrell"
wrote:


wrote:

On Sat, 06 Nov 2010 11:45:19 -0700, Rich Grise
? wrote:

?Wild_Bill wrote:
??
?? In the DIY article I saw, there was a step-down transformer utilized, and
?? numerous comments about how some dimmers won't last long when attempting
?? to drive (the primary winding of) a transformer, instead of driving a
?? resistive load of incandescent lamp.
??
?I'm an electronics guy by experience and training, and I'd be terribly
?reluctant to use SCR control on the primary of a transformer. I'm "only" a
?tech, so I can't quote numbers; it's just sort of a gut feeling that the
?inductive reactance could cause a phase shift and upset the firing sequence
?of the SCRs.
?
?Of course, I'd be happy to hear of anyone else's experience with this kind
?of setup.
?
?Thanks,
?Rich
I have used lamp dimmers on transformers in the past with mixed
results. Generally a 600 watt or better dimmer handles a 50-100 watt
transformer load reasonably well. The control at the low end, IIRC, is
far from linear and predictable.


The transformer core can saturate from the DC bias caused by cheap
dimmers.
Like I said - far from linear and predictable control.
I favour Variacs and have about 5 different sizes - as well as a
couple good-sized isolation transformers- very usefull for
troubleshooting old radio equipment etc - prevents getting shocks from
"live" chassis, and combined allows ramping up power to see what
happens instead of just plugging it in and letting all the "magic
smoke" escape at once.
Have a "buck-boost" transformer as well that can add or subtact 6 or
12 volts from line voltafe - but I like the variac better for that
too. (4 terminal, not 3)


You can use a handful of old filament transformers & switches to get
a lot more choices from a Buck/Boost configuration. 240V variacs are
usually cheap, but have to be derated becasue you are using them at half
voltage.
My old buck-boost is one I built from a heavy fillament transformer
many many years ago, that has followed me around like a well trained
puppy.

As for the 240 volt variacs, they are good for the nameplate AMPERAGE
on 120, which reduces the power (va or wattage) rating significantly.

And I see my fat-finger syndrome spelt voltage wrong again. Thought I
better correct it before someone else jumped on it.

Joseph Gwinn wrote:

(...)

If only I could patent it, I could be rich and famous.
Boundless power source. I would have settled for rich.

Luckily, we both settled for "devilishly handsome".

More seriousy, models can be dangerous.

Yup.


--Winston

wrote:

On Sun, 07 Nov 2010 21:28:49 -0500, "Michael A. Terrell"
wrote:


wrote:

On Sun, 07 Nov 2010 03:20:13 -0500, "Michael A. Terrell"
wrote:


wrote:

On Sat, 06 Nov 2010 11:45:19 -0700, Rich Grise
? wrote:

?Wild_Bill wrote:
??
?? In the DIY article I saw, there was a step-down transformer utilized, and
?? numerous comments about how some dimmers won't last long when attempting
?? to drive (the primary winding of) a transformer, instead of driving a
?? resistive load of incandescent lamp.
??
?I'm an electronics guy by experience and training, and I'd be terribly
?reluctant to use SCR control on the primary of a transformer. I'm "only" a
?tech, so I can't quote numbers; it's just sort of a gut feeling that the
?inductive reactance could cause a phase shift and upset the firing sequence
?of the SCRs.
?
?Of course, I'd be happy to hear of anyone else's experience with this kind
?of setup.
?
?Thanks,
?Rich
I have used lamp dimmers on transformers in the past with mixed
results. Generally a 600 watt or better dimmer handles a 50-100 watt
transformer load reasonably well. The control at the low end, IIRC, is
far from linear and predictable.


