Microcurrent gold - Reprise - Page 1
Forum section: electronics / repair

author: Moriambar
author_rank: Unknown
timestamp: May 16, 2021, 01:08:16 pm
content: TIFU (well yesterday but it's the same) shorting VCC to gnd through my uCurrent Gold albeit for a couple of seconds.Although I set a limit of 20mA on the psu, it seems that I have damaged the thing.Since I have no skill in diagnosing and repairing such things (and have a limited supply of tools), I thought this could be a good "project" but I need your guidance.Here's a recap:SymptomsAfter doing the wrong thing, the microcurrent (it was in the milliamp range) ALWAYS shows up negative values upon powerup with the PSU disconnected.Nothing appears blown or damaged upon visual inspection. The LED still turns on when switching on.I tried a couple of measurements and they seem kinda off, but I don't know how to determine whether the microcurrent is really cooked or if it's fine.First of all it gives very low readings without taking the delta for the negative starting offset. Even after doing the delta It is off wrt my EEVBlog BM235 and Keithley DM6500 of 2-3mA over a range from 5 to 20mA. Both the other meters show the same value more or less.The same applies for the microcurrent range where the uCurrent shows values of ~100uA less than the other multimeters (around 350 uA vs 470uA). The measurements have been done using a npn current sink roughly programmed.Anyhow the microcurrent values are always wobbly and never stable.They were not this way before.QuestionsCan this be my paranoia and the discrepancies justified by burden voltage of the meters? I probed with either meter and the micro current separately.What can I do to see whether there's something wrong?Can you help/guide me step by step plz?Tools I haveDM6500BM235Scope->SDS1104X-e (iirc, it's the 100MHz 4 Channel Siglent)Linear siglent PSU (I don't recall the model unfortunately)DER EE LCR meterHakko FX888D soldering iron with some tipsI think other stuff don't matter, but notably i do not have a hot air gun/station.Cheers

author: LA7SJA
author_rank: Regular Contributor
timestamp: May 16, 2021, 05:04:03 pm
content: The schematic:https://www.eevblog.com/files/uCurrentRev5schematic.pdfIf you short the input pin, what do you measure on the output in the different ranges?I do remember a problem with one batch of the ground-splitting op-amp oscillating with some multimeter.Can you measure the AC component (use the scope, Luke) on the negativ output with reference to the negative pole on the battery when using one of the "BAD" reading multimeter?You should read this tread:https://www.eevblog.com/forum/testgear/questions-regarding-ucurrents-rail-splitter-subcircuit/msg2756182/#msg2756182and thishttps://www.eevblog.com/forum/blog/eevblog-1057-current-murphy/?allJohan-Fredrik

author: Moriambar
author_rank: Unknown
timestamp: May 16, 2021, 07:00:34 pm
content: Quote from: LA7SJA on May 16, 2021, 05:04:03 pmHello and thanks for your reply.I'm very aware of the issue but I wasn't effected either because I got it way before or way after (couldn't remember) the incident.Anyhow:QuotemA=>-0.21uA=>-0.7nA=>-4and alsoQuoteI get a high frequency noise basically, ~100mVppThanks

author: LA7SJA
author_rank: Regular Contributor
timestamp: May 16, 2021, 08:12:51 pm
content: Hi "Moriambar"Is this noise present if you remove the multimeter?Do you have a picture of the scope image with and without the multimeter?Any way you could try to remove the rail splitting op-amp and use a split powersupply (2 x 2V or two 1.5V battery)?I think it is the LMV321 thats broken.Johan-Fredrik

author: Moriambar
author_rank: Unknown
timestamp: May 17, 2021, 07:50:51 am
content: Quote from: LA7SJA on May 16, 2021, 08:12:51 pmHello, so here are the two waveforms:with multimeter attachedwithout multimeter attachedI used the BM235 btw.I can try to remove the splitting opamp later tomorrow I think, and I can attach it to a split power supply.Thanks for now.Just to learn: what makes you think it's the LMV321 that's gone bad?Cheers

