25-30Arms amplifier (50-60Hz) ideas? - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: August 15, 2021, 08:24:36 pm
content: I’m looking for a low cost regulated ac current source capable of delivering up to say 30Arms for say 60 seconds at a time. This will be used for calibration of current measurement circuits typically a CT or in some cases a shunt. The burden should be fairly low so compliance voltage can be fairly low, 15Vrms or so should do it.Prefer something OTS possibly class D audio amp adapted? Or roll your own?Let the ideas fly......

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 15, 2021, 08:58:33 pm
content: Variac and transformer, with a known accurate ammeter for reference.

author: Jester
author_rank: Frequent Contributor
timestamp: August 15, 2021, 09:02:11 pm
content: Quote from: NiHaoMike on August 15, 2021, 08:58:33 pm“Regulated”I have a large variac and low voltage transformer, it happily makes 30-40A, however the output follows the line voltage and changes in wiring resistance changes vs. Temperature so it dancea all over the place. Regulated is a must.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 15, 2021, 09:27:43 pm
content: Bingo. I suppose you could try with a ferroresonant transformer, but those aren't perfect, it helps but it's still somewhat sensitive. Maybe a, Idunno, 20dB improvement is adequate? But also, the output harmonic distortion isn't great, and that's probably just as important?An audio amp is a fine start, or a true sine wave / synthesized inverter. Audio amps aren't usually that low of an impedance but a trans/autoformer will do a fine job with that.Wait, 15V compliance range? What the hell for, that's 450VA!Do these sensors go on distribution lines or something?!Well, you can certainly get audio amps that size, you're not screwed. Just need a largish one.And if that's largely reactive, can always resonate it out a bit, with some adjustable taps for tuning say. Then the amplifier can be smaller, and the transformer ratio higher or lower as the case may be. The output won't be well calibrated then; you'll want to do a simultaneous, ratiometric calibration of the DUT in series with a standard sensor.Tim

author: Jester
author_rank: Frequent Contributor
timestamp: August 15, 2021, 09:38:32 pm
content: Quote from: T3sl4co1l on August 15, 2021, 09:27:43 pmI actually do have a ferroresonant transformer and used it the last time I was doing current calibrations., it helps but small changes in wiring resistance have a significant effect on current without feedback. I’m looking for a closed loop solution.Years ago I had a current amplifier from a protective relay test set (Manta test systems). It worked really well at 30A, I wish I still had that amplifier. Anyone have a current amplifier from a Manta, Doble, Omicron, multiAmp? Those were the manufacturers of that type of equipment back in the day.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 15, 2021, 11:19:14 pm
content: For similar purposes I made a Hovland current pump based AC current sourse. Look at The Art of Electronics, for example.I needed 10-15 A RMS source. I used OPA189, two TO220 Mosfets and 0.01 R (possibly 0.02 R) current shunt, two 5V 20-30 A ac-dc power supplies (Mean Well, but any other may do the job), and a low-distortion oscillator.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2021, 12:32:30 am
content: Once used a, Amp-Line AC source I think it was? -- though I kinda doubt you need anywhere near the cost of such a unit. They are programmable, synthetic sources, so it's all the flexibility you could want.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 16, 2021, 01:51:53 am
content: Couple the variac to a stepper motor and use an Arduino to automatically adjust it to get the current just right?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 16, 2021, 05:52:50 am
content: I don't really think one would actually need 15 V just for a current measurement. Chances are some 3 or 5 V should be sufficient. This would reduce the power requitements quite a bit, still some 100-150 VA. For short time us one could use a transformer with a lower rating (e.g. 50 VA). A 110 V input transformer would be a bit high - so one may need a little higher power rating because of the slight mismatch to an audio amplifier.

author: Jester
author_rank: Frequent Contributor
timestamp: August 16, 2021, 11:17:37 am
content: Quote from: Vovk_Z on August 15, 2021, 11:19:14 pmI could not find Hovland or Howland current pump in AOE, what page or section is that in?Quick google search leads to ma type circuits.

author: Jester
author_rank: Frequent Contributor
timestamp: August 16, 2021, 11:30:09 am
content: Quote from: Kleinstein on August 16, 2021, 05:52:50 amAgreed sort of. milli-Ohms add up pretty quickly at 30A, leads, shunt resistance and then the Output FET resistance, in most cases 10-12V rails should be adequate.

author: Berni
author_rank: Super Contributor
timestamp: August 16, 2021, 11:37:50 am
content: I would say get a big powerful audio amplifier and a signal generator.This will be stable in regards to the mains voltage. The audio amplifier should have a stable gain since they normally have feedback from the output and it takes care of the most difficult part of the job, actually providing all of power. If you want closed loop control of the output current then you could also close your own feedback loop around it and use the amplifier as just a power output stage.The load sounds like its pretty low impedance however, so it might be a good idea to add an impedance matching transformer to the output, letting the amplifier turn the extra voltage swing into more output current.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 16, 2021, 12:44:51 pm
content: With the transformer behind the amplifer / generator the high current part would not have any FET: just a shunt, cables, the transformer secondary and the DUT. With the transformer the amplifier would have to drive some 30 V and 3 A (which is reasonable in the audio world) instead of 3 V and 30A.One may still need to stabilze the current (could be on the primary side), as the copper wires and winding will warm up and this make the current drop when the whole thing gets warm.

