Hot wire cutter question, power supply
 

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

Hot wire cutter question, power supply
 

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

Hot wire cutter question, power supply
 

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.

If you plug your 12v supply into an autotransformer (Variac) and read
it's DC output voltage (across a 1.2K ohm 1/2 W resistor) with various
input voltages, you will find out if your supply has an internal
regulator or is unregulated.

If it is unregulated, Bobs Your Uncle. Just run the supply at the
appropriate power level using your autotransformer.

If it is internally regulated, you get to insert an external
variable regulator between the supply and your cutter or you get
to replace it with a 12 V variable bench supply.

--Winston

Hot wire cutter question, power supply
 

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

That's something I usually cobble together when needed.

Safety wire, a board and some nails are all it takes.
(ok, and a battery charger)

Nail some nails in the board, zig-zag some wire between them (wrap
around the nail, of course).

That's the voltage regulator.

Stretch the cutter wire tight (in a frame - probably more of that board.

Clip the charger to one end of the cutter, the other end to the last
nail (farthest from the cutter) and turn it on.

Move the clip to a closer nail to make it hotter.

The right temperature will make crisp bacon sizzle when the wire touches
the foam.

It doesn't smell as tasty though...



Richard

Hot wire cutter question, power supply
 

On Fri, 05 Nov 2010 21:04:12 -0800, 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.

As Bill Noble and Winston have noted, you could find out if the
power supply (PS) includes a voltage regulator. Then you could
find out if it's a linear or switching regulator. A 12V 4A linear
PS would usually include a 60Hz transformer weighing 3 or 4 pounds.
If you wanted to de-regulate, you could make an output connection
after the rectifiers and filter caps but before the regulator.

Some 60Hz transformer VA vs size vs weight stuff:
http://amfone.net/Amforum/index.php?action=printpage;topic=16537.0

If the supply is light weight it probably has a switching regulator
and may be unsuitable for de-regulating.

An alternative to changing voltage is changing wire gauge or length.
http://www.heatersplus.com/nichrome.htm has four worked examples
of computing wire gauge, length, voltage, and amperage to get
desired temperatures etc, about 1/3 thru the page.

For example, that 24V 2A PS would need thin wire, say #28 or #30,
to avoid excessive current while still heating up enough. I have
a 21" long hot wire cutter with #24 nichrome (about 0.020 diameter)
powered by a battery charger. Draw is 1.5 - 2A at 6V setting and
3.5 - 4A at 12V, which from above link would be about 500F and 900F
respectively. It works well at 12V except that the section of wire
that is cutting cools off too much, which slows down cutting speed.

A better setup would be #21 wire with a 6 or 7 amp 20V PS -- it would
run a few hundred degrees hotter (to cut much faster), would have
more metal in the cut (hence more heat capacity, to cool off more
slowly in the cutting zone), and could be subject to higher spring
tension without breaking (thus bowing less, giving a straighter cut).

Another nichrome wire properties chart:
http://www.interfacebus.com/properties-of-nichrome-wire.html

--
jiw

Hot wire cutter question, power supply
 

The hot-wire foam cutter that I looked at several years ago used a wall box
lamp dimmer module on the primary 120VAC side of a step-down transformer.
An actual AC-DC power supply isn't required, but could be used if the design
is a simple AC/DC adapter without extra circuitry.

The dimmer module performs the task of a variable temperature control for
the heated wire.

A similar device is a triac circuit that will vary the duty cycle of the
120VAC input to the transformer primary. With only a triac, possibly a small
heatsink, variable resistor/pot, capacitor and inductor, the circuit is
really compact and simple. This same circuit has been used in a variety of
heating devices and soldering iron variable temp controls.

The most important part of the wiring will be the safety ground. If the
safety ground is ignored or done improperly, a fatally hazardous condition
may exist.

The input line needs to be properly fused, and have a reliable, easy to
reach power switch.

A good quality step-down transformer "should" provide adequate line
isolation.
A variac (autotransformer) does not provide isolation, and the output will
be a direct, fatally hazardous connection to the utility power in the
building.

