Problem with DIY CC Load - Page 1
Forum section: electronics / beginners

author: cichy45
author_rank: Contributor
timestamp: March 03, 2022, 10:06:49 pm
content: Hi everyone!I build some simple DC CC load basing on this source:http://www.kerrywong.com/2013/10/24/building-a-constant-currentconstant-power-electronic-load/But without micro controller control, just simply set reference voltage with 10 turn pot and all good. It works without issue.Here is my schematic:As you see, to allow opamp to go true 0V I have symmetrical +/- 12V.My issue is: when I short amp meter terminals - so my DUT goes directly to ground all is fine and dandy. My mutimeter shows (for example) 1 Amp (connected from +DUT to mosfet drain, so just simply on high side).Now I have this kind of panel meter:https://pl.aliexpress.com/item/4000112135774.html?algo_exp_id=ee694896-4860-406b-a0b8-86c5128b1479-31&pdp_ext_f=%7B%22sku_id%22%3A%2210000000289861578%22%7D&pdp_pi=-1%3B17.77%3B-1%3B-1%40salePrice%3BPLN%3Bsearch-mainSearchCommon ground between thin power wires (vcc, gnd, vsense) and thick current wires.When I connect my panel amp meter in series in AMPMETER terminals, sense to DUT+, VCC to +12V, my multimeter still shows 1A but my panel meter acts wanky. Its measures jump +/- few % up and down and are a few % offset from multimeter. So, multimeter show 1A, panel meter shows 0,955A, 0,978A, 0,931A.The strange thing is, that the same panel meter works perfectly fine with CC DC load named xhdz-fz-2g. More information:https://www.eevblog.com/forum/beginners/understanding-this-dc-load-circuit/https://www.eevblog.com/forum/beginners/xhdz-fz-2g-electronic-load-comparator-function/I desoldered solder bridge and connected my panel meter as in threat above and it works just OK, measurements are on spot with my multimeter and solid (no jumping).If I understand correctly, that xhdz-fz-2g "virtual ground" (+0.7V) is equivalent to my ground, xhdz-fz-2g true GND is my -12V and xhdz-fz-2g +12V is my +12V. Panel meter amp shunt shorts DUT to GND/virtualground and power supply ground is also tied to GND/virtualground.So any idea why this panel meter works with xhdz-fz-2g and not with my DC CC load?

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 03, 2022, 10:24:17 pm
content: 2. Those panel meters usually have loose connectors so it is one of the reasons of current jumps. The current meter shunt have some resistanse (0.01 R possibly), and it's wires and connectors (connection force veries). Sometimes I solder wires directly to avoide those loose connectors and use better wires and much more strong connectors like XT30 (XT60) with steady and ultra-low connection resistance.1. As I see from first sight (I may be wrong) the input signal gnd has to be connected with Ampmeter+ for your exact design. So it needs isolated power supplies for a load with ampmeter and signal source.But it can be avoided if you swap ampmeter and a shunt. You only has to be carefull with ampmeter gnd connection - it has to be connected to circuit only with one wire ('Ampmeter-'). Do not connect power supply gnd of an ampmeter.I may wrong because I haven't investigated your case closely, it just seems to me you have a typical problem and I write typical desision.I've done it several times but don't remember at the moment my exact decision and can't look inside my devices because of a real war here.

author: cichy45
author_rank: Contributor
timestamp: March 03, 2022, 10:35:54 pm
content: Thanks for the answer.Quote from: Vovk_Z on March 03, 2022, 10:24:17 pmThis is rather not the case. It works OK with xhdz-fz-2g. I move it and current measurement is solid.Quote from: Vovk_Z on March 03, 2022, 10:24:17 pmI already tried connecting with only negative lead of panel amp meter to GND and leave panel power supply GND floating (as they are internally connected) but the same issue is present, no change in behaviour.

author: MarkF
author_rank: Super Contributor
timestamp: March 04, 2022, 02:18:00 am
content: I would recommend two changes:1) Make a few small circuit changes to improve stability. The series resistor to the MOSFET should be smaller. In the range of 50 to 100 ohms in order to drive the internal gate capacitance. And the resistor from the sense should be larger. The feedback current will be small and a large resistance will protect the op-amp input. Last, a larger feedback capacitance will help eliminate oscillations. I found that 4nF to be the very minimum.2) To your Amp meter. Having the meter where you do will cause errors in your current sensing. Your set current from the pot to the (+) op-amp input is referenced to ground and your sense resistor (R43) then is also referenced to ground. Therefore, the error in you sense voltage will increase as you increase the current setting because the voltage across the Amp meter will increase and interfere with the voltage being sensed. I would try to put the Amp meter between the MOSFET and sense resistor but THIS REQUIRES MODIFYING THE AMP METER. See this video by Peter Oaks:Note: The op-amp will make the voltage at your sense resistor (R43) the same as the voltage from your pot (WIPER). Knowing the max pot voltage and the value of your sense resistor, you set the max current your load is capable of. Imax= Vwiper/ Rsense

