Will this UPS Transformer work for this HOT WIRE FOAM CUTTER application?

Thread Starter


Joined Nov 10, 2018
Circuit design for transformer based foam cutter.JPG

I have a couple APC UPS transformers that I salvaged when the systems died. I now actually have a use for one in the circuit diagram shown above.
I am using 15 gauge Nichrome wire so it'll be thick enough to cut a specific shape in foam to create a Carbon fiber reinforcing hoop for my Paramotor trike. According to the Nichrome calculator, I'll need about 2 volts and 10 Amps to heat the 15 gauge Nichrome wire to 600 degrees to cut the foam.
Nichrome Calculator.JPG

Here's a picture of the salvaged UPS transformer. APC does not provide any specs for their components.

UPS Transformer-sm.jpg

I think my son might have 'borrowed' my good multimeter or CRS is kicking in -- so I had to try to use a crummy little tester that probably is only good for continuity.

Crappy Multimeter.jpg

Resistance (ohm) testing between leads 1&2 (white-yellow)/1&4 (white-blue)/2&3 (yellow-black)/3&4 (black-blue) were INFINITE(Open) on this tester.
Resistance (ohm) testing between leads 1&3 (white-black) AND leads 2&4 (yellow-blue) were close to ZERO OHM on this tester.

Even though I have two boxes of wallwarts collected over the years, they all appear to be AC-to-DC converters. I have having trouble finding a low voltage AC source to test the primary to secondary windings.

Can some of you very knowledgeable folks please weigh in and tell me if I'm on the right track or simply confirm that I'm good to go in using this transformer for this purpose?


Joined Sep 17, 2013
Do you know the voltages those windings were generating when previously in use, and what current the load was drawing?
Have you tested the primary (mains) winding?

Thread Starter


Joined Nov 10, 2018
The answer, Alec_t, is No I don't know those values.

I just remembered that I have a AC transformer from a Coffee roaster project that outputs 22 VAC I can test with.

Rahulk70 suggested the following testing:
"How to determine the primary 120V input wires? There are two ways:

1.This is the easiest way. Get a cheap 12VAC or 9VAC 500mA transformer and use the 12 or 9VAC to connect to the UPS transformer low voltage side(thicker winding with 3 wires, you can leave the centre wire.You have only two wires BLK/WHITE so its fine). You will get high voltage at the other side. Use a multimeter to measure the voltage across the four wires.Beware this will be higher voltage,70-100V if it's a 120V transformer(I assume your trans is 120V like mine) or 180-200V if it is a 220V transformer."​

Thread Starter


Joined Nov 10, 2018
I found the Coffee Roaster controller and loosened a couple twist caps to access the 12-0-12 volt taps (yellow-black-yellow). In the coffee roaster I use yellow-yellow which provides 24VAC output.

Testing Step-up function:
All testing was into the primary side (heavy 10ga wire) of the UPS Transformer to test the voltage out of the 4 secondary winding wires.

Using 12v input:
UPS Transformer secondary wires 1 & 3 output 100 volts
-wires 1 & 2 output 1 volts (deflection on tester stayed at same pt even while scale changed from 500v-50v-10v)
-wires 1 & 4 output 1 volts (same as 1&2)

-wires 2 & 4 output 12 volts
-wires 3 & 4 output 0 volts (no movement)

Using 19v input (transformer output of 24v dropped to 19v under load of the UPS transformer connection)
-UPS Transformer secondary wires 1 & 3 output 150 volts

Now, how do we interpret the readings above to guestimate expected results when testing Step-Down function with power into the four side secondary wires?

Which wire input combination yields a low voltage, high amperage output on the primary side?


Joined Sep 7, 2010
That transformer is plenty big enough .... you have a dimmer on the input so there can be no reason for it not to work ....(as long as the dimmer is not overloaded by the transformer)

Try it . Make sure when you turn on the dimmer is at low setting , then turn it up to get the right temperature in the wire .


Joined Sep 30, 2009
Before doing this in real life, I would lose the Green(ground) wire going to the resistance wire, cutter wire. The transformer give a galvanic separation from the rest of your circuit.