The transformer frame and one secondary winding connection should definitely
be safety earth ground connections. These connections will guard the user
from any transformer leakage issues.
Standard insulating and connection practices, including a grounded metal
enclosure for the electrical connections, will result in a safe device.

If any of the connection or grounding info is unclear, seek qualified,
knowledgeable and experienced help from someone that can check your assembly
methods before using a DIY home-built line voltage operated device.

Secondly, you may find it very helpful to look up info regarding
vapors/fumes from heated synthetic materials.
Some plastics, probably more than the plastics industry would want us to
know, will give off toxic fumes when heated or burned.. maybe even small
amounts of sarin, for example.

I generally don't even use plastics in a microwave oven, but not because I'm
particularly paranoid about it, but I've seen the damage that MW heating
does to plastics.

--
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

Hot wire cutter question, power supply
 

Jon,
There are some good suggestions here, but I would use a heavier wire for thermal mass reasons and a PWM (Pulse Width Modulation),
DC, PM speed control. They are inexpensive and do not use high voltage, so is much safer. The real driver for me would be the
trigger operated pot to instantly control the wire heat when in use to compensate for the cooling effect on the wire by the
substrate.
Steve

"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

Hot wire cutter question, power supply
 

"Steve Lusardi" wrote in
:

Jon,
There are some good suggestions here, but I would use a heavier wire
for thermal mass reasons and a PWM (Pulse Width Modulation), DC, PM
speed control. They are inexpensive and do not use high voltage, so is
much safer. The real driver for me would be the trigger operated pot
to instantly control the wire heat when in use to compensate for the
cooling effect on the wire by the substrate.
Steve

"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.

There's a ton of info he

http://www.ar15.com/forums/topic.html?b=3&f=4&t=292695

A lot of photos of folks work, but also a lot of different hardware set
ups to consider.

Doug White

Hot wire cutter question, power supply
 

I had to make one of those to make long triangles to insulate the
inside corners of the pump house, that reminds me, got to set that up
for winter today.

I went to a hobby shop and bought a beat up old train transformer with
the big variable switch on top for $10 IIRC. Then I found some of
those old ceramic rings light bulbs use to be screwed into and hooked
a spring to each one and strung stainless steel aircraft safety wire
between the two insulators. I guess one on a spring would work also,
can't recall why I did that. Anyhow worked great.

I even set up a 10' long table and a fence that I could move to either
side of the wire to make "crown molding" out of 4"X 4' X 8' foam.

Thinking of setting it back up soon to make lost foam molds for
aluminum.

SW

Hot wire cutter question, power supply
 

On Fri, 05 Nov 2010 21:04:12 -0800, 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.


As an alternative, a stick/TIG welder is an adjustable constant
current supply. I've used mine to power a quick and dirty hot wire
cutter. The welder's minimum current may require using a wire heavier
than the typical cutter. Mine will go down to about 5A.

--
Ned Simmons

Hot wire cutter question, power supply
 

On 11/6/2010 1:08 AM, Wild_Bill wrote:
The hot-wire foam cutter that I looked at several years ago used a wall
box lamp dimmer module on the primary 120VAC side of a step-down
transformer.


snip---------

danger - copying the above into your own design is likely to be both
risky and unsatisfactory - please consider using a proper transformer
with isolated primary and secondary circuits

Hot wire cutter question, power supply
 

On 11/6/2010 7:08 AM, Bill Noble wrote:

On 11/6/2010 1:08 AM, Wild_Bill wrote:
The hot-wire foam cutter that I looked at several years ago used a wall
box lamp dimmer module on the primary 120VAC side of a step-down
transformer.


snip---------

danger - copying the above into your own design is likely to be both
risky and unsatisfactory - please consider using a proper transformer
with isolated primary and secondary circuits

That in fact, is precisely the version I saw on Instructables....

I appreciate all the great input. I should have given a bit more detail.
I'm quite certain the power supply I have is a linear. I plan to control
input voltage with a variac.

