Hi folks,

I haven't come to ask for help without doing my homework - I wouldn't waste your time like that. In this forum alone there are over a dozen similar questions asked and answered. I've spent several evenings googling elsewhere as well - most lead to a variac solution or a sales pitch.

I'm hoping to find some advice on building a variable - or at least, switchable - AC power source, with 0 to say..., 12VAC. It will be powered by a 24VAC 2.4A power supply I already have.

The purpose is to 'mark' various metals, using a low voltage and an electrolyte (ie: "etch" but leaving an almost black finish). In a nutshell, DC will etch metal, AC will mark it. A general rule of thumb is "the higher the voltage, the faster the etch"..., with downside of raggedy edges. A lower voltage makes cleaner images, but the times could stretch into days instead of hours.

A continuously variable source would be great (ideally, an AC version of an LM317 or LM338 - how cool would that be!) A multi-pole switch with resistors, zener diodes, or some such circuitry would also be a good way to go - I can always play with the values to get fractional voltages, once I know the proper way to set it up.

There are many circuits that will do this ( https://hackaday.com/2015/01/15/etching-steel-with-a-dc-wall-wort/ ), but only for a single voltage. I've built several of them over the years including that one, as well as numerous variable DC power supplies. However, different metals react differently to different voltages, so I'd like try a range of voltages to find the optimum voltage for various high carbon steel alloys (knives and tool making), brasses and bronzes (signage and tool making), etc.

I've found a lot of those projects are trying to lower 128VAC to 110VAC and similar ranges though, where I'm looking only at low voltages.

Efficiency isn't my largest concern, although I wouldn't discount it. I know Ohm's Law, but I'm not sure of the best way to apply it in this situation - simple resistance seems like a bad approach. I'm aware of the dangers of not using an isolated transformer.
I know it can be done with a variac, but I don't have one of those. Plus, I'd like the finer resolution that comes from a range of 0 - 7VAC (or so), instead of 1 - 110VAC, and I'd like to build it out of my parts collection. I know there are cheaper options than a variac, because there are commercial options starting around $59.99.

Any help would be gratefully received, and I'd gladly post my progress and the results, giving credit to those who made it happen. If it's possible, of course.

Thanks
davegsc

 

I think a simple approach is to use a small variac to power a 12VAC transformer. That would give 0 to 12 volts with fairly fine resolution. It would be an efficient method.

Alternately, use a small transformer, like say 12v with a 1.4VA power rating, and input the output of that into a small audio amplifier with a volume control. To get a maximum of 12Vac output, the amplifier should operate off of about ±16Vdc.