Power supply part 4

A project log for Hot wire foam cutter

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

QuinnQuinn 02/19/2021 at 22:510 Comments

After considering the options, I've been leaning recently towards just using stock of things I already have, so am going to further explore the diy solution with a variac and large transformer.

While the transformer supplies 17.1VAC, wiring the variac for it to give 110% of the input on the output, this provides 18.8V output.  Further, if I rectify if and smooth with a large cap, the no load voltage would be 26.6V.  Drawing current from that will make the output more pulsing with that peak, and the minimum depending on how large of a capacitor and how much current is drawn.  Needing the higher voltages only happens with the thinner gauge wires, which require less current.  This may allow a moderate size capacitor to ensure an average higher voltage when needed, and when running at higher currents and lower voltage the output won't be completely DC, but that isn't important for a heater resistive load.  Estimate is that I need 10000uF to keep the droop down to 1.7v when drawing 2a, though this is worth testing to confirm.

I will need to take care about filtering so that the meter can measure the average voltage and current.  A simple R-C should suffice.  The meter has a 909kohm input impedance, so there is plenty of room for the RC.  The time constant just needs to be a bunch longer than the 1/120 sec period pulses.