The transformer core can saturate from the DC bias caused by cheap
dimmers.
Like I said - far from linear and predictable control.
I favour Variacs and have about 5 different sizes - as well as a
couple good-sized isolation transformers- very usefull for
troubleshooting old radio equipment etc - prevents getting shocks from
"live" chassis, and combined allows ramping up power to see what
happens instead of just plugging it in and letting all the "magic
smoke" escape at once.
Have a "buck-boost" transformer as well that can add or subtact 6 or
12 volts from line voltafe - but I like the variac better for that
too. (4 terminal, not 3)


You can use a handful of old filament transformers & switches to get
a lot more choices from a Buck/Boost configuration. 240V variacs are
usually cheap, but have to be derated becasue you are using them at half
voltage.
My old buck-boost is one I built from a heavy fillament transformer
many many years ago, that has followed me around like a well trained
puppy.

As for the 240 volt variacs, they are good for the nameplate AMPERAGE
on 120, which reduces the power (va or wattage) rating significantly.


Yes, but small ones can be dirt cheap. Most people seem to think
they can handle double the current when one is used at half the
nameplate voltage, and can't be convinced otherwise till smoke pours out
of their recently departed variac.



And I see my fat-finger syndrome spelt voltage wrong again. Thought I
better correct it before someone else jumped on it.


--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

In article ,
wrote:

On Sun, 07 Nov 2010 16:26:14 -0500, Joseph Gwinn
wrote:

In article ,
Bill Noble wrote:

On 11/7/2010 8:56 AM, Joseph Gwinn wrote:
In ,
[snip]

But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).

The presented load is in fact largely resistive, and I think the point
made by another poster about DC unbalance saturating the transformer
core is a key issue. If one is in fact driving an inductor, one can
have both problems at once; the problems are independent.

In any event, Lutron et al make semiconductor dimmers intended to drive
resistive loads through step-down transformers.

Joe Gwinn


Joe - not correct because of the way triacs work - if driven by a pure
sine wave, it is true (or nearly so). But if you turn on say 90 deg
after zero crossing you have a huge current spike - that is due to the
inductive nature of the transformer and the fact that there is no flux
in the core at turn on.

It's true that the triac turns on abruptly, in a microsecond or two. At
90 degrees, this imposes a full voltage step on the transformer input.
But, wouldn't inductance slow the spike down, whatever the state of the
core? This is a classic homework problem. The current rises linearly
from zero, and the higher the inductance the slower the rate of rise.

You would, ideally, use a reverse phase dimmer/controller. They turn
the current OFF at a particular phase angle, instead of on. Some of
the high end dimmers for magnetic transformer low voltage lighting are
reverse phase dimmers. They are NOT cheap. They do, however, reduce
the EMI/RFI emissions significantly and almost totally eliminate the
filament hum common to standard Thyrister dimmers (which Variacs also
totally eliminate , which is why they are still used in many studio
lighting systems). Darn electronic theatre lighting systems are pretty
noisy - I work next to one several hours a day.

Reduce filament noise and EMI? That certainly makes sense. There are
many patents on reverse-phase dimmers as well, with reduced noise and
EMI given as advantages.


As for variacs and EMI, I have a war story: In the 1970s, I lived in
Baltimore. A friend of mine was an electronics tech at Johns Hopkins
University Hospital. The hospital did much basic research, and the
electrophysiology department had a screened room (big walk-in box made
of copper window screen fabric and having a metal door), within which
they attempted to record nerve potentials and pulses, but were greatly
hindered by electrical noise despite the screenroom.

My friend was asked to solve this problem. It developed that the
overhead lights, which shone through to copper screen fabric, were
fluorescent. Ouch. No wonder. Replaced the fluorescent lamps with
incandescent lamps controlled by a wall-mounted variac dimmer. The
noise level inside the screenroom dropped to insignificance (relative to
the inherent noise of the instruments). One could also dim the lights,
making the instrument displays easier to see. The doctors were very
pleased.

I would guess that the EMI from the fluorescent lamps was enough to get
through the power-line EMI filters where power enters the screenroom to
supply the instruments. The good doctors probably also left the door
open, but the powerline path was probably by far the most important, as
only noise below a megahertz or so can affect electrophysiology
experiments.