author: cichy45
author_rank: Contributor
timestamp: March 04, 2022, 06:46:20 am
content: Thank you MarkF for the support.Quote from: MarkF on March 04, 2022, 02:18:00 amThanks! I will make these adjustments in next iteration.Quote from: MarkF on March 04, 2022, 02:18:00 amThat is something I was thinking of, but ignored it because: IF my panel amp meter would work, I do not really care about Vset at opamp, because I look at the readout of my panel meter. Current should be stabilized either way, if opamp is sensing voltage between R43 and GND or R43+Rshunt and GND. Just the ratio between Vset and Vsense would be disturbed?Could it be fixed, to connect 10 turn pot low terminal (GND) to point between R43 and Rshund (amp meter)? Then both, Wiper and sense would rise up from ground when current increases?Either way, I sill do not understand why this panel amp meter does not work correctly, when both - meter and DUT are connected at ground.I tried the trick with separating grounds in amp meter, but it seems not to work (after separating them at amp meter PCB amp meter will not turn on unless current wire is connected to GND, power supply gnd is then "cut off", it seems that circuitry is tied to ground via current wire and only power supply wire is for referencing VCC to current?).

author: magic
author_rank: Super Contributor
timestamp: March 04, 2022, 06:48:42 am
content: I wouldn't be surprised if this load is oscillating and that's what throws the Chinese ammeter out of whack.As shown by MarkF, C42 and R42 should be swapped, I don't know what that kerrywong guy was smokingR41 being 10k makes no sense too.Regarding C42 value, it depends on what R42 is. What matters is their product - you could leave it 1nF and increase R42 to 10kΩ and the effect is about the same.BTW, I don't think you need a negative supply here. LM358 will go down to ground, low enough to turn off the MOSFET if you set zero current.

author: cichy45
author_rank: Contributor
timestamp: March 04, 2022, 06:57:34 am
content: Thanks magic.Unfortunately I have no oscilloscope (yet) so I have no way of checking for oscillations, but that could be the case. My multimeter however shows rock solid 1A while panel amp meter is jumping. But maybe my multimeter is too slow to pickup any current variations, I can not determine that without oscilloscope.If you say that LM358 can go close enough, then I will remove negative supply. It should simplify the circuit a bit.

author: magic
author_rank: Super Contributor
timestamp: March 04, 2022, 07:37:54 am
content: Gate threshold voltage: 2.5V~4.5V for 250uA.LM358 can go down to 1V easily and to <0.1V if sourcing current or sinking <0.1mA.You could add some sort of protection from overdriving the output FET under no input voltage. All those naive dummy loads apply full 12V to the gate and then when some input voltage appears, the load pulls many amps from it for a fraction of a second it takes to discharge the gate to a reasonable level.I suppose something like that could work. Q1,R1,D1 should activate when drain voltage is less than ~1.5V and make the opamp think that load current is already exceeded so the FET stays off. 2.5V can be generated with TL431, or some other reference voltage could be generated with some other regulator. The same voltage can also be used for the potentiometer, to reduce dependence on the 12V supply.

author: not1xor1
author_rank: Frequent Contributor
timestamp: March 04, 2022, 09:11:38 am
content: Quote from: cichy45 on March 03, 2022, 10:06:49 pmThis might work (I do not know how the voltage/power reading might be affected):1) connect R43 directly to ground2) remove the shunt resistor from the panel meter3) put a 100 Ω multi-turn trimmer in parallel to R434) connect the cursor to ampmeter+5) regulate the trimmer for the proper current readingin any case you would need a low thermal drift resistor for R43

author: cichy45
author_rank: Contributor
timestamp: March 04, 2022, 09:24:38 am
content: Or I guess, I can simply skip R43 and use amp meter shunt as reference for measuring current and mosfet control via opamp?But this will not allow me to parallel multiple stages, as they need to have separate shunt for current measurements. I could use multiple panel meters heh.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2022, 09:43:08 am
content: To reduce the tendency to oscillate with an inductive one should consider an RC series element (some 100 Ohms and 100 nF) in parallel to the MOSFET (DS). This does hamper fast response a little, but reduces the chance to get oscillation.The ampmeter at the ground side of R43 would add to R43 quite a bit and the cabeling would add temperature dependence. One would use the shunt of the amp-meter instead of R43, so sense the voltage from the ampmeter circuit. It still depends on the quality of the shunt there - the cheap panel meters may also use a cheap, rather low value shunt that is not really good.The idea of limiting the current / reponse if the voltage at drain drops too far is good. Without it, there can be quite some initial current spike that can be possibly damaging to the load or DUT. The version shown by magic is simple, but a but limiting when testing low voltage sources (e.g. single NiCd / NIMH cells).

