DIY AC Current reference? - Page 1
Forum section: electronics / projects

author: Jeroen3
author_rank: Super Contributor
timestamp: January 31, 2024, 08:00:54 pm
content: To verify an AC current 50hz measurement (5A) I am looking for a reference standard.Ideally a portable box with a switch 1A and 5A at 50 Hz to run through a small shunt device I need to verify.Looking at available gear this is really expensive, really big (or both) and not very portable. High power as well, not really intended for sensing inputs.Doing these references for DC is trivial and also readily available, AC not that much.Right now a GW Instek source is used.So I was brainstorming a bit if I could make one. I was thinking maybe a amplifier circuit fed from a sensing transformer, compensated by a RMS to DC converter. A bit of heat should not be a problem. But maybe there are better ways?1% should be enough, it will be accompanied by a DMM anyway, those aren't much better in rms current.I am trying to avoid using any digital elements, eg an mcu as waveform gen or measurement, because that raises the complexity *a lot* and makes it potentially hard to maintain. (for test gear)What do you think?

author: enut11
author_rank: Super Contributor
timestamp: January 31, 2024, 10:41:07 pm
content: You need a stable 50Hz audio generator and an audio amplifier. The DUT is connected in series with a suitable resistor, say 4R, and a DMM in amps mode. The accuracy obtained will be that of the DMM.

author: bdunham7
author_rank: Super Contributor
timestamp: January 31, 2024, 10:52:18 pm
content: If you design a closed-loop system with feedback like a calibrator, that will be an ambitous project and would require something like a very large audio amplifier for output. If you can live with open-loop, meaning you'll have to dial it in using your DMM, simply use a variac followed by a step-down transformer power your DUT, a suitable resistor and your reference DMM all in series. I'd use as large a step-down transformer as you can afford along with a large (10A if possible) variac for better thermal stability.

author: Zero999
author_rank: Super Contributor
timestamp: January 31, 2024, 11:13:15 pm
content: A Howland current pump can be used to convert an AC voltage source, into a current source.At what voltage?If it's very low, then you could use a current transformer run in reverse, that way your current source would only need to supply a few hundred mA.https://www.eevblog.com/forum/projects/need-help-with-bi-directional-constant-current-source-(100ma)/msg1972685/#msg1972685

author: Benta
author_rank: Super Contributor
timestamp: February 01, 2024, 12:05:26 am
content: Quote from: Zero999 on January 31, 2024, 11:13:15 pmExactly.I'm totally missing context here. 10 V or 10 kV drop/input range?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 01, 2024, 05:30:32 am
content: Quote from: Jeroen3 on January 31, 2024, 08:00:54 pmAs above, your simplest path is to use an Audio Amplifier, 50Hz & 5A is inside common Audio.If you want standalone, you also need a 50Hz sine generator.The simplest there is probably a AliExpress ICL8038 generator IC module. These use shaped triangles as the sine, good enough for AC calibrate.Because you only want 50Hz, you can add some low pass filtering to the output.For 5A you have a choice of larger batteries, or a plug pack, or a 50Hz transformer (also heavy).

author: MathWizard
author_rank: Super Contributor
timestamp: February 01, 2024, 06:51:24 am
content: If it was me as a hobbyist, I would buy an old stereo with huge transformer, and discrete output transistors, or add all your own, etc.I saved 1 stereo from the scrap yard, and so far used it in a variable 20V 1A PSU, and unfinished Curve Tracer. That was an old basic PSU, I plan to make some better, more powerful ones. I already added connectors on all the transformer taps, so I can use it in other stuff pretty easily. Someday I should make some low voltage AC PSU too. If I wanted, I could drive it from a real sig.gen.

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2024, 07:25:46 am
content: Is an AC square wave instead of sine wave acceptable?If so, then a DC current source can drive an h-bridge to produce a AC square wave current source.

author: MarkT
author_rank: Frequent Contributor
timestamp: February 01, 2024, 09:55:32 am
content: Quote from: Jeroen3 on January 31, 2024, 08:00:54 pmI think it could be easier to do this the normal way, by comparing with a known calibrated ac current meter or current shunt at regular calibration intervals. Then the test source can be uncalibrated.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 01, 2024, 09:59:19 am
content: It doesn't have to be battery portable, it can have a mains lead.The audio amplifier I did before but with an MCU as source for a breaker primary injection tests, however this couldn't do it longer than around 10 seconds before overheating. 50 Hz is a very low for frequency standard audio amplifiers and they don't like it. Luckily breakers do trip so it doesn't need to maintain it that long.Load resistance is a shunt or hall effect sensor input typically <100 mOhm, so a total power of 2.5W, say 5W.Audio amplifiers typically go for subwoofers that are 4 or 8 ohm, that's >100W for 5A. Probably why they're not that portable.I was hoping to be able to be able to fabricate something the size of a small korad psu instead of a 3u rack.Quote from: David Hess on February 01, 2024, 07:25:46 amInteresting, I can try this. It would be trivial to turn any DC source into an 50 hz square with an h-bridge.Maybe run it though a current transformer and obtain something stable and not totally distorted...Then perhaps I can use a stepper motor driver board this?Anyway, here goes hoping this is a components you could buy...