author: LA7SJA
author_rank: Regular Contributor
timestamp: May 17, 2021, 08:32:02 am
content: Quote from: Moriambar on May 17, 2021, 07:50:51 amThe LMV321 is responsible for making a voltage division ground reference with a resistive voltage division of the battery voltage and the operational amplifier (configured as a 1: 1 buffer) to stabilize the artificial ground point.Pin 1 on the LMV321 (U2) is VBat/2 and the op-amp is configured as X1 buffer (U2 pin 4 connected directly to U2 pin 3).The LMV321 is the only active part in the ground circuit (the only component that can oscillate)Johan-Fredrik

author: Moriambar
author_rank: Unknown
timestamp: May 17, 2021, 08:27:51 pm
content: Quote from: LA7SJA on May 16, 2021, 08:12:51 pmDesoldered. Noise has not any ringing shape anymore and it does not change with or without the meter attached. Scopes measures about 20mVpp of noise.The shorted readings are off though, displaying around -110 to -130 mV on the multimeter, with the selector on short. It is kind of consistent regardless of the range although it seems a bit bigger in absolute value on the mA.So, should I buy a new LMV321?Thanks for your help

author: LA7SJA
author_rank: Regular Contributor
timestamp: May 17, 2021, 09:10:18 pm
content: Hi!Is the output a clean dc (check with a scope)?What is your powersource, batteries or splitt power?How is the power feed to the cirquit?Is the ofset only present on pin 1 of U4 or on both U1 and U4?Don't buy the LMV321 for now, you may need more parts.What do you measure from GND black output pin (J4) to pin 2 and 5 on one of the op-amps?Johan-Fredrik

author: Moriambar
author_rank: Unknown
timestamp: May 17, 2021, 09:26:39 pm
content: Quote from: LA7SJA on May 17, 2021, 09:10:18 pmSo, I lied… the noise is more in the 200mVpp ballpark now. I installed the new firmware on the scope today and basically all probes were set to 1x.I attached the waveform with dc coupling across the output terminals.The circuit is powered via my SPD3303X set in serial mod. The most negative terminal is set to the minus of the battery, the most positive to the plus and one of the other two (midpoint) are directly connected to the output - post.QuoteI will have to check this tomorrowthanks for now

author: LA7SJA
author_rank: Regular Contributor
timestamp: May 17, 2021, 09:41:13 pm
content: Hi MoriambarYou did remove the coin cell? didn't you?How was things connected and what voltages was aplied when the acident happend?Johan-Fredrik

author: fzabkar
author_rank: Super Contributor
timestamp: May 18, 2021, 05:49:00 am
content: FWIW, the MAX4239AUT amps are configured with a gain of 10. The datasheet specifies a Minimum Stable Closed-LoopGain of 10V/V. This means that the amp is operating right at the limit of its spec.https://datasheets.maximintegrated.com/en/ds/MAX4238-MAX4239.pdfQuoteI suspect that this poses no problem, but I thought I'd just make the observation.It might be interesting to replace one of the amps with a MAX4238.

author: Moriambar
author_rank: Unknown
timestamp: May 18, 2021, 06:37:43 am
content: Quote from: LA7SJA on May 17, 2021, 09:41:13 pmYes I removed the battery, but thanks for asking.Well when the accident happened the input+ terminal was connected to VCC and input - directly to GND.I don't remember how much voltage was supposed to be there tbh (I was doing too many things at once) but definitely between 5 and 9V, but the PSU should've been CC limited to 20 or 40mA of current (again: too many things at the same time and I got distracted).Later today/tomorrow (depending on your timezone too) I'll do the measurements you asked.Thanks!

author: Moriambar
author_rank: Unknown
timestamp: May 18, 2021, 06:42:17 am
content: Quote from: fzabkar on May 18, 2021, 05:49:00 amHello and thanks for your interest.Unfortunately I have no available MAX4238.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 18, 2021, 08:33:19 am
content: The max4239 should be OK with a gain of 10. It is chosen because of higher bandwidth. For just use with a DMM and DC the max4238 would be OK too, but for the use with a scope the higher BW is very nice.Oscillation of the virtual ground is a known issue with some OPs used in the circuit - there are many OPs similar to the LMV321 and some are more prone to ocillation than others. So one batch of the µCurrent got OPs prone to oscillation.The linked schematics (https://www.eevblog.com/files/uCurrentRev5schematic.pdf) is already for the updated circuit that fixed the oscillation problem of the LMV321 by adding R10. The older version may not have R10 there. Ideally one would add an extra cap from the batery neg side to the virtual ground - but only with R10 in place.At the other end R8 at the output is inside the loop and making things harder for the signal output and one may get oscillation there. Shorting out R8 (for the may actually help.