author: cichy45
author_rank: Contributor
timestamp: March 04, 2022, 09:57:36 am
content: I do not need fast response load. 90% of DUT are li-ion batteries, I do not plan to test transient or pulsating load, just simple CC to measure Ah capacity.Ideally, I would parallel a bunch of these stages to achieve high power dissipation, so maybe 10 stages so I can have 1 kW of dissipation. Of course, one stage = opamp, mosfet, shunt as I have read that paralleling mosfet for linear operation without control over every single mosfet is asking for trouble.I will update my schematic and post it here in the evening, so maybe you can share some more suggestions.

author: magic
author_rank: Super Contributor
timestamp: March 04, 2022, 10:05:27 am
content: At 1kW and DC it could be significantly cheaper to use a few big-ass resistors / heaters / etc in parallel and arrange things so that the active load only sinks the last remaining few amps and compensates for thermal drift of the "genuinely dummy" loads.It's a no-brainer to average voltages from a few shunt resistors and feed that to the opamp.Quote from: Kleinstein on March 04, 2022, 09:43:08 amIt's simple, but it may not work well after all.It has occurred to me that if the DUT becomes current limited for whatever reason, my circuit will be a shunt constant voltage regulator. And given that there is a common source and a common emitter stage in the loop, I wouldn't bet on its stability under such conditionsBut it could be a starting point to design something truly reliable. Perhaps the protection loop should bypass the opamp and clamp gate voltage directly.Regarding protection activation threshold, it seems to me it should ideally occur right before the FET transitions to triode region because that's the point when the opamp will start to raise gate voltage in response to insufficient drain current which could backfire when drain voltage increases suddenly for any reason. OP's part's datasheet isn't very helpful in this regard, but for <10A chances are that 1V could be safe.

author: cichy45
author_rank: Contributor
timestamp: March 04, 2022, 10:35:34 am
content: Quote from: magic on March 04, 2022, 10:05:27 amI was thinking about it but decided against. In my opinion it would complicate the device, as resistors would need to be swapped when working with different DUT voltages to have 10 or 20A for example. With all power going via mosfets, DUT voltage does not matter if it is 24, 32V or 80V.As heatsink I use PC CPU heatsinks, mosfets are soldered to copper base using low tem 138 degree Celsius tin-bismuth solder thus having very low thermal resistance, better that paste I suppose (minus side, that whole heatsink is DUT+ voltage).I tested AOT13N50 and FQA32N20C, fairy cheap and despite not being declared for linear operation, can handle almost 100W dissipation. I have also IXYS Linear and those are good as well.Quote from: magic on March 04, 2022, 09:43:08 amFor protection I might just simply rip off design from aforementioned xhdz-fz-2g. It would complicate control stage but would give all safety I suppose? Given that I have symmetrical power supply.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2022, 10:41:57 am
content: A way to protect for the case of saturation of the FET would be to reduce the set signal. The regulation would than cross over from a constant current regulation to a simulated resistance, larger than the minimum resistance the circuit can actually provide.Just limiting the gate voltage would be a first step.With low voltage and rather high current, the shunt (R43) can be quite important and I would consider having the shunt at the circuit and have a panel meter that supports an external shunt. Some series resistance can help reducing the power dissipation of the FET, but the cable resistance could also get too much.For low voltage, like < 10 V the MOSFET selection is not that critical, for > 20 V the SOA part can become important. A FET failing short can be quite bad. So at least plan for an additional fuse.

author: magic
author_rank: Super Contributor
timestamp: March 05, 2022, 07:15:25 pm
content: Quote from: Kleinstein on March 04, 2022, 10:41:57 amSimulating a resistance slightly higher than Rds(on) may actually be a good idea with MOSFETs.Is there any particular known circuit you have in mind?I suppose one would have to look at the ratio of drain voltage to shunt voltage and compare against Rds(on)/Rshunt.Quote from: cichy45 on March 04, 2022, 10:35:34 amThat circuit you linked in the first post looks weird. Using a comparator seems even less stableCorrect me if I'm wrong, but the way it works when the DUT current-limits is:- drain voltage drops too low, the comparator sets current to zero- drain voltage rises, the comparator restores normal operationI also think there is some error on the schematic altogether, the connections to LM393 inputs make no sense.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 05, 2022, 09:58:53 pm
content: constant resistance mode is realized by getting the set voltage for the current from the drain voltage.For an automatic cross over one would need to use a kind of minimum ciruit (e.g. ideal diode clamp circuit), so the the current can not get higher than the set resistance. One would need to find a suitable minimum resistance once (e.g. Shunt + R_on + some head-room). Chances are the use could be allowed to set a higher resistance as well.