author: Kleinstein
author_rank: Super Contributor
timestamp: February 01, 2024, 10:32:15 am
content: Quote from: PCB.Wiz on February 01, 2024, 05:30:32 amThe function genrator chips like ICL8038 and XR2206 are obsolete. For the xr2206 there seems to be a Chinese copy - still not a reliable source.For generating a 50 Hz signal one could start with a square wave and than use a low pass filter to reduce the harmonics. A reasonable meter should not be too picky about some harminics.A not too sharp low pass and notch filter can have a quite stable amplitude response.A Wien bridge type generator cold give a cleaner sine, but needs somewhat tricky amplitude regulation.A pure square wave could be an issue because of the high slew rate needed. Many meters are somewhat OK with this, but one can't say for sure - so not could for a calibration signal that should provide a well behaved signal, but not a borderline case that may or may not work.I would not use a RMS converter chip for the feedback - they are rather slow to react and would thus need an extra slow multiplicative feedback loop (e.g. with a Wien bridge). The RMS chips are also not that stable. For a target of only some 1% a stable amplitude generator should be good enough.For a current regulation the choice is still between using directly a 5 A shunt and a current transformer. For 1 A the shunt may be easier, with 5 A a current transformer may be easier choice. The current transformer could allow to also check at a more conveient current: the DMMs are often not that great at high currents.For the drive side a audio amplifier may be a suitable choice. Here especially types for car audio may help, as they are for a low 12 V supply and sometimes a 2 Ohm load.In a DIY solution may may be able to start with an even lower supply (e.g. 6V).A transformer also for the drive signal is possible, but needs extra care with the feedback and one would likely wind / modify the transformer, as the voltages would be unusually low. For more than 5 A (e.g. 10 A or 20 A) I would consider that path as it simplifies the amplifier.

author: voltsandjolts
author_rank: Unknown
timestamp: February 01, 2024, 10:43:15 am
content: Quote from: Jeroen3 on January 31, 2024, 08:00:54 pmIf it's not for production testing, you could increase the shunt resistance temporarily for test, reducing the 1A/5A test current requirement, perhaps down to a level your sig gen or small amp could supply.

author: mawyatt
author_rank: Super Contributor
timestamp: February 01, 2024, 02:45:55 pm
content: Quote from: David Hess on February 01, 2024, 07:25:46 amExactly along the lines we've been thinking since developing the AC Volts Reference concept here, altho delinquent in producing a AC Current version:https://www.eevblog.com/forum/testgear/ac-rms-dmm-tests/msg3940957/#msg3940957The AC Current version would incorporate an accurate stable unipolar DC current source (simple PMOS with precision sense resistor, precision 5V reference and op-amp) and verified with a DC current reading from an accurate DMM. The DC output current source would feed an CMOS H-Bridge (either discrete or chip) which is clocked with a flip-flop to guarantee 50% duty cycle. So for 50Hz one could use a simple 555 timer @ 100Hz and CMOS FF (74HC74) with CMOS buffer (if needed) to drive the H-Bridge. The AC current would develop between the H Bridge terminals as sort of a Floating BiPolar Current Source and be directly linked to the DC Current Source amplitude and stability.Would expect this to work quite well, since we've shown how good the AC square-wave voltage concept works, see link above.Best,

author: Terry Bites
author_rank: Super Contributor
timestamp: February 01, 2024, 03:16:46 pm
content: AC current sources are not so complcated but you have to watch the compliance limit, the maxumum voltage it can supply across the load.Bipolar current sources can be made with an opamp and a power amp. A floating psu/ battery supply lets you ground one side of the load. The See attched. Compensation is needed and it may be more complex than I've scetched it. Check that the power amp has the correct polarity. If the power amp is phase inverting you swap over the opamp inputs.An output transformer could be used to convert a class D automotive audio amp from diff to single ended

