I have assembled an electric foam cutter.
This is the hardware list:

- Transformer
220 Volts AC current INPUT (Italy)
5 to 12 Volts DC OUTPUT
Rated 1 Amp (1000 mmA)

- Steel wire 10,5" long, about 0,018" gauge.

This setup works fine with a smooth cut trought the foam, but after using it for 3-5 minutes the transformer gets really hot, and beeing scared to burn it, I switch it off.
NB : this happens at 12 DC volts output setting.

Were is the bug?
1) Is the transformer going to burn?
2) Too much volts or too much draining amps?
3) What is going to happen reducing the voltage output (5-7), is this going to reduce the burning hazard of the transformer?
4) Things will work better using a light dimmer connected to the transformer?

I know too much questions but I am not an electrician expert.
Thank you for the incoming help reply.

I'm using small steel wire from the craft department. Might be 28-30 gauge. I use about 2-2.5 amps for the heat setting I like to cut at, so that may well be the problem. Some other wires (and smaller gauges), like Niachrome get hotter with lower currents. The voltage is dependant on the length of wire and the current required for the heat you need.

I would suggest getting a larger transformer, perhaps 20-24 volts at 3 amp secondary, and use a dimmer on the primary to adjust the heat for your preference. You WILL burn up the one you are using if it gets too hot. You can use AC right off the transformer secondary, the wire really doesn't care if it's AC or DC...

Thank you Gene.
But if [amps (I) = Volts/Resistance (ohms)], then keeping the resistance constant (steel wire length and gauge), reducing the voltage we should get a decrease in amps.
Is it correct isn't it ?

Like GB says, get a bigger transformer, else th eone you have is going to burn out.

My set up is this:
Transformer, Primary 240 vac ( I'm in the UK )
Secondary 20 vac rated at 5 amps
Bow is approx 27 ins using Nichrome wire around .02 in

Wire has a unit resistance per unit length, viz the longer the wire the more resistance, if you cant change the length then you need to change the voltage applied to it to reduce the current, but in doing so you reduce the heating effect. Selecting the correct type of wire is important, you need max heat for minimum current/voltage...this is where Nichrome wire is best afaik

Originally Posted by frankyfly

Thank you Gene.
But if [amps (I) = Volts/Resistance (ohms)], then keeping the resistance constant (steel wire length and gauge), reducing the voltage we should get a decrease in amps.
Is it correct isn't it ?

That's correct, Franky. However, as Gene says, you need more than 1A current to cut foam. You are exceeding the rating of your transformer - that's why it's getting hot.

The current is what heats the wire (watts = IČR). The voltage is the "pressure" that forces the current through the wire. If you reduce the pressure, you will reduce the current - and you won't be able to cut foam! That defeats the purpose.

I use a transformer rated at 24VAC/100VA output (that's about 4 amps). A 1 amp transformer "just won't cut it" to put it mildly....

10,5" long??? About 26 cm? My bow is about 54"/138cm long.....I have shorter ones too. They all need the same amount of current to cut properly - the shorter ones just need less voltage to do so.

Brad

The wire resistance will not remain constant. It changes with temperature. I've heard that 1W/inch is a goot estimate for foamcutting wire.

Thank you all guys.
See what I have found.

INPUT 230-400 volts
OUT 12 - 24 volts / 50 Volts Amp.
What means 50 volts/amp?
The last picture is the bow I'm working with.
The light dimmer is an adjustable resistor, can I put it in the circuit between the transformer and the steel wire of the bow.

Originally Posted by frankyfly

See what I have found.

INPUT 230-400 volts
OUT 12 - 24 volts / 50 Volts Amp.
What means 50 volts/amp?

Basically it means that the transfo you have has a current rating of approx 2amps......
VA is the product of Volts * Amps ( disregarding the losses ) but it depends on how the secondary of the transfo is configured........ If its just a single winding with a tapping to give 12v then it is rated for 2 amps, if it has 2 windings then its possible that it may be rated at 4 amps......Try it and see if your bow cuts cleanly

50 VA / 12V = 4.166666 Amps
50 VA / 24V = 2.08333 Amps

Use the 12V taps for your short bow - this should be all that is necessary. Use the 24V taps if you are using a long bow. 50 VA is marginal at best for foam cutting - you are pushing that transformer to its limits. A 100VA transformer would be better. However, what you have will work - just remember to shut it down and let it cool off after each cut - don't push it too hard for very long, or more than just your foam will be smoking!

Brad

Thank you all for your advice on the subject.
Back to chop the foam !