Need advise making hot wire foam cutter - RC Groups
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Nov 21, 2008, 09:48 PM
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Need advise making hot wire foam cutter

Guys, I'm a chemist, not an electrical engineer. But I can figure out many simple things electricity and am an outstanding solderer.

I want to make a foam cutting bow. I have braided stainless steel .020 wire and a computer 250W power supply that I have gotten the 12V side jumpered to work great. I can easily make a tension bow.

I know I need a way to regulate the current going to the foam cutting wire so I can regulate the temp to foam conditions. So, what do I need and how do I need to wire it up.
I figured I need a potentiometer or something similar, but I need somebodies advise that has done this a few times or knows what they are doing.

Thanks in advance guys.
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Nov 21, 2008, 11:01 PM
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alexcmag's Avatar
A potentiometer don't work in such high currents.

According to my data, .020" stainess steel wire is 3.616 ohm/m, so it is the resistance for a 1m (40") wire, a good length for most wings.

Current (in Ampéres) = tension (in Volts) / Ohms , so in this case current = 12 / 3.616 = 3.3A = +-40W

A common rule is to use 1W/inche for a wire cutter, so if you make a 40" bow with 0.020" stainless steel wire it is almost perfect for use, so you need little adjust.

My suggestion is to add diodes in series with the power supply and keys for turning diodes on or off. With 5 diodes in series you will 50%-100% scale. Any standard silicon diode rated for 4A or more is enough.

0.010"= 0.25mm =14.464ohm/m
0.014"= 0.36mm =06.975ohm/m
0.015"= 0.38mm =06.260ohm/m
0.020"= 0.50mm =03.616ohm/m
0.025"= 0.64mm =02.207ohm/m
0.032"= 0.81mm =01.378ohm/m
0.040"= 1.0mm =00.904ohm/m
0.063"= 1.6mm =00.353ohm/m
Last edited by alexcmag; Nov 21, 2008 at 11:10 PM.
Nov 21, 2008, 11:21 PM
Registered User
This came up a little earlier. a speed controller for a brushed motor will usually work fine when coupled with a servo driver, and so will one of the older mechanic ones used on RC cars.Another approach is to use a dimmer that can handle inductive loads ahead of the transformer.
Nov 22, 2008, 12:03 AM
Registered User
The diodes seem an interesting way, is there any way it could be simpler with something that does not require more than one or two solder joints and a simple dial?
I did not want to invest in a servo driver but I do have a couple used speed controls handy. is there a way to make it mechanical?

So I gather that no dimmer switches or similar from a big hardware store would be suitable?

Thanks guys
Nov 22, 2008, 03:39 AM
supreme being of leisure
ZAGNUT's Avatar
regulating the PC power supply will be a pain IMO. since you're willing to go to the hardware store then go to someplace that specializes in lighting and pick up a dimmer and a toroid transformer designed for low voltage lighting that'll handle 100 watts or so. simple, cheap and works great.

here's a recent thread on the subject:

Nov 22, 2008, 12:02 PM
Registered User
By mechanical speed control I meant something like this:
these are just large high power variable resistors, driven by servos, and were commonplace in RC cars. I think they are still common in most car kits, but they are probably replaced with their electronic equivalent by now in order to save weight.
Nov 22, 2008, 12:53 PM
supreme being of leisure
ZAGNUT's Avatar
Originally Posted by Brandano
large high power variable resistors
or more commonly referred to as a "rheostat"
Nov 22, 2008, 01:04 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Originally Posted by Fuelman
... I want to make a foam cutting bow ...
See this issue of RS Soaring Digest Online
or this issue, I forgot which one:

Prettig weekend Ron

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