wyflyer

My Feedback: (7)

I have built a foam cutter that I plan to run off a modified computer power supply. It puts out 12 volts at 12 amps. I was wondering what you guys use to regulate the juice coming into the wire to regulate the wire heat?

Thanks in advance,
Ryan

Irish R.Seer

My Feedback: (1)

I use a simple, cheap rotary dimmer switch.

BMatthews

THAT WON'T WORK WITH A COMPUTER SUPPLY! ! ! ! The computer supplies are not like a transformer or a transformer based power supply. If you try to use a dimmer on the front end of it something WILL go bang or not work right.

You'll need a simple wire wound rehostat or other high current, high wattage capable variable resistor on the output. Also you should be aware that these power supplies are often configured such that they need about 1 or 2 amps minimum flowing out the 5 volt side of the supply in order to function. If your's is working with current coming out the 12 volt side then you're lucky.

In any event high wattage wire wound rehostats are rare and aren't cheap. But you can make your own using a offcut of floor tile and some soft STEEL wire or stainless steel safety wire. It needs to be steel for the resistance. Another option would be the nichrome element from a toaster. Get an 8 inch long by about 1 to 1 1/2 inch wide tile cutting from a floor tile shop. Either get them to drill 3/16 holes at the ends or do it yourself with a carbide drill. The holes are for the bolts to act as binding post points. Don't overtighten or you'll shatter the tile. Wind the wire in a spiral with about a 1/8 to 3/16 spacing. Then mount this to a board with metal L brackets and set up a slider of some sort from 1/8 music wire with a little copper shoe at the end to ride on the spiral wire with a fair bit of tension.

And just remember that the power supply MUST have a load on it at all times. If you switch out the wire then the suppy either stops working, leaks through a high voltage or may go into a failsafe no output condition depending on the design. Adding a shunt resistor to set an idle current of about 1 t 2 amps on the 5 volt output will prevent this from happening and is probably a wise thing to do in any event.

Good luck with this. For the wire I'm using I found that 12 volts wasn't enough. I'm lucky enough to have a high current multi tapped transformer and I can configure whatever I need. But if the computer supply works then there sure are a lot of them for cheap.

Irish R.Seer

My Feedback: (1)

I stand corrected. I am using a 24volt transformer. Thank you for the info.

BMatthews

No problem Irish, I figured that was probably the case.

FWIW, I prefer the transformer option as it ensures you're properley isolated from any line voltage references. But then I don't make it a habit too touch the hot (possibly in more ways than one) wire in any event.... [X(]

Andrew

If you already have a 24v transformer, here is a simple variable heat foam cutter -- we have built a couple for use in our robotics lab and have been pleased with the results.

http://www.nsrca.org/technical/tip_t...wer_supply.htm

Here is a bit of information on PC power supplies -- like Bruce said, they are not good sources for direct power foam cutters since by design the PS attempts to maintain voltage output within 10 to 20% of specified values, depending on the output level. If pushed, the internal overload circuitry will kick in and shut the supply down.

http://web2.murraystate.edu/andy.bat...owersupply.htm

Andrew

Lile

I have had pretty good luck with simple 12 volt transformers. There are variable transformers available that can sometimes be found on surplus sites, but WATCH OUT they are NOT isolated from the AC power line. You definitely want isolation as you are dealing with a bare wire here. A lamp dimmer may work OK.

Lawrence Lile
Lile Engineering
http://www.lile.biz
providers of Nichrome wire for the hobbyist and experimenter
Free Nichrome Hot wire foam cutter plans

patternwannabee

My Feedback: (1)

Here's a low-tech solution.

Forget about the box at the top. It's just nichrome wire. You would insert this in series with the bow.