This is nothing special or long term, it's part of my project to line my
Vidmar drawers with foam and cut pockets for micrometers and other
tools. I found the perfect foam, lining the cases used to ship a medical
device one of my customers manufactures. They gave me the contact info
for the company that does the custom foam inserts, and I ended up with 6
sheets of nice dense black Polyurethane foam, 1/2" thick, cut to fit my
drawers exactly (25" square, where most foam I found came in 24" widths)

After cutting a pocket, I am going to hot-wire about a 3/16 thick slice
off the piece I just removed, and put it back in the cutout.


Thanks everyone, great info as always!


Jon

Hot wire cutter question, power supply
 

I wasn't suggesting that a lamp dimmer could heat the wire directly, if
that's the way you interpreted it, Bill.

I described the dimmer going to the primary side of a step-down transformer.
I think most people that know transformers, understand that a step-down type
consists of a primary and a secondary winding.

It's not easy describing electrical circuits in a way for everyone to
understand them.. that's why I always try to remember to include the seek
help statement.

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.

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


"Bill Noble" wrote in message
...
On 11/6/2010 1:08 AM, Wild_Bill wrote:
The hot-wire foam cutter that I looked at several years ago used a wall
box lamp dimmer module on the primary 120VAC side of a step-down
transformer.


snip---------

danger - copying the above into your own design is likely to be both risky
and unsatisfactory - please consider using a proper transformer with
isolated primary and secondary circuits

Hot wire cutter question, power supply
 

On Fri, 05 Nov 2010 21:04:12 -0800, 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.

Jon,

I started with a goodly length of nichrome (dunno what gauge, it was a
heating element I picked up) and a 12V battery charger. Decreased the
length of nichrome til it was just right for that output.
--
Best -- Terry

Hot wire cutter question, power supply
 

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

Hot wire cutter question, power supply
 

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.

--Winston

Hot wire cutter question, power supply
 

On 11/6/2010 11:45 AM, 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


it depends on whether the SCR (triac really) is chosen to drive an
inductive load or only a resistive load - the motor speed control in a
variable speed drill is sized and rated for an inductive load, a lamp
dimmer, more particularly an older lamp dimmer, is very much not sized
that way

Hot wire cutter question, power supply
 

On Sat, 06 Nov 2010 09:26:03 -0500, Sunworshipper wrote:



I had to make one of those to make long triangles to insulate the
inside corners of the pump house, that reminds me, got to set that up
for winter today.

I went to a hobby shop and bought a beat up old train transformer with
the big variable switch on top for $10 IIRC. Then I found some of
those old ceramic rings light bulbs use to be screwed into and hooked
a spring to each one and strung stainless steel aircraft safety wire
between the two insulators. I guess one on a spring would work also,
can't recall why I did that. Anyhow worked great.

I even set up a 10' long table and a fence that I could move to either
side of the wire to make "crown molding" out of 4"X 4' X 8' foam.

Thinking of setting it back up soon to make lost foam molds for
aluminum.

SW

Ive got a bunch of brand new rolls of hi-temp wire, if anybody wants any

Gunner


--
"Confiscating wealth from those who have earned it, inherited it,
or got lucky is never going to help 'the poor.' Poverty isn't
caused by some people having more money than others, just as obesity
isn't caused by McDonald's serving super-sized orders of French fries
Poverty, like obesity, is caused by the life choices that dictate
results." - John Tucci,

Hot wire cutter question, power supply
 

On Sat, 06 Nov 2010 09:56:02 -0500, Ned Simmons wrote:

On Fri, 05 Nov 2010 21:04:12 -0800, 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.


As an alternative, a stick/TIG welder is an adjustable constant
current supply. I've used mine to power a quick and dirty hot wire
cutter. The welder's minimum current may require using a wire heavier
than the typical cutter. Mine will go down to about 5A.


OOOH!! Excellent idea! I never thought about doing that.

Very nicely Done!

Crom but I like reading the smart guys here!!

Gunner

--
"Confiscating wealth from those who have earned it, inherited it,
or got lucky is never going to help 'the poor.' Poverty isn't
caused by some people having more money than others, just as obesity
isn't caused by McDonald's serving super-sized orders of French fries
Poverty, like obesity, is caused by the life choices that dictate
results." - John Tucci,

Hot wire cutter question, power supply
 

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.

Er. That's page '9/16' for component values.