author: richard.cs
author_rank: Super Contributor
timestamp: August 16, 2021, 01:53:34 pm
content: Doing this for a CT is considerably easier than doing it for a current shunt, because for the CT you can have a lower current and compensate by using multiple turns through the core. I have done this to calibrate a Rogowski at 2 kA with an amplifier driving 4 A through a 500 turn coil which the Rogowski was placed around. The amplifier was one of thesehttps://www.caltest.co.uk/product/bop-series/and needed to output about 100 V to drive 4 A through the coil inductance.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 16, 2021, 02:39:09 pm
content: You may find this interestinghttps://testguy.net/content/264-6-electrical-tests-for-Current-Transformers-explainedEasier than going down the brute force road.

author: Jester
author_rank: Frequent Contributor
timestamp: August 16, 2021, 03:19:51 pm
content: Quote from: Terry Bites on August 16, 2021, 02:39:09 pmI’m pretty familiar with the CT tests described, as a young engineer I worked for a utility commissioning large transformers (300MVA) etc, they have both metering and protection CT’s, I tested plenty. The ones I need to calibrate now are small and dirt cheap in comparison, and not as linear.

author: Zero999
author_rank: Super Contributor
timestamp: August 16, 2021, 04:08:48 pm
content: You need an amplifer with current output, i.e. a voltage to current converter. 30A is too higher current. Use a 10:1 transformer so the amplifier only has to supply 3A. Here's a link to a similar thread, where I poster a design, which can be adapted.https://www.eevblog.com/forum/projects/30a-precision-current-source/

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 16, 2021, 05:58:44 pm
content: Quote from: Jester on August 16, 2021, 11:17:37 amIt is a Howland current source, it is usually described in the operational amplifiers section.For the Third Edition of The Art Of Electronix it is in 4.2.5 "Current sources", page 230.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 16, 2021, 06:07:41 pm
content: I used once a design of 10A current source with TDA7294 a current transformer as a current source and too. (with 10:1 custom power transformers). But it came larger and not really fine as with low-voltage high current CS (like Howland current source).I mean I have two 10A rated designs: one with current transformers and audio power amplifiers (TDA7294), and the second one without CTs - the second one is much simpler and I think much cheaper too.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 16, 2021, 06:11:27 pm
content: Three phase 10A rated current source (voltage controlled, without oscillator) is at the photo. We may see that it is very simple:I used two 3.3V 60A rated AC-DC power supplies in that design, but I found later that 5V power supplies are more widespread and cheaper.A small PCB on the center is +-(12-15) VDC regulated PS for OPamps (and +12V for a fan). Three current sources are on the three 60x80 mm PCBs mounted to the 80x80 mm heatsink with a 12VDC fan.

author: Zero999
author_rank: Super Contributor
timestamp: August 16, 2021, 06:18:16 pm
content: Quote from: Vovk_Z on August 16, 2021, 06:07:41 pmDid you try configuring the TDA7294 as a Howland current source? It should be possible, going from the data sheet, but bear in mind it has a minimum closed-loop gain of 24dB. I would try it, with the noise gain set for >24dB and the signal gain very low, so a low value current sense resistor can be used.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 16, 2021, 06:23:16 pm
content: I have only pre-design v0 sketch at my hands, the final design is not present as a file. That v0 design uses INA163 as two opamps (actually three) in one case. But it has too large DC offcet, which varies with the temperature a lot (more than I wanted).I found that it is much better to use OPA189 (two, with four precision 0.1% resistors). It is much more precise (about an order better than INA163) and more widespread and even cheaper.A1 part is not needed for OPA189. C7, C8 are not needed at all. A 3296- or other type rheostat has to be added in series with R10 or R11 to adjust the bias current for MOSFETS.I put that circuit in two variants.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 16, 2021, 06:37:53 pm
content: Possibly it can be easier to use TDA7294 with a custom 10:1 power transformer as a current transformer for 30 A rated device. I can't say exactly.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 16, 2021, 06:45:38 pm
content: Quote from: Zero999 on August 16, 2021, 06:18:16 pmNo, as TDA7294 is not unity-gain stable, and I didn't want to solve this problem too.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 16, 2021, 06:50:34 pm
content: Quote from: Zero999 on August 16, 2021, 04:08:48 pmIt (your Reply #10 in that thread) is very similar to my design, in general, with a slight difference that I used Mosfets but not bipolar transistors.The other differences are that I used smaller value Howland pump resistors (2k-4k-10k), and one more opamp as an input repeater with zero output impedance.

author: Zero999
author_rank: Super Contributor
timestamp: August 16, 2021, 06:58:57 pm
content: Quote from: Vovk_Z on August 16, 2021, 06:45:38 pmFortunately, that's fairly easy to solve. Set the noise gain to >24dB and all will be well. The tiny increase in high frequency noise is way above the frequency of interest and will be removed by the current transformer.https://www.eevblog.com/forum/beginners/op-amp-spec-noise-gain-configuration/

author: fcb
author_rank: Super Contributor
timestamp: August 16, 2021, 06:59:21 pm
content: Fluke 5220 / 55120 transconductance amplifier. Schematics online for the 5220. Worth studying. Another unit that is pretty nice is the 1620A made by Ballantine Labs, these come up occasionally on eBay - haven't seen the schematics online.The big problem is the size of the compliance voltage you want (15V) - this is pretty big. If you build your amplifier own, watch-out for inductance in your current sense resistor.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 16, 2021, 07:09:57 pm
content: Quote from: Zero999 on August 16, 2021, 06:58:57 pmI understand this, but as it can't work with low power supply voltages (It likes to have something like > +-15VDC power supply) it doesn't suit fine a design with a direct current source without CT. But if I use additional CTs then I don't need a Howland current source (with a general ground).The main reason for a Howland current source is that it provides a common ground (for an input and an output) so we don't need CT but need a low-voltage high current power supply instead.