author: Moriambar
author_rank: Unknown
timestamp: May 18, 2021, 11:05:05 am
content: Hello!Here are the measurements with the device on short and split psu inputQuote from: LA7SJA on May 17, 2021, 09:10:18 pmU1 has an offset of around +2mV, while U4 displays -30mVQuote-1.997V and +1.995V respectively.ThanksM.

author: LA7SJA
author_rank: Regular Contributor
timestamp: May 18, 2021, 04:49:46 pm
content: Hi M.If you lift pin 3 U4 and bridge it to GND, what du you measure on the output?Johan-Fredrik

author: Moriambar
author_rank: Unknown
timestamp: May 18, 2021, 05:45:02 pm
content: Quote from: LA7SJA on May 18, 2021, 04:49:46 pmI think the task of lifting one pin of such a tiny ic are well beyond my soldering capabilities.Perhaps I'll give it a try but I smell failure all the way

author: LA7SJA
author_rank: Regular Contributor
timestamp: May 18, 2021, 06:45:30 pm
content: Hi Moriambar don't give up!I just tryed on a scrap pcb with an rather large soldering iron and a needle, it went greatI agree I think U1,U2 and U4 are bad and maybe R1 and or R9, check if the green LED goes out when the voltage is lower than 2.5V. Maybe you can ask Dave to qoute you the spare parts.Johan-Fredrik

author: Kleinstein
author_rank: Super Contributor
timestamp: May 18, 2021, 07:32:10 pm
content: A first point would be to check if the oscillation comes from the max4239 or the virtual ground. The measured frequency is near 12 kHz which is about where the AZ frequency of the max4239 is. There is a kind of beat frequency visible, indicating 2 near by frequencies.An oscillating OP usually oscillates at 1 frequency and often more close (still below) to the upper loop bandwidth. So with the LMV321 as a follower this would be more like a little (e.g maybe a factor 2 or 3) below 1 MHz. 12 kHz would be rather slow for a LM321 with unity gain.This kind of points to seeing the AZ frequencies of the 2 max4239 and not the virtual ground oscillating. There may still be some contribution of the virtual ground increasing the amplitude.A first point to check would be the µCurrent revision - it helps when you know the actual circuit and not just approximately.It it is really the Rev 5 circuit as in the plan (with R10 as shown), adding some capacitance (e.g. 100 nF-50 µF) from the battery neg side to the virtual ground could help, as it would lower the virtual ground impedance quite a bit.With some AC cuperimposed, not all meters may correctly measure to the mV range.The DC voltage is still at least approximately OK - so I would expect the OPs not to be really bad.For a test of the OPs rough working may apply a AC signal of suitable size to the input and check the signal output and intermediate (pin1 of U1) with the scope.

author: Moriambar
author_rank: Unknown
timestamp: May 18, 2021, 07:53:27 pm
content: Quote from: LA7SJA on May 18, 2021, 06:45:30 pmWell you're god, Johan-Fredrik. I can BARELY solder packages suck as those of the amplifiers on the microcurrent and it takes a lot of time and effort for me to do so!QuoteWell the resistors are too small, 0603 packages are too difficult, I can solder them but it's something really tasking for me. Imagine the tiny packages on the microcurrent!Btw the led goes out around 2.65V IIRC.As for the spare parts, I guess it's way cheaper imo to get them locally than from australia since I'm in Italy… I could try to replace U1, U2 and U4, but if the resistors are involved, then it's a new microcurrent for me.Well I don't know how to proceed now, should I order the components? The lift and ground is just a ridiculous maneuver for me lol. I tried but it's pointless lol.