--Winston

Hot wire cutter question, power supply
 

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

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

Thanks!
Rich

Hot wire cutter question, power supply
 

Jon Anderson wrote in
:

polyurethane foam

DO NOT CUT POLYURETHANE FOAM WITH A HOT WIRE !!!

The fumes are very toxic... unlike polystyrene...including
hydrogen cynide (think gas chamber).

Check the MSDS's.

Ken

Hot wire cutter question, power supply
 

In article ,
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.

Your instincts are correct - most thyrister dimmers don't like to work
through a transformer. I don't recall the exact reason, but I don't
think it was the primary inductance. (They *really* don't like driving
a big capacitor through a rectifier bridge either.)

But it doesn't matter. Lutron and the other dimmer manufacturers offer
dimmers intended to work through a transformer. Look for dimmers
intended to drive low voltage lamps, such as task lights under kitchen
cabinets.

If you want to know the reason transformers require special handling,
read the patents listed on the dimmer or its box or instructions.

Joe Gwinn

Hot wire cutter question, power supply
 

On 11/6/2010 2:04 PM, Ken Moffett wrote:

The fumes are very toxic... unlike polystyrene...including
hydrogen cynide (think gas chamber).

Check the MSDS's.

A BIG Thank You! I'd planned to do it outdoors, but I don't like messing
with anything toxic I don't -have to-. I have an alternative method to
slice down the foam, and I think I'll go that route...


Jon

Hot wire cutter question, power supply
 

Jon Anderson wrote:
On 11/6/2010 2:04 PM, Ken Moffett wrote:

The fumes are very toxic... unlike polystyrene...including
hydrogen cynide (think gas chamber).

Check the MSDS's.

A BIG Thank You! I'd planned to do it outdoors, but I don't like messing
with anything toxic I don't -have to-. I have an alternative method to
slice down the foam, and I think I'll go that route...

Electric carving knife? I had a project once where I had to cut some
closed-cell PE (pipe insulation) and neoprene, like they use on roll bars.

I used a bread knife. It went through them like butter. :-)

Cheers!
Rich

Hot wire cutter question, power supply
 

If a transformer input - then use a vari-ac or adjustable transformer
that allows you to dial in the voltage you want or need.

If a DC supply, then the output has be controlled - likely with a
series power resistor that can be adjusted.

Martin

On 11/6/2010 1:45 PM, 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

Hot wire cutter question, power supply
 

Martin Eastburn fired this volley in
:

If a transformer input - then use a vari-ac or adjustable transformer
that allows you to dial in the voltage you want or need.

If a DC supply, then the output has be controlled - likely with a
series power resistor that can be adjusted.

Martin

There are some sophisticated control methods that will allow you to keep
the heat of the wire constant under load. Modern hot wire foam cutters
generally use them.

We buy some 50K little foam disks every year -- slabs are "sawn" from
blocks of foam, and the disks cut by CNC equipment similar to a CNC
router. I have not explored it myself (not having the need), but the
vendor told me that they keep the wire temperature constant during all
phases of traversing and cutting.

LLoyd

Hot wire cutter question, power supply
 

Sure one could do a complex servo system in analog transistors
and do as you say. That wasn't what was being tried on the kitchen
table. That is a basic process by someone that isn't into electronics.

Martin

On 11/6/2010 7:22 PM, Lloyd E. Sponenburgh wrote:
Martin fired this volley in
:

If a transformer input - then use a vari-ac or adjustable transformer
that allows you to dial in the voltage you want or need.

If a DC supply, then the output has be controlled - likely with a
series power resistor that can be adjusted.

Martin

There are some sophisticated control methods that will allow you to keep
the heat of the wire constant under load. Modern hot wire foam cutters
generally use them.

We buy some 50K little foam disks every year -- slabs are "sawn" from
blocks of foam, and the disks cut by CNC equipment similar to a CNC
router. I have not explored it myself (not having the need), but the
vendor told me that they keep the wire temperature constant during all
phases of traversing and cutting.

LLoyd

Hot wire cutter question, power supply
 

On Sat, 06 Nov 2010 15:07:38 -0800, Jon Anderson
wrote:

On 11/6/2010 2:04 PM, Ken Moffett wrote:

The fumes are very toxic... unlike polystyrene...including
hydrogen cynide (think gas chamber).

