Power supply options: unregulated diy

A project log for Hot wire foam cutter

A power supply, bow and table for cutting foam with a hot wire

QuinnQuinn 02/15/2021 at 19:460 Comments

I do have sufficient parts on hand to build a suitable supply, though it's likely to be bigger and heavier than I'd like. This would be unregulated, transformer based.

Main transformer

I have a number of higher current output ones.

A 6.3v, 20A, which isn't terribly large. This won't work great for the bow however with such a low voltage. It would be great for a rigid wire cutter though.

From a large UPS, I have a transformer that I expected to be 120v to 24v, probably 50A. The backup used a 24v battery, though I don't remember if the drive was a full bridge or single bridge.  I ended up testing it.  There is a 120v coil, and a lower current secondary 14v coil. The main output is 14v however.  Given the UPS power, this must be 70A.  It's huge and heavy, overpowered and lower voltage than desired.

From a second large UPS, I tested it's transformer to be 13.6 or 15.7V output depending on which 120 input tap is used.  I don't have a way to know the output current, but based on the wire, it's probably 90+A.  Given that this is a 24V battery system, I'm again surprised the low voltage side is this low, but I guess it must work better for these high power systems.

From a smaller UPS, I have a transformer I tested to be 120 to 17.5v, center tapped, probably 17A. I know this circuit switched the 12v back and forth between each side of the center tap in order to get sufficient voltage from a 12v battery.

Another one from a UPS also has 17.5vac output, center tapped, likely 23A.  There is also a smaller 24V output coil.  It also has a second tap on the higher voltage end, likely used to get a higher output boost when on battery.


To adjust the output voltage, I have a number of variable transformers that could be used. These could feed the input of the transformer for smooth voltage adjustment.  All of these can be wired to output 0-100% or 0-110% of the input.

I have a bunch of 1.75A variacs. These are reasonable size, and would handle up to 210W at the maximum transformer output voltage. Equivalently, with that smaller UPS transformer, output up to 12A, which is sufficient for the direct needs. With the larger transformer, it would be lower current.

I have a pair of 3A variacs, a little bigger, and would handle a bit more power.

Past that, I have a couple stand-alone variacs about 7 or 8 amps.

And I have a pair of 20A variacs, which are huge and heavy.


I think regardless, I'd like to rectify the transformer output and put some caps on it to smooth it out. I should have a handful of 15A bridges, but also some much higher power diodes if I wanted to go higher.


To help be more consistent with cutting, I'd like voltage and current metering on the supply. At first I dug through my analog meters, but I didn't have a good matched pair.

Instead I found a set of these modules I unsoldered from some power product. Thankfully I worked out the pinout and wrote a note so it was pretty easy to get going.

These use 0-200mV voltmeter modules, Datel/Murata DMS-20PC-0-YS. The volt side directly connects to the module input pins, so I simply need to scale with a resistive divider.

The current side has a OPA234PA op-amp to drive the meter input. Input pins feed the op-amp. I didn't work out the values in the op-amp feedback, so I'm not certain the gain. It was originally set to measure over a 600A/50mV shunt, so 83uohm. Based on the original application, this implies it's configured for a 1.2x gain. There are a couple multi-turn pots, one in the feedback loop which can adjust gain, and one as a zero adjust. The op-amp I assume is in this circuit in order to scale based on the odd value current shunt.

I was able to find a 0.01ohm 5W resistor in stock I can use as my shunt, which can directly feed the meter (without the op-amp). Given that the existing circuit gain could not reach a useful value, I will just disconnect the op-amp output from the meter and feed it directly from my shunt. This shunt will work for 0-19.99A on the meter. The meter does support negative, but I don't think it's useful here. If I wanted to use a different shunt for higher current, I could adjust the op-amp feedback.  Without using the op-amp, the board only needs 5v.  The op-amp requires +/-12V.


I think this leaves 1 option I'm considering for building the supply  so far.

One of the 1.75A variacs feeding one of the smaller UPS transformers.  This would result in 0 to 17.5 V output, 12A max. While the 12A is fine, 17.5 V will prevent long higher gauge wires.  Using the 110% wiring for the variac, would result in 19.25V, which is a bit better.

I could use the 3A variac to increase the current a bit, but that isn't really important in this situation.