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2024, 05:01:06 pm
content: Quote from: Jeroen3 on February 01, 2024, 09:59:19 amThe idea is that starting from a regulated DC current, which is easy to produce, an h-bridge switching at 50/60 Hz can produce the same RMS output current as a square wave *without* monitoring the AC output, which is more difficult.Since it is a current going through the h-bridge, the resistance of the power MOSFETs is irrelevant to the AC output current. Smaller MOSFETs should be used to limit charge pumping through the gate capacitance, but the error contribution from this should be insignificant.Now since the output is an AC current square wave instead of a sine wave, calibration of the reading it a little bit different. A current measurement using RMS conversion will read correctly, but an average responding current measurement will read 11.1% high and needs to be taken into account.The output from the h-bridge is differential so the whole procedure assumes that either the source or device under test is floating, which should not be difficult to arrange. If this is a problem, then a single ended AC current source using the same method is possible with a half-bridge, but the circuit is more complex because a pair of matched DC high current sources are required.Quote from: mawyatt on February 01, 2024, 02:45:55 pmI have gotten good results with the h-bridge or half-bridge method of AC voltage calibration before to better than 0.1%. I have never done it myself for current, but know others have. The limitation is that since the output is a square wave instead of a sine wave, it assumes that the frequency response of the measurement is already flat and correct. If the frequency response of the dividers needs to be adjusted, then calibration with a leveled sine source is required first.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 01, 2024, 07:06:11 pm
content: Quote from: Jeroen3 on February 01, 2024, 09:59:19 amIt's not so much 50Hz as 5 Amps that is the challenge for Audio Amps.Quote from: Jeroen3 on February 01, 2024, 09:59:19 amolder/simpler 12V to 230V inverters used a quasi square, where they only drive some portion of the time, giving a three stage waveform.Your easiest power source for low voltage is likely to be 5V, so a ~0.7 ohm 25 watt resistor and some MOSFETS should give 5A ballpark.Addit: This claims 71% duty cycle gives RMS match.https://www.quora.com/What-is-a-modified-sine-wave-power-inverter

author: mawyatt
author_rank: Super Contributor
timestamp: February 01, 2024, 07:32:57 pm
content: Quote from: David Hess on February 01, 2024, 05:01:06 pmIf you follow the link above we provided you'll see we achieved much better than 0.1% for a unipolar voltage square-wave considering the ideal value is just Vdd/2, there's even a small correction factor for the rise/fall times of the non-ideal squarewave if you really want to get precise.We haven't used this concept for current either (yet), but agree it should be pretty good for AC RMS Current as well. Tempting to roll one just for funBTW, for the OP, the 50Hz may cause a slight error in the readings since the DMMs use may be AC coupled.Best,

author: Micke
author_rank: Regular Contributor
timestamp: February 01, 2024, 08:03:05 pm
content: I have built two of these with very good result!I should have a spare PCB you could gethttps://www.eevblog.com/forum/testgear/external-amplifier-for-laboratory-generator/msg2956398/#msg2956398Edit: Forgot to mention what it is!It is a precision constant current source DC-100kHz 8ABased on a NIST paper "A wide band transconductance amplifier for Current calibrations", link in the original post above.

author: bdunham7
author_rank: Super Contributor
timestamp: February 01, 2024, 08:11:20 pm
content: Quote from: mawyatt on February 01, 2024, 07:32:57 pmThe OP is trying to "verify a small shunt device" and without knowing the characteristics of that device nobody can say how well a 50Hz square wave will serve that purpose.

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2024, 11:00:37 pm
content: Quote from: PCB.Wiz on February 01, 2024, 07:06:11 pmAudio amplifiers are designed for a higher impedance, like 4 or 8 ohms, which at 5 amps implies a supply voltage of 20 or 40 volts and low voltage across the pass transistor. A discrete 5 amp linear amplifier with low output voltage compliance is not difficult to design and build, but it would be a difficult load for a properly designed audio amplifier.

author: bdunham7
author_rank: Super Contributor
timestamp: February 01, 2024, 11:45:04 pm
content: Quote from: David Hess on February 01, 2024, 11:00:37 pmIndeed, if you start from a CC design it would be reasonably easy, but an audio amplifier is a voltage source not a current source and the feedback portion isn't well suited for the purpose. I can do it as I have some pretty high power audio amplifiers, but I would use a substantial series resistance like 5 ohms. That's a lot of dissipated power.

author: David Hess
author_rank: Super Contributor
timestamp: February 02, 2024, 01:15:02 am
content: Quote from: bdunham7 on February 01, 2024, 11:45:04 pmAt low currents I would use two current outputs in parallel with one being fixed and the other variable at up to twice the current, yielding a symmetrical bipolar output current. It is a little wasteful but works great. And of course a Howland current pump is an option.At high currents I have never come up with a satisfactory solution, other than the Howland current pump. Tektronix had an inscrutable solution for their curve tracers for driving the base, but I am not sure if it could be converted to high precision.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 02, 2024, 06:56:58 am
content: Quote from: David Hess on February 01, 2024, 11:00:37 pmI can always add some dummy load in series.