Challenges to build a stable <0.3% 60Hz variable 250V source? - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: March 22, 2019, 04:53:25 pm
content: 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?

author: DaJMasta
author_rank: Super Contributor
timestamp: March 22, 2019, 06:18:23 pm
content: What's you're home power's stability like? If it's 0.3% or better, you just need a variac.

author: Jester
author_rank: Frequent Contributor
timestamp: March 22, 2019, 07:44:01 pm
content: Quote from: DaJMasta on March 22, 2019, 06:18:23 pmI 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.

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2019, 10:53:03 pm
content: 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.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 22, 2019, 11:28:12 pm
content: 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.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 23, 2019, 12:55:07 am
content: Quote from: Jester on March 22, 2019, 07:44:01 pmI wouldn't claim that it's a particularly stable source, but it's really not far off of your original specification. 0.3% of 250V is 750mV, a fair bit looser than 10mV steps, and with my 34461A's probes straight into a mains outlet at my bench, I've recorded +- 0.4V or so, which is just a smidge over your 0.3% specified tolerance.What kind of current do you need? If it's very little, you could just cram a function generator through an audio transformer or similar and get to the right ballpark, but I'd be expecting 10s of uA on the output. May be easier to send the function generator through a BJT to get some current going, then step up the voltage by 50 or 100 in a transformer. Since the function generator is variable, it should give you pretty good tolerances as long as the output spec was better than 0.3% base accuracy and you calibrated out the transistor's, base resistor's, and transformer's losses in the system and had a good and stable supply for the transistor.

author: Jester
author_rank: Frequent Contributor
timestamp: March 23, 2019, 01:24:14 am
content: Quote from: DaJMasta on March 23, 2019, 12:55:07 amI’m out of country at the moment and don’t have my note book but from what I recall my cobbled together version is fairly stable perhaps +/-100mV @ 250V, just observing a 34401A display over say 1 minute, and that’s with the Transformer open loop. There is an observable noise element and a separate drift element. I have not characterized over a longer period. On my to-do list is determine root cause of the noise and drift. To see if this is even a possible approach or if I need to look at other approaches.For current output 100mA should be adequate. The audio amplifier I’m using now is somewhere in the 30W ballpark and the transformer is far from ideal and much lower power but has been adequate because burden was next to nothing for use so far.

author: David Hess
author_rank: Super Contributor
timestamp: March 23, 2019, 02:21:38 am
content: Something along the lines of what Zero999 showed is what I would suggest although I would include current limiting in some form. If better efficiency for more power is needed, then a class-d audio amplifier can replace the linear output stage.