Joe Gwinn


Work out the math if you don't believe me, or
try it with a transformer and a battery and a scope - measure the
current pulse at turn on with say 12V applied to the 12V winding of a
transformer - you are looking for the first 1 ms of current

This sounds like we would have significant DC current through the
transformer winding, which could saturate the core. A saturated core
will certainly cause a current spike. Maybe I don't understand the
proposed test circuit.


I recall that Lutron had patents in this area, and these patents
provided a summary of the issues to be solved, so I did some searching.

A good discussion appears in US patent 4,876,498. A later version of
the same patent is 4,954,768. Leviton reacted with their own solution,
in patent 7,482,758. And 5,477,111 speaks directly to control of
inductive loads like motors. (Go to http://www.pat2pdf.org to get
copies.)

Anyway, the big issue in these patents is DC causing saturation, and (in
two-wire circuits) inductance-caused errors in knowing when to trigger
the triacs.


Joe Gwinn

On Mon, 08 Nov 2010 09:54:06 -0500, Joseph Gwinn
wrote:

In article ,
wrote:

On Sun, 07 Nov 2010 16:26:14 -0500, Joseph Gwinn
wrote:

In article ,
Bill Noble wrote:

On 11/7/2010 8:56 AM, Joseph Gwinn wrote:
In ,
[snip]

But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).

The presented load is in fact largely resistive, and I think the point
made by another poster about DC unbalance saturating the transformer
core is a key issue. If one is in fact driving an inductor, one can
have both problems at once; the problems are independent.

In any event, Lutron et al make semiconductor dimmers intended to drive
resistive loads through step-down transformers.

Joe Gwinn


Joe - not correct because of the way triacs work - if driven by a pure
sine wave, it is true (or nearly so). But if you turn on say 90 deg
after zero crossing you have a huge current spike - that is due to the
inductive nature of the transformer and the fact that there is no flux
in the core at turn on.

It's true that the triac turns on abruptly, in a microsecond or two. At
90 degrees, this imposes a full voltage step on the transformer input.
But, wouldn't inductance slow the spike down, whatever the state of the
core? This is a classic homework problem. The current rises linearly
from zero, and the higher the inductance the slower the rate of rise.

You would, ideally, use a reverse phase dimmer/controller. They turn
the current OFF at a particular phase angle, instead of on. Some of
the high end dimmers for magnetic transformer low voltage lighting are
reverse phase dimmers. They are NOT cheap. They do, however, reduce
the EMI/RFI emissions significantly and almost totally eliminate the
filament hum common to standard Thyrister dimmers (which Variacs also
totally eliminate , which is why they are still used in many studio
lighting systems). Darn electronic theatre lighting systems are pretty
noisy - I work next to one several hours a day.

Reduce filament noise and EMI? That certainly makes sense. There are
many patents on reverse-phase dimmers as well, with reduced noise and
EMI given as advantages.


As for variacs and EMI, I have a war story: In the 1970s, I lived in
Baltimore. A friend of mine was an electronics tech at Johns Hopkins
University Hospital. The hospital did much basic research, and the
electrophysiology department had a screened room (big walk-in box made
of copper window screen fabric and having a metal door), within which
they attempted to record nerve potentials and pulses, but were greatly
hindered by electrical noise despite the screenroom.

My friend was asked to solve this problem. It developed that the
overhead lights, which shone through to copper screen fabric, were
fluorescent. Ouch. No wonder. Replaced the fluorescent lamps with
incandescent lamps controlled by a wall-mounted variac dimmer. The
noise level inside the screenroom dropped to insignificance (relative to
the inherent noise of the instruments). One could also dim the lights,
making the instrument displays easier to see. The doctors were very
pleased.

I would guess that the EMI from the fluorescent lamps was enough to get
through the power-line EMI filters where power enters the screenroom to
supply the instruments. The good doctors probably also left the door
open, but the powerline path was probably by far the most important, as
only noise below a megahertz or so can affect electrophysiology
experiments.


Joe Gwinn


Work out the math if you don't believe me, or
try it with a transformer and a battery and a scope - measure the
current pulse at turn on with say 12V applied to the 12V winding of a
transformer - you are looking for the first 1 ms of current

This sounds like we would have significant DC current through the
transformer winding, which could saturate the core. A saturated core
will certainly cause a current spike. Maybe I don't understand the
proposed test circuit.