Check the MSDS's.

A BIG Thank You! I'd planned to do it outdoors, but I don't like messing
with anything toxic I don't -have to-. I have an alternative method to
slice down the foam, and I think I'll go that route...

I use an electric carving knife (Proctor Silex, $8.88 at BigLots) for
furniture cushion foam, if something like that will work for you.

--
If we attend continually and promptly to the little that
we can do, we shall ere long be surprised to find how
little remains that we cannot do. -- Samuel Butler

Hot wire cutter question, power supply
 

Jon Anderson wrote:

On 11/6/2010 2:04 PM, Ken Moffett wrote:

? The fumes are very toxic... unlike polystyrene...including
? hydrogen cynide (think gas chamber).
?
? Check the MSDS's.

A BIG Thank You! I'd planned to do it outdoors, but I don't like messing
with anything toxic I don't -have to-. I have an alternative method to
slice down the foam, and I think I'll go that route...


Meat slicer with a rotating knife? :)


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

Hot wire cutter question, power supply
 

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.

Hot wire cutter question, power supply
 

On Sat, 06 Nov 2010 08:08:00 -0700, Bill Noble
wrote:

On 11/6/2010 1:08 AM, Wild_Bill wrote:
The hot-wire foam cutter that I looked at several years ago used a wall
box lamp dimmer module on the primary 120VAC side of a step-down
transformer.


snip---------

danger - copying the above into your own design is likely to be both
risky and unsatisfactory - please consider using a proper transformer
with isolated primary and secondary circuits
The circuit he is looking at HAS an isolating step down transformer -
and is generally a safe design - IF his dimmer will handle an
inductive load.

Using the dimmer alone does not provide either fine enough control OR
isolation.

Hot wire cutter question, power supply
 

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.

Hot wire cutter question, power supply
 

On Sat, 06 Nov 2010 13:44:23 -0700, Rich Grise
wrote:

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

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

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

Hot wire cutter question, power supply
 

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

Hot wire cutter question, power supply
 

On 11/6/2010 3:22 PM, Rich Grise wrote:

Electric carving knife? I had a project once where I had to cut some
closed-cell PE (pipe insulation) and neoprene, like they use on roll bars.

Not far off, I have a blade that will fix a hacksaw, but looks more like
a bread slicer blade. Maybe it is... Anyway, I'll just screw it to a
piece of scrap plastic with shims to set height. A little side to side
wiggling of the foam as I advance it into the blade should do nicely.
I was looking at the hot wire as I just thought it would be cool to make
and use one.


Jon

Hot wire cutter question, power supply
 

On 11/6/2010 5:59 PM, Michael A. Terrell wrote:

Meat slicer with a rotating knife? :)

Those things scare me! lol...


Jon

Hot wire cutter question, power supply
 

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.


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

Hot wire cutter question, power supply
 

Jon Anderson wrote:

On 11/6/2010 5:59 PM, Michael A. Terrell wrote:

? Meat slicer with a rotating knife? :)

Those things scare me! lol...


I bought one, but haven't used it yet. No room on the kitchen
counter until the remodeling is done. :)


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

Hot wire cutter question, power supply
 

On Sat, 06 Nov 2010 15:07:38 -0800, Jon Anderson
wrote:

On 11/6/2010 2:04 PM, Ken Moffett wrote:

The fumes are very toxic... unlike polystyrene...including
hydrogen cynide (think gas chamber).

Check the MSDS's.

A BIG Thank You! I'd planned to do it outdoors, but I don't like messing
with anything toxic I don't -have to-. I have an alternative method to
slice down the foam, and I think I'll go that route...


Jon

A cheap electric carving knife from Goodwill works quite well for poly
foams

Gunner

--
"Confiscating wealth from those who have earned it, inherited it,
or got lucky is never going to help 'the poor.' Poverty isn't
caused by some people having more money than others, just as obesity
isn't caused by McDonald's serving super-sized orders of French fries
Poverty, like obesity, is caused by the life choices that dictate
results." - John Tucci,