Electronics > Projects, Designs, and Technical Stuff
Challenges to build a stable <0.3% 60Hz variable 250V source?
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Jester:
I’m in need of a stable source as described in the title for calibration of low burden ac circuits. Looking at the used Flukes on eBay, besides their undesirable size and weight, I don’t need the variable frequency aspect.

I’m wondering with all the newer/better components now available if it might be better to build one?

Has anyone here tried? What was your basic approach and what are the tricky aspects to surmount?
DaJMasta:
What's you're home power's stability like?  If it's 0.3% or better, you just need a variac.
Jester:

--- Quote from: DaJMasta on March 22, 2019, 06:18:23 pm ---What's you're home power's stability like?  If it's 0.3% or better, you just need a variac.

--- End quote ---

I was thinking 10mV resolution, Mains move more than 0.3% every second or two, so variac not even close to a solution.

I actually cobbled together an audio amplifier driving a step up transformer and I drive that with an HP signal generator and it’s infinitely better than a variac. I’m thinking of making more of an effort to come up with something more precise.
Zero999:
Your tolerance is ambiguous: are you referring to the frequency or voltage?

Presumably you're talking about a very low current?

All you need is a signal generator with an accurate frequency and voltage output and a suitable amplifier. The Art of Electronics book has a schematic of an amplifier with a transformer in the feedback loop to get tightly regulated 120VAC at 60Hz from an unregulated +/16DC source. It's fairly easy to modify it for 250VAC and since you don't need 15W, a smaller, lighter transformer can be used.
Conrad Hoffman:
Not quite good enough, but General Radio made the 1570-A automatic Variac. It would hold voltage to 0.25% and could change at 10 V/s. It's luck to find one these days, but they work quite well. I believe there were several models. I used one in my darkroom for several years to hold the enlarger voltage constant. They don't have the hum and heat of a Sola regulating transformer, though you might be happy with one of those for low power. Usually cheap used, expensive new.
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