I recall that Lutron had patents in this area, and these patents
provided a summary of the issues to be solved, so I did some searching.

A good discussion appears in US patent 4,876,498. A later version of
the same patent is 4,954,768. Leviton reacted with their own solution,
in patent 7,482,758. And 5,477,111 speaks directly to control of
inductive loads like motors. (Go to http://www.pat2pdf.org to get
copies.)

Anyway, the big issue in these patents is DC causing saturation, and (in
two-wire circuits) inductance-caused errors in knowing when to trigger
the triacs.


Joe Gwinn
The emi from the flourescent bulbs was not just going back the power
lines. The radiation from the bulbs themselves was significant, and
anything short of a full faraday cage wasn't going to catch it all.
The copper screen was not sufficient (either not full enough coverage,
or not grounded, etc)

Joseph Gwinn wrote in
:

In article ,
Winston wrote:

Joseph Gwinn wrote:
In ,
wrote:

(...)

That all looks inductively reactive to me.

Then there is problem with the spice model. More below.

Joe, I did the experiment in the real world just now.

You are right. With a resistive load on the secondary
I saw undetectable phase shift between voltage and
current on the primary. With an inductively reactive
load on the secondary, I saw the expected significant
lag in the current peak in relation to the voltage peak
on the primary.

That will be what I learned today and I thank you.

(...)

War story: Many years ago, I was interested in the physics of
xenon flash lamps. The physical model is pretty simple, a charged
capacitor discharging into an arc, which arc heated the xenon to
incandescence, the resulting light and heat radiation carrying the
energy away. This yields a set of coupled ordinary differential
equations that one solves numerically, time step by time step. I
got it all working, and all was well. Then I changed the duration
of the time steps, and more energy came out as light and heat than
was stored in the capacitor. Oops. Violated the conservation of
energy.

Turns out I had made two mathematical mistakes, which mistakes
cancelled one another only for the original step duration.

Wow!
That one must have had you scratching your head for a while!

It did. If only I could patent it, I could be rich and famous.
Boundless power source. I would have settled for rich.

More seriousy, models can be dangerous.

What's worse is a model in the hands of a freshly minted graduate who has
no common sense. Designs for things like high gain omnidirectional
antennas with 150% efficiency is what you get. They tend to get more
excited than suspicious, which is very depressing. They've been taught
that anything that comes out of an expensive computer program MUST be the
truth, and they have no horse sense to tell them otherwise.

I spend a lot of my time at work raining on kid's parades...

Doug White

On Mon, 08 Nov 2010 14:29:13 -0500, wrote:

The emi from the flourescent bulbs was not just going back the power
lines. The radiation from the bulbs themselves was significant, and
anything short of a full faraday cage wasn't going to catch it all.
The copper screen was not sufficient (either not full enough coverage,
or not grounded, etc)
I'll go along with that - I have problems with a wireless mouse 24"
below a single 48" tube fixture, I need to go to a corded mouse.
Gerry :-)}
London, Canada

Jon Anderson wrote:
Want to make a very simple hot wire cutter to cut polyurethane foam.
Probably not more than 12-16" span, max. Found a simple setup on
Instructables using a 24v 2a transformer and a wall light dimmer.
Well, I've got a 12v 4a DC power supply, seems to me that this ought to
be enough, but thought I'd ask.

Thanks,

Jon


Jon,

I'm real curious (after all this discussion) about what you decided
to do?



--

Richard Lamb
email me:
web site: www.home.earthlink.net/~cavelamb

On 11/8/2010 4:35 PM, CaveLamb wrote:

I'm real curious (after all this discussion) about what you decided
to do?

It was in one of my replies. After reading about the toxic fumes I'd get
from polyurethane, I suddenly remembered a hacksaw blade I have that
looks like it came from a bread slicing machine, and maybe it did.
Going to make a simple fixture to hold the blade under tension at the
required height, and oscillate the foam past the blade.
I'll post a few pictures to the Drop Box when I get back to fitting the
drawer for my micrometers.