author: Moriambar
author_rank: Unknown
timestamp: May 18, 2021, 07:57:47 pm
content: Quote from: Kleinstein on May 18, 2021, 07:32:10 pmHello, and thanks for your reply. Yes it's Rev5 as advertised on the board too. Please do mind that my beginnitude prevents me to understand a single thing of the rest you said above.QuoteEhr… so… what should I do?Thanks

author: Moriambar
author_rank: Unknown
timestamp: May 19, 2021, 04:35:16 pm
content: Ok I just checked R1 and R9 in circuit using an LCR meter on DCR and on my keithley. Those are within my ability to replace. I tried for a couple of hours the pin-lifting, but I cannot do it.Btw R1 measures 0.01 Ohms, while R9 measures 9.99 Ohms.So should I get U1, U3 and U4 replaced or should I throw away the microcurrent gold? Or are there any other tests that do not involve pin-lifting?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 19, 2021, 06:33:34 pm
content: The LMV321 seems to be already removed - so not really much sense in chicking it in circuit in a non destructive way. Chances are it would be replaced with a new one at the end. The LMV321 or a similar replacement is very cheap, so little sense in putting the old one back. It is more the shipping that could hurt.The 10 Ohms resistor still measures OK - at least good enough. The othere 2 shunts at the input are relatively difficult to damage and thus very likels still OK.One could also check R12 in circuit with the switch set to the nA position. I would not really expect it to be bad, but just in case.One point one could test is checking the switch contacts, with an ohms meter.Another possible point may be just to short out R8 - as in the plan it does more harm than good and may increase the ringing at the output.One could test the amplifiers with an AC signal to the input and the scop at the output and maybe pint 1 of U1. This should tell if the amplifiers still works, at least roughly.Usually OPs are Ok or fully broken - partially working is a rare case.The offset readings look odd - how are they measurend. One should get 1 offset reading, at least for the mA and µA range. There is no need for an external shunt. the 0.01 Ohms resistor is essentially like a short. An external short is more like a possible source for errors (e.g. thermal EMF from the cable and connectors) - zero current is best realized with open inputs.

author: LA7SJA
author_rank: Regular Contributor
timestamp: May 19, 2021, 07:46:35 pm
content: QuoteIf you by this mean R2 (10K) you would need about 50V to destroy it (in a year). But wy notI agree regarding ordenary op-amps but this op-amp(s) are quit a different beast. I think it has been proven that the virtual ground op-amp is oscilating earlyer in the diagnostic.Moriambar! do not giv up. (This is what's called learning)If you send Dave an PM and ask if he could sell you the spare part (I would be suprised if he turn you down)Johan-FredrikPS Sorry I'm so slow to respond but the chemotherapy makes me so slow and lazy.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 19, 2021, 08:03:31 pm
content: With the given frequency the oscillations shown don'T look like the LMV321 is oscillating. This was a possible problem with the earlier version of the µCurrent, but should be fixed with version 5. The frequency and beat frequency very much looks like the maxa4239 internal AZ frequencies. The questing is a little why so strong.R12 is the 270 ohms resistor just in front of the OP. It is unlikely that it is bad, but in theory possible. Choosing the nA range is suggsted as the 10 K resistor would not effect the testing of R12 much.

author: Moriambar
author_rank: Unknown
timestamp: May 19, 2021, 09:11:14 pm
content: Quote from: Kleinstein on May 19, 2021, 06:33:34 pmOk so R12 is 2686 Ohms, while R2 is 9.99kQuoteAll well under a ohm when connected, open when not.QuoteThis I can do tomorrow, perhaps but I don't know what to measure then…QuoteBy AC signal you mean like something from my arb? If so can you specify some parameters, please? I have no idea which can be goodQuoteYeah, I don't know what to make of that.Thanks for your kind replies and interestQuote from: LA7SJA on May 19, 2021, 07:46:35 pmWell I haven't got the setup, the eyesight, the tools, and the experience for such a job as lifting one pin of a smt package, especially without room to maneuver! Also I think that spending two hours getting sour trying it is enough for meThe problem is that, unfortunately, up until now I don't feel like I learned basically anything about the failure and the reasons behind all of the measuring. But if you guys are still willing to help I will give it my bestQuoteWhich spare part are you talking about? I don't know what I have to replace is the lmv? Or the two op amps? Or just one?Btw I'm in Italy, and I think I can get a better deal from some distributor than from Dave, since he ships from the other side of the world... I also have some things to order so I may get free shipping from digikey and such! As long as I get the proper replacement.QuoteNo problem, my man, keep strong. And thanks for all of the help you're willing to give regardless.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 20, 2021, 06:47:04 pm
content: Having R12 at some 2.7 K is odd - the plan has 270 Ohms. 2.7 K would offer more protection to the OP - so it could be an additional improvement over the plan. I very much doubt a 270 ohms resistor has changed from overload to 2.7 K - this is more like an intentional different part.An AC signal could com from a function genrator / ARB. One good test signal is a triangle at some 1 kHz and than likely a resistor (e.g. some 100 K) to scale down the signal / make it a current output, e.g. for the µA range. One may have to be carefull with the ground for the input an output side, as both the ARB and scope may have a connection to PE. So make sure to have ground at the common black terminals. The trinagle makes it easy to see the linear part of an amplifier and where clipping starts. Another good test could be a small square signal to see if there is excessive ringing.Shorting and maybe measuring first R8 would be usfull with the AC test (it may improve the response / reduce ringing and maybe reduce the AC background seen with no input signal. Especially if R8 is also at 2.7 K - this could explain poor performance. R8 may already be gone ( 0 ohms) in an improved version.Future offset readings should be done with open inputs, not with an external short.If one order parts from DK anyway, I would definitely get a replacement for the LMV321 and maybe also the 2 max4239 - though I doubt they are both broken.Replacing the LMV321 may be seen as practice - it's a cheap part.

