OT: Need help from the electricity minded...

"BCRandy" wrote in news:DFdPg.9$P% snipped-for-privacy@bignews2.bellsouth.net:

No, not really. The wire you're using to cut the foam has a specific resistance. The resistance causes it to heat up as the current passes through it. If enough current passes through it it will get red hot and cut the foam, if too little current flows it won't get hot enough and if too much flows it will burn up.

W/o knowing the specs of the wire you're using it's just a guess. Try the 8v output and see if it's hot enough, if so, stop. If it's not hot enough go to the 16v output and try again, if it's still not hot enough try the 24v output.

The VA rating is how much power the transformer can put out before all the smoke leaks out of it and we all know electic stuff won't work it the smoke leaks out...

*IF* we knew the specifics of the wire you're heating (the resistance per foot and the length you're using) we could tell you exactly.

The 24v one looks wrong as unless its a seperate winding it would be 10VA as well, the transformer as a whole will have a VA rating. The best one to pick would be the 8volt output as it should provide 1.25 amps for a 10VA (8x1.25) rating and current is what you want more of.. If the resistance of the wire is such that at 8 volts you don't get enough current you'll need to up the voltage but theoretically the current should be limited to ~625mA@16v (probably not enough). Ideally you should cut the wire to work on the 8volt supply as this will give the most current while staying within the transformer rating.

Thanks for the quick responses. I am using a 30 inch long steel piano wire that is 0.015" diameter. The resistance I measured was 2 ohms per foot.

After reading the responses, am I correct in thinking that, with all else constant, a shorter wire will be hotter?

Randy

"BCRandy" wrote in news:IXwPg.128$ snipped-for-privacy@bignews7.bellsouth.net:

Top posting corrected.

Right. Let's see now.... if the wire is 30" long the resistance should be 5 ohms so using the 8 volt output you'll have a current flow of 1.6 Amperes (current = Volts/Restance) which may damage the transformer (1.6A x 8V = 12.8VA) and using the other taps will just get worse. I think you need to go to a longer wire or higher resistance wire. Check around for Nichrome (sp?) wire, that's the stuff they use in toasters, etc. and should have much higher resistance/foot.

I can't use a longer wire (space limitations), but I can try to find one with a higher resistance. In some of the plans I found on the net, they used a guitar string for the wire. I wonder if it's resistance is higher? OTOH, I could spend a couple of bucks at the Goodwill for a used toaster and try using the heating wire from it. It should have a higher resistance. Here in central Florida, Ni-Chrome wire may be hard to come by. If I can't find any, I may have to try these alternatives. Thanks for the assistance - I'll let you all know how this works out.

Randy

You may find the existing is OK but not for continuous use. The transformer should take some overload just not for a long period. You can probably assume the 10VA is a continuous rating so overloading by 20% shouldn't be a issue for a few minutes.