I may yet make a hot wire cutter for some other project, but I don't
have many cuts to make, so the blade should do fine.

Jon

Jon Anderson wrote:
On 11/8/2010 4:35 PM, CaveLamb wrote:

I'm real curious (after all this discussion) about what you decided
to do?

It was in one of my replies. After reading about the toxic fumes I'd get
from polyurethane, I suddenly remembered a hacksaw blade I have that
looks like it came from a bread slicing machine, and maybe it did.
Going to make a simple fixture to hold the blade under tension at the
required height, and oscillate the foam past the blade.
I'll post a few pictures to the Drop Box when I get back to fitting the
drawer for my micrometers.

I may yet make a hot wire cutter for some other project, but I don't
have many cuts to make, so the blade should do fine.

Jon


You WILL get a smoother cut with a hot wire.

A NIOSH mask solves the fumes problem nicely.


--

Richard Lamb
email me:
web site: www.home.earthlink.net/~cavelamb

CaveLamb wrote:
Jon Anderson wrote:
On 11/8/2010 4:35 PM, CaveLamb wrote:

I'm real curious (after all this discussion) about what you decided
to do?

It was in one of my replies. After reading about the toxic fumes I'd
get from polyurethane, I suddenly remembered a hacksaw blade I have
that looks like it came from a bread slicing machine, and maybe it did.
Going to make a simple fixture to hold the blade under tension at the
required height, and oscillate the foam past the blade.
I'll post a few pictures to the Drop Box when I get back to fitting
the drawer for my micrometers.

I may yet make a hot wire cutter for some other project, but I don't
have many cuts to make, so the blade should do fine.

Jon


You WILL get a smoother cut with a hot wire.

A NIOSH mask solves the fumes problem nicely.

The 1/2" - thick closed - cell foam cuts easily, quickly and
cleanly with a hobby (X-Acto) knife. Tight radii are easy to make.

AMHIKT (Go ahead!)

--Winston

In article ,
Doug White wrote:

Joseph Gwinn wrote in
:

In article ,
Winston wrote:

Joseph Gwinn wrote:
In ,
wrote:

(...)

That all looks inductively reactive to me.

Then there is problem with the spice model. More below.

Joe, I did the experiment in the real world just now.

You are right. With a resistive load on the secondary
I saw undetectable phase shift between voltage and
current on the primary. With an inductively reactive
load on the secondary, I saw the expected significant
lag in the current peak in relation to the voltage peak
on the primary.

That will be what I learned today and I thank you.

(...)

War story: Many years ago, I was interested in the physics of
xenon flash lamps. The physical model is pretty simple, a charged
capacitor discharging into an arc, which arc heated the xenon to
incandescence, the resulting light and heat radiation carrying the
energy away. This yields a set of coupled ordinary differential
equations that one solves numerically, time step by time step. I
got it all working, and all was well. Then I changed the duration
of the time steps, and more energy came out as light and heat than
was stored in the capacitor. Oops. Violated the conservation of
energy.

Turns out I had made two mathematical mistakes, which mistakes
cancelled one another only for the original step duration.

Wow!
That one must have had you scratching your head for a while!

It did. If only I could patent it, I could be rich and famous.
Boundless power source. I would have settled for rich.

More seriousy, models can be dangerous.

What's worse is a model in the hands of a freshly minted graduate who has
no common sense. Designs for things like high gain omnidirectional
antennas with 150% efficiency is what you get. They tend to get more
excited than suspicious, which is very depressing. They've been taught
that anything that comes out of an expensive computer program MUST be the
truth, and they have no horse sense to tell them otherwise.

I spend a lot of my time at work raining on kid's parades...

Same here, although I have young engineers not necessarily fresh minted.
Some people are born with common sense, some learn it (prompted by the
pain of accumulating scars), and some never acquire it.

The most effective approach I have found is to ask people to do simple
crosscheck analyses based largely on first principles. Like
conservation of energy.


