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Too much current Answered

I built a hotwire device to cut foam insulation. If I connect it directly to a 12 volt battery, it gets 'way too hot very quickly--the hotwire turns red almost immediately. However, if I connect the device to a wall charger rated at 5 volt/2amps, it works fine,if a little too cool for a good cutting speed. What's happening here?


Double voltage means 4 times the power.

The resistance wire will draw as much current from the source as it needs OR the source can supply. Your 12 volt battery had a huge current capacity and could easily melt the wire if allowed to.

Your lower voltage wallwart at 5 volts and limited to 2 amps is controlling the amount of current flowing through the wire.

I suggest you look at a PWM (pulse width modulation) control board, very cheap on ebay or similiar site. Get one to run off 12 volts and use it to control the voltage to the wire.

Power (watts) is Current x Volts, make the volts lower and you reduce the watts.

That sounds like what I'd need for this. I'll have a look and thanks very much for all replies.

A length of resistance wire behaves almost like an ideal resistor, and an ideal resistor is one that obeys Ohm's law; i.e. I=V/R.

Also for an ideal resistor, doubling the voltage across it, quadruples the power dissipated, since,

P = V*I = V^2/R = I^2*R

That seems to be pretty consistent with your observation that a 12 volt source makes the wire get a lot hotter than a 5 volt one.

Of course that is not the whole story. Another big part is loading of the voltage source. By that I mean it is naive to assume a wall charger rated at 5 volts, or a battery that is nominally a 12 volt battery, will succeed in dropping 5 volts or 12 volts, respectively, across any given load.

If you want to know the actual voltage across your hot wire, then measure it with a voltmeter.

For real voltage sources, there is loading. That is to say the voltage, measured at the output terminals, decreases as the current it is supplying increases.

Theres a relationship between volts, amps and resistance.

The greater the voltage the more power will flow through the wire.

So either use a midway voltage between 5v and 12v that works for your application or increase the resistance of the wire by making it longer.