Hot Wire Foam cutter power supply - Page 5 - RC Groups
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Oct 10, 2012, 11:38 PM
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f14tom's Avatar
Originally Posted by vtdiy
If this is a regulated DC supply (check to make sure) you probably can use it if you put an adjustable DC regulator on the output. These are sold as LED lamp dimmers on Ebay. NOTE: that is NOT the same as an AC lamp dimmer switch.

Here's an example of a DC LED dimmer:

In this case you would wire the DC output of your power supply to the input of the DC LED dimmer. Then the output of the LED dimmer would go to the hot wire.

But you MUST be sure that your power supply is a DC 12 volt supply for the LED dimmer to work.
Thanks so much for the detailed explanation, vtdiy. The power supply I have is "GT Power 20 Amp Switching DC Power Supply":

The description of the power supply says: "It has 2 sets of output jacks for a total output of up to 20A @ 12V, and has a digital display that shows the output current."

So, I think that I should get an LED dimmer and connect it between the output of the power supply and the cutting wire. I should be able to regulate the amount of voltage going into the wire in order to get an optimal amount of voltage.

Does that sound correct?

Thanks again!!
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Oct 11, 2012, 12:19 AM
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f14tom's Avatar
Originally Posted by vtdiy
Here's an example of a DC LED dimmer:
Ok, one last question, hopefully. I searched the internet for DC LED light dimmer. It appeared that the highest output amp I could find was 12V 8A ones.

If I understand correctly, these dimmers will accept 12V DC input and output variable amount of voltage, up to 12V. Also, the maximum amperage of the current coming out of the dimmers will be 8A.

So, if that's the case, will my power supply that could provide up to 20A hurt the dimmer? Also, would 8A (maximum current?) be sufficient in heat up any type (or just some particular types) of wire?

Oct 11, 2012, 07:34 AM
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I want to jump in on this discussion too. I've been using a 12v battery charger to cut foam wing cores and it works well enough, but I want a nicer, adjustable system. My charger has a switch for either 10 amp charging or 50 amps for starting. I've been using the 50 amp setting with about 40" of wires with good results. I want to build or buy something more adjustable. I need to be very budget minded and I won't be cutting tons of cores, but I want something decent. Things I've considered include :
A dimmer switch on the input side of the charger?
A variac going to the charger? (expensive)
Variable power supply 30v 20 amp. About $65 on ebay
Dimmer switch and transformer
(probably the same as the dimmer/charger option)

What would be my best option for an inexpensive, decent quality power supply? I want to have the ability to cut 48" wings or heat a few inches of wires with a "table saw" configuration.
Oct 11, 2012, 08:13 AM
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flypaper 2's Avatar
Couple of other things to look into for controllers. Sewing machine pedal speed controller. Dremel tool speed controller, Bathroom fan dial control Electric train transformer. Router controller. I use a Variac myself,

Oct 11, 2012, 08:39 AM
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Would these be installed ahead of the charger, power supply, transformer, etc? Which would be the best option, or are they all the same?
Oct 11, 2012, 01:27 PM
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vtdiy's Avatar
Lots of confusion is natural here because of all of the various options and equipment.

Here are some first principles:

1.) ALL regulated DC power supplies which you want to add adjustment to would require an adjuster on their OUTPUT side. In other words something that would adjust the DC output.

This type of adjustment method includes the DC LED lamp dimmer mentioned earlier. Or a longer wire, or a potentiometer (but a high wattage one -- not cheap or easy to find).

2.) The maximum amperage of a power supply is the most it can put out without damage. It is not what it will actually put into your hot wire (or your DC adjuster going to the hot wire).

What goes through the hot wire (and adjuster) is dependent on the resistance of the hot wire. If your hot wire was copper and 1/2 inch thick, it would have very low resistance, and could draw hundreds of amps (frying your power supply, and yes, the LED dimmer).

On the other hand if it was stainless steel and .011" thick and normal bow length I might draw a couple of amps. So to answer the first question, even though the 12V power supply is rated at 20 amps, a proper size type and size cutting wire would easily draw much less than the 8 amp rating of an added LED dimmer. Usual amp draws on reasonable sized bows with reasonable length and thickness wire are 2 to 6 amps.