Which elicits yet another war story: A now retired optical engineering
colleague of mine would evaluate requests for proposals by applying
conservation of (optical) energy to the requested optical system.
Typically, the minimum scene brightness, maximum allowed entrance
aperture size, and minimum image brightness would be specified, often
indirectly. Optical systems are passive (unless there is an image
intensifier), so the total energy in the image cannot exceed that
falling on the aperture. In many cases, the requested system was
physically impossible. The art is writing the proposal to explain and
solve this without tipping the competition off or embarrassing the
customer.


Joe Gwinn

In article ,
wrote:

On Mon, 08 Nov 2010 09:54:06 -0500, Joseph Gwinn
wrote:

In article ,
wrote:

On Sun, 07 Nov 2010 16:26:14 -0500, Joseph Gwinn
wrote:

In article ,
Bill Noble wrote:

On 11/7/2010 8:56 AM, Joseph Gwinn wrote:
In ,
[snip]

But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer
in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).

The presented load is in fact largely resistive, and I think the
point
made by another poster about DC unbalance saturating the transformer
core is a key issue. If one is in fact driving an inductor, one can
have both problems at once; the problems are independent.

In any event, Lutron et al make semiconductor dimmers intended to
drive
resistive loads through step-down transformers.

Joe Gwinn


Joe - not correct because of the way triacs work - if driven by a pure
sine wave, it is true (or nearly so). But if you turn on say 90 deg
after zero crossing you have a huge current spike - that is due to the
inductive nature of the transformer and the fact that there is no flux
in the core at turn on.

It's true that the triac turns on abruptly, in a microsecond or two. At
90 degrees, this imposes a full voltage step on the transformer input.
But, wouldn't inductance slow the spike down, whatever the state of the
core? This is a classic homework problem. The current rises linearly
from zero, and the higher the inductance the slower the rate of rise.

You would, ideally, use a reverse phase dimmer/controller. They turn
the current OFF at a particular phase angle, instead of on. Some of
the high end dimmers for magnetic transformer low voltage lighting are
reverse phase dimmers. They are NOT cheap. They do, however, reduce
the EMI/RFI emissions significantly and almost totally eliminate the
filament hum common to standard Thyrister dimmers (which Variacs also
totally eliminate , which is why they are still used in many studio
lighting systems). Darn electronic theatre lighting systems are pretty
noisy - I work next to one several hours a day.

Reduce filament noise and EMI? That certainly makes sense. There are
many patents on reverse-phase dimmers as well, with reduced noise and
EMI given as advantages.


As for variacs and EMI, I have a war story: In the 1970s, I lived in
Baltimore. A friend of mine was an electronics tech at Johns Hopkins
University Hospital. The hospital did much basic research, and the
electrophysiology department had a screened room (big walk-in box made
of copper window screen fabric and having a metal door), within which
they attempted to record nerve potentials and pulses, but were greatly
hindered by electrical noise despite the screenroom.

My friend was asked to solve this problem. It developed that the
overhead lights, which shone through to copper screen fabric, were
fluorescent. Ouch. No wonder. Replaced the fluorescent lamps with
incandescent lamps controlled by a wall-mounted variac dimmer. The
noise level inside the screenroom dropped to insignificance (relative to
the inherent noise of the instruments). One could also dim the lights,
making the instrument displays easier to see. The doctors were very
pleased.

I would guess that the EMI from the fluorescent lamps was enough to get
through the power-line EMI filters where power enters the screenroom to
supply the instruments. The good doctors probably also left the door
open, but the powerline path was probably by far the most important, as
only noise below a megahertz or so can affect electrophysiology
experiments.


Joe Gwinn


Work out the math if you don't believe me, or
try it with a transformer and a battery and a scope - measure the
current pulse at turn on with say 12V applied to the 12V winding of a
transformer - you are looking for the first 1 ms of current

This sounds like we would have significant DC current through the
transformer winding, which could saturate the core. A saturated core
will certainly cause a current spike. Maybe I don't understand the
proposed test circuit.


