AC arbitrary source/amp - Page 1
Forum section: electronics / testgear

author: Harvs
author_rank: Super Contributor
timestamp: July 29, 2019, 08:17:47 am
content: Hi all,I'm looking for a amplifier to take an arbitrary signal from an AWG or similar to bring it up to AC line voltages (max >250AC RMS L-N 3ph, so at least +/-350Vp 3 channel). I'm building a test system for characterising pieces of power measurement and control equipment with complex waveforms.So far I seem to have just found fairly exotic piezo driver amps and high power options, where as it's really only 1 or 2mA of drive signal I'll need.There's HV opamps available that could be a DIY option, but I'd rather buy something off the shelf with output protection if possible.Something like the BK 9801 with an analog input would have been perfect (using 3 of).Any thoughts?Thanks

author: Kleinstein
author_rank: Super Contributor
timestamp: July 29, 2019, 09:41:22 am
content: There are power amplifiers available, though not sure they have a 350 V model. I have used a 200 V version.https://www.kepcopower.com/bop.htmThe units are niice 4 quadrant power supplies too, though a bit pricey.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 29, 2019, 09:58:05 am
content: The name of product you are looking for is "relay tester". There are very few companies making them, and they are prices like very few people need them.Omicronis one of the companies making them. You may be able to find used ones on eBay once every now and then.For high prices obviously.The cheapest method, I kid you not, is a function generator feeding a surround amplifier and a stack of transformers.(the ebay link will be dead soon, so a screenshot is attached)

author: capt bullshot
author_rank: Super Contributor
timestamp: July 29, 2019, 09:58:48 am
content: BK 8901 is just a rebranded unit from ITech, did you check their product line?http://www.itech.sh/en/Anyway, I had similar problems in a former job, the chosen way back then was:Use a low (signal level, e.g. 2V ... 10Vpp) level signal fed into the DUT (which was switched or modified to an appropriate input voltage range) to do all the testing / simulation / validating stuff using a custom (homebrew in that small company) 3 phase U / I signal generator. That signal generator could simulate all the special waveforms for the test cases. This way we could verify the signal processing etc.Final calibration of the units using a standard multifunction calibrator (the Fluke 5520A afair) that could provide volts, amperes and watt calibration at the required accuracy and nominal voltage / current levels.

author: Harvs
author_rank: Super Contributor
timestamp: July 29, 2019, 10:10:59 am
content: Quote from: Kleinstein on July 29, 2019, 09:41:22 amThis is essentially what I'm after, and they do have a 1kV model, and there's a couple of brands like it out there. It's just weird the 90's vintage website and a front panel that looks like it came from a physics lab of the same erra. I would have thought there was something more mainstream.Quote from: Jeroen3 on July 29, 2019, 09:58:05 amNot quite what I'm after. We effectively already have test waveforms that we've recorded from parts of the distribution grid that we wish to test out on different makes and models of equipment. We don't want to carry out "normal" tests in compliance with a standard, instead we wish to test edge cases that generally aren't considered when designing meters.Already had a go at using an iron core toroid on the output of an audio power amp, but it didn't have enough freq response. The magnetizing current went through the roof whenever "high" freq content hit it. Probably need 5-10kHz high roll off.Quote from: capt bullshot on July 29, 2019, 09:58:48 amUnfortunately we can't do that, we're testing final products that have to remain as "black boxes" and don't have input ranges that extend down that far. Building the signal generator is no issue, it's just getting the voltages up.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 29, 2019, 10:24:07 am
content: Quote from: Harvs on July 29, 2019, 10:10:59 amMost probably you won't get there with an off-the-shelf mains transformer. Try audio transformers or ask your favourite transformer supplier to wind one matching your specifications. 20Hz ... 20kHz, and 1:10 step-up should be achieveable, maybe you'd need a few design iterations and a defined load on the transformer. There are some examples in the literature that I vaguely remember for such stuff, at least one of them placed the transformer inside the feedback loop of the amplifier to get high signal fidelity. Another one (I believe it's one of Jim Williams' app notes) used a tube audio power amplifier as a near-to-indestructible driver amp for the step-up transformer.Edit: Watch out for low frequency content in your test waveforms. That might drive the transformer core into saturation, resulting in quite high a primary current. High frequency usually just gets smoothed and filtered, or causes ringing / peaking / resonant peaks (from minor to excessive, depending on transformer construction) on the secondary.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 29, 2019, 10:47:05 am
content: Quote from: Harvs on July 29, 2019, 10:10:59 amDid you look at omicrons product line? I believe they offer arbitrary distortion in some of their devices. Maybe some EMC test lab has them so you can get some time with them.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2019, 09:07:23 pm
content: They used to use big McIntosh tube amplifiers for this sort of thing because they were extra rugged.Big audio power amplifiers get into the right impedance range to not require an output transformer.If I were building something from scratch, I would at least consider a class-D output stage because linear amplifiers at that power level are not trivial.