Just don't put too thick a wire on, too short a wire on, or a wire made of a good conductor, like copper. Stainless stel has much more resistance then copper (or mild steel).

3.) @Calla969, Because your battery charger is of unknown type, it is impossible to answer whether a variable unit should be put before it (on the AC side) or after it (on the DC side).

If your charger is of the old unregulated (transformer and rectifier) type -- usually has an old fashioned meter, and NOT a digital display or LEDs -- then you could put a $19.95 Harbor Freight router speed controller on the AC side, by just plugging it in.

BUT, if you have a "digital," "smart", "computer", "PWM" or "regulated" automotive charger -- usually without a meter but with LEDs and/or a digital display, then you must use a variable control on the output DC side. That might be a DC LED dimmer, as mentioned before IF the battery charger's output voltage did not exceed the dimmer's acceptable voltage range. And conceivably a really smart charger might not like adjustment at all -- if it is trying to regulate a current charge rate and sees the dimmer as a "non-battery".

If using an automotive battery charger, simple old fashioned chargers are the easiest and work best -- an example of a suitable 6 amp Harbor Freight charger that costs $29.95 was referenced earlier. This in combination with the 19.95 HF Router speed controller will give you a plug and play system.

In any case it's a good idea to check your bow length and wire type on the Hot Wire Calculator.
Last edited by vtdiy; Oct 11, 2012 at 01:48 PM.
Oct 11, 2012, 03:13 PM
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f14tom's Avatar

Thank you, vtdiy!!

As usual- very detailed, but informative answers from vtdiy. Thank you so much!!
Oct 11, 2012, 06:41 PM
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Thanks for the great information! The charger I'm using is the old type. What would the router speed control do that a dimmer would not do? I've seen the dimmers used with a transformer which is pretty much all that's in the charger.
Oct 12, 2012, 09:13 AM
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vtdiy's Avatar
Originally Posted by Calla969
Thanks for the great information! The charger I'm using is the old type. What would the router speed control do that a dimmer would not do? I've seen the dimmers used with a transformer which is pretty much all that's in the charger.
You could use a dimmer switch, and I have, with a transformer, as you mention. But I also have the router speed controller and batery charger, and prefer that one.

Why? the Router speed controller is plug and play. It contains a fuse and fuse holder. It contains a cord and an outlet socket, It has a decent knob compared to a dimmer switch. It has a case. And the case is pre- drilled with all components installed. It has a nice 3 way toggle switch with FULL, OFF, and VARIABLE power settings. It costs $19.95. It can be used to control the speed of any brushed motor, including a router, dremel, drill, etc. Also a plastic welder (I have one), and transformers in general.

Now, if you price out a dimmer switch, case, fuse holder, fuse, cord, outlet, strain relief and factor in a couple hours of construction, I think the router speed controller is a great bargain.

Up to you -- if you like building things go for it.
Oct 12, 2012, 12:55 PM
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Thanks for the help!
Oct 15, 2012, 05:00 PM
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richie967's Avatar

An update....

I am glad this thread is being put to good use! After many cuts and practicing with my cutter, I would recommend to anyone getting into the realms of hot wire cutting to use a nice pre built adjustable supply...

i have also made a nice jig to cut 90 and 45 degree angles.... (pictured below) The jig is great to lay blocks of foam, and move wire over the top.

I have also found g-clamping the hot wire bow to the bench to be very successful.... Thank you to an Earlier poster for the suggestion.. worked a treat!

As promised, here is what i've done with my cutter...One cathedral tail cut and fibreglassed..... job done...

I have also attached a pic of the plane....

Oct 29, 2012, 04:09 PM
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f14tom's Avatar

Failure at first attempt...sort of

I finally got all my components together and built and temporary hot wire foam cutter. First of all, here is the list of my components:

Power supply: 12 Volt GT Power 20 Amp Switching DC Power Supply (

Dimmer switch: 12-24 Volt, 8 Amp Dimming Controller For LED Lights ( (also see Panel A of the attached image, which is at the "lowest" level setting in the picture)

Hot wire: ~ 36 inch long 0.023" Carbon Steel Welding Wire (

Wire used to connect between power supply, dimmer and hot wire: salvaged power cable from an old sander (~ 14 AWG? of twisted copper wire), ~ 6" between power supply and dimmer; ~ 36" between dimmer and the hot wire

I included the links above because they give the images of those components and they were the places where I got them from.