I recall that Lutron had patents in this area, and these patents
provided a summary of the issues to be solved, so I did some searching.

A good discussion appears in US patent 4,876,498. A later version of
the same patent is 4,954,768. Leviton reacted with their own solution,
in patent 7,482,758. And 5,477,111 speaks directly to control of
inductive loads like motors. (Go to http://www.pat2pdf.org to get
copies.)

Anyway, the big issue in these patents is DC causing saturation, and (in
two-wire circuits) inductance-caused errors in knowing when to trigger
the triacs.


Joe Gwinn
The emi from the flourescent bulbs was not just going back the power
lines. The radiation from the bulbs themselves was significant, and
anything short of a full faraday cage wasn't going to catch it all.
The copper screen was not sufficient (either not full enough coverage,
or not grounded, etc)

It was a commercial faraday cage, and I'm pretty sure it was grounded.
If closing the cage door solved the problem, my friend would not have
been given the assignment.

My reason to think that conducted EMI was the issue is that those lamps
are far too small physically to be very good at emitting noise at 1 MHz
and below. Nor would 1 MHz waves fit through the door, so leaving it
open would have little effect, as observed. At 1 MHz, the wavelength is
300 meters.


Joe Gwinn

On 11/8/2010 10:26 PM, CaveLamb wrote:

You WILL get a smoother cut with a hot wire.

For the application, not an issue. The cut surface will be facing down
and not visible.

A NIOSH mask solves the fumes problem nicely.

Sure, but an expense I don't really wish to incur. I'm only lining 3-4
drawers with foam, it's not a huge project. If I'm getting into
something new and going to do a lot of it, I'll do it right. When it's
just a small project, I'll look to cut corners where I can get away with
it. Frankly if I'd remembered the blade in the first place, I wouldn't
even have looked up hot wire cutters.

Funny now that I think about it, I don't even remember where I got this
blade. Best I can recall, I've had it for 15+ years and never used it... G

Jon

On 11/9/2010 6:18 AM, Winston wrote:

The 1/2" - thick closed - cell foam cuts easily, quickly and
cleanly with a hobby (X-Acto) knife. Tight radii are easy to make.

That's how I'm cutting the pockets for the tools. What I want to do with
the piece I've cut out is slice 3/16 off and stuff that back into the
pocket to cushion the mics. That will also help as a 1/2" deep pocket is
a bit much for most of my mics.

Jon

Jon Anderson wrote:
On 11/9/2010 6:18 AM, Winston wrote:

The 1/2" - thick closed - cell foam cuts easily, quickly and
cleanly with a hobby (X-Acto) knife. Tight radii are easy to make.

That's how I'm cutting the pockets for the tools. What I want to do with
the piece I've cut out is slice 3/16 off and stuff that back into the
pocket to cushion the mics.

Straightedge slice. Very pretty!

That will also help as a 1/2" deep pocket is
a bit much for most of my mics.

That's why I cut additional recesses on
both sides of the tools for my fingers.

It works great!

--Winston

Jon Anderson wrote:

On 11/9/2010 6:18 AM, Winston wrote:

The 1/2" - thick closed - cell foam cuts easily, quickly and
cleanly with a hobby (X-Acto) knife. Tight radii are easy to make.

That's how I'm cutting the pockets for the tools. What I want to do with
the piece I've cut out is slice 3/16 off and stuff that back into the
pocket to cushion the mics. That will also help as a 1/2" deep pocket is
a bit much for most of my mics.

Bread knife. :-)

Cheers!
Rich

Put the variac on the primary of a transformer. e.g. a 24V
then the variac can be from 0v to say 24v depending on how
it is set up.

Martin

On 11/6/2010 10:48 PM, wrote:
On Fri, 05 Nov 2010 22:22:15 -0700, Bill Noble
wrote:

On 11/5/2010 10:04 PM, Jon Anderson wrote:
Want to make a very simple hot wire cutter to cut polyurethane foam.
Probably not more than 12-16" span, max. Found a simple setup on
Instructables using a 24v 2a transformer and a wall light dimmer.
Well, I've got a 12v 4a DC power supply, seems to me that this ought to
be enough, but thought I'd ask.

Thanks,

Jon

how are you going to control the voltage on the power supply - you don't
want the wire glowing white hot, you want it just warm enough to do the
job. Many 12V power supplies are regulated, so it would take a bit of
futzing around to make the supply adjustable
GENERALLY an AC transformer is a better choice - you know there is
no fancy regulation, and no rectifiers to pop.
A lamp dimmer can work for controll, but some do not like inductive
loads like transformers. A simple powerstat / variac controlling the
transformer works great - and you can use any transformer with
adequate current output because you have full control of the voltage.

A variac alone will do the job, but DO NOT be tempted to go that route
as it has no isolation - the cutter will be "live" in relation to
ground. You NEED an isolation transformer of some type in the circuit.

For cutting thin foam, most of us would just use our benchtop water jet
machines.

An old sewing macine could possibly be adapted/modified to cut thin, soft
materials.

When I was making straps and various other accessories for boats that I had
years ago, I stopped in a local furniture upholstery shop to find out what
types of needles they used for thick, multiple layers of fabric, and the
friendly owner was happy to inform me of all sorts of interesting info
regarding thick materials (straps, parachute harnesses etc).
They use needles that cut as they pass thru thick materials, where common
single-pointed needles will get jammed in dense, thick materials.
I was able to gently grind the shanks of the commercial machine cutting
needles so they would fit an old Necchi home-type sewing machine, and the
cutting points made multiple layers of strap material and various fabrics a
breeze for the tiny Necchi motor.

I believe the cutting points were ground similar to an endmill, with the tip
ground as two bevels in opposing directions and with a greater relief angle,
which would likely require a good magnifier and possibly a mini abrasive
disk or stone in a rotary tool.

A cutting point that reciprocates rapidly could be very useful for cutting
fine lines in synthetic materials such as foam. While feeding the material
by hand (without using the sewing machine feed pawls, or thread), closely
spaced perforations/holes will essentially become a cut line.
The freehand feed mode is used for embroidery-type work with thread, such as
creating fabric patches with graphics on them (I watched this being done
years ago).

--
WB
..........


"Jon Anderson" wrote in message
...
Want to make a very simple hot wire cutter to cut polyurethane foam.
Probably not more than 12-16" span, max. Found a simple setup on
Instructables using a 24v 2a transformer and a wall light dimmer.
Well, I've got a 12v 4a DC power supply, seems to me that this ought to be
enough, but thought I'd ask.

Thanks,

Jon

Turkey cutting knife does a good job - back and forth opposing knives.
Martin

On 11/10/2010 7:12 AM, Wild_Bill wrote:
For cutting thin foam, most of us would just use our benchtop water jet
machines.

An old sewing macine could possibly be adapted/modified to cut thin,
soft materials.

When I was making straps and various other accessories for boats that I
had years ago, I stopped in a local furniture upholstery shop to find
out what types of needles they used for thick, multiple layers of
fabric, and the friendly owner was happy to inform me of all sorts of
interesting info regarding thick materials (straps, parachute harnesses
etc).
They use needles that cut as they pass thru thick materials, where
common single-pointed needles will get jammed in dense, thick materials.
I was able to gently grind the shanks of the commercial machine cutting
needles so they would fit an old Necchi home-type sewing machine, and
the cutting points made multiple layers of strap material and various
fabrics a breeze for the tiny Necchi motor.

I believe the cutting points were ground similar to an endmill, with the
tip ground as two bevels in opposing directions and with a greater
relief angle, which would likely require a good magnifier and possibly a
mini abrasive disk or stone in a rotary tool.

A cutting point that reciprocates rapidly could be very useful for
cutting fine lines in synthetic materials such as foam. While feeding
the material by hand (without using the sewing machine feed pawls, or
thread), closely spaced perforations/holes will essentially become a cut
line.
The freehand feed mode is used for embroidery-type work with thread,
such as creating fabric patches with graphics on them (I watched this
being done years ago).