So, I connected the Negative output port (Black) on the power supply to the "V-" port of the "IN" on the dimmer switch and the Positive output port (Red) to the "V+" port of the "IN".

In order to make sure that my setup works, I secured the Carbon Steel Welding wire to a wooden bow to form a 36 inch long bow. I had 2 wires coming out of the "OUT" ports of the dimmer switch with alligator clips on the end of the wires. Then, I clipped the alligator clips at each end of the bow.

With the dimmer at its lowest setting (for some reason, this unit show the indicator line to be passed OFF when it's at the lowest setting), I turned on the power supply. Slowly turning up the dimmer switch to just a bit above "OFF", the power supply indicated ~ 2.1 Amp output. The wire started to heat up within 20 seconds and it started to stretch. I was able to cut a piece of foam with it.

After about 1 min, the amperage on the power supply started to go up to 4 - 5 amp. I then turned down the dimmer to 2 amp. But, soon after, the amperage started to go up again. I then turned the dimmer all the way to the lowest setting (past OFF). The amperage on the power supply dropped again but then went back up to 5 amp quickly. At that time, I also smelled something burning. I then turned the power supply off.

Came to find out, the burning smell was coming from the dimmer switch. When I opened it up (see Panel B), I was able to trace the burned smell to a small black square on the circuit board (Panel C), which also appeared to be burned visually.

I am puzzled as to why the dimmer burned. I did not think that either the voltage nor amperage output from the power supply to cause an issue. Could have the dimmer been a faulty one? Or, just not to the advertised specs? Or, could the power supply be the issue?

Also, according to the calculation worksheet by vtdiy (20 volt, 20 amp, 38 inch carbon steel wire), the amp draw at max heat was estimated at 1.9 amp. One thing I am confused at was the "calculated wire diameter", which was at 0.011 inch. The wire I was using was 0.033 inch. Could that also be an issue?

If the dimmer switch or power supply was not faulty, then one way I thought that I could make a functional cutter with my power supply is to increase the hot wire length in-between the two alligator clips by having excess wire, but still retain the cutting portion of the hot wire to 36 inches (which is about the maximum length I need).

Any help would be greatly appreciated.
Oct 29, 2012, 06:40 PM
Registered User
Basically your wire is far too thick. If you use wire three times the intended diameter it will take approximately TEN TIMES the current. You were quite possibly putting the full 20A current of your power supply through an 8A dimmer so it's not really surprising that it didn't do it much good.

It's really good idea to MEASURE the current rather than just guessing at it. A wattmeter or multimeter work well for that.

Oct 29, 2012, 08:01 PM
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flypaper 2's Avatar
I have read where a light dimmer couldn't handle the amps. Those guys reverted to the bathroom fan controller which will handle more amps. I use a .020 Mig welding wire. Takes an average about 6 to 8 amps on the variac.

Oct 30, 2012, 09:59 AM
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vtdiy's Avatar
Originally Posted by f14tom
Also, according to the calculation worksheet by vtdiy (20 volt, 20 amp, 38 inch carbon steel wire), the amp draw at max heat was estimated at 1.9 amp. One thing I am confused at was the "calculated wire diameter", which was at 0.011 inch. The wire I was using was 0.033 inch. Could that also be an issue?
Yes, it is an issue. The calculator said 1.9 amps if you used the suggested .011 diameter wire. You chose to ignore that and use much thicker wire (.023").

I substituted your .023" mig wire in the calculator and it shows you drawing 6.6 amps, which is close to the LED dimmer's maximum rating of 8 amps Since the actual composition and resistance of the Harbor freight MIG wire you chose is unknown, it could exceed the max for the LED dimmer. Also as Flypaper said they may not work well, especially near their max output rating.

I suspect that it would have worked well if you'd got some .011 stainless fishing leader, and your amp draw was around 1.9 amps as suggested in the calculator. Rather than nearly 7amps.


Another problem -- your specs are confusing -- important to keep these straight. In the first part of your post you say .023" dia wire and 36" length. In the above quote from the last part of your post you say .033" wire and 38" length. Which is it?

I used the more conservative first numbers in my reply above. The second set would have been far worse and would easily exceed the LED dimmers rating..

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