author: Moriambar
author_rank: Unknown
timestamp: May 20, 2021, 06:52:19 pm
content: Quote from: Kleinstein on May 20, 2021, 06:47:04 pmsorry, my bad! it read 0.2686k and I forgot how to write it, though!QuoteI'll measure R8, but I think I'll get the same 270 ohms, not 2.7k I feel like a big fail. and then try the arb method with resistor.QuoteOk cool I'll get you back with the other tests as soon as I can

author: Moriambar
author_rank: Unknown
timestamp: May 20, 2021, 09:17:50 pm
content: Quote from: Kleinstein on May 20, 2021, 06:47:04 pmSo,here is output from the thing on the conditions you asked:Whilw this is the output of pin 1 of U1, although for me it's almost impossible to measure it and at the same timee print it on the scope.Notice that it's on a very different scale…So, what to do now? do you think that replacing the amps and the lmv should fix it?But most of all: what is broken and how did you understand that?Thanks

author: Moriambar
author_rank: Unknown
timestamp: May 25, 2021, 08:02:41 am
content: Ok, just as a follow up I added the two ops and the lmv in my digikey basket and, as soon as I reach the free shipping threshold I'll order and attempt the repair!Thanks for now, I'll followup when I've done it!EDIT: Digikey won't cut it 'cause they don't have the max opamp, also the lmv321 is flagged as "end of life" and they just have 10 in stock!I'll try and switch to another catalog

author: Kleinstein
author_rank: Super Contributor
timestamp: May 25, 2021, 01:10:26 pm
content: For the virtual ground driver there are plenty of alternatives. A MCP6001 would be a relativly 1:1 replacement - make sure to get the same pinout, as there are different pinout versions in sot23-5. There are lower power ones too.The max4239 is a bit unique as beinning relatively fast for the low power and also very low offset. There are alternative zero drift OPs, but usually slower and also often higher bias and offset. So one has to compromise. Possible candidates are OPA335 and MCP6V76.

author: Moriambar
author_rank: Unknown
timestamp: May 25, 2021, 01:26:05 pm
content: Quote from: Kleinstein on May 25, 2021, 01:10:26 pmWe'll see, the max4239 will be available eventually, and the lmv I'll try your suggestions thanks

author: Moriambar
author_rank: Unknown
timestamp: May 31, 2021, 11:04:03 am
content: Ok I substituted the LMV and the two max ops (with much difficulty since there's no space around the maxs)That yielded a complete fail. Now, both open and short in any range give -320 mV.I don't know whether the situation is recoverable, so I think I'll place an order for a new microcurrent next week or thereabouts.Thanks for all your help.EDIT:actually I thought to measure it again, since it probably was the isopropanol that wasn't completely dry.Now the inputs read exactly 0 mV BUT on the ON setting and with a load attached too (an LED with current limiting resistor).

author: Moriambar
author_rank: Unknown
timestamp: May 31, 2021, 07:06:44 pm
content: So ultimately my soldering wasn't very good and I had to redo the op amps.Being in a cramped area, and with me having almost no skill whatsoever, it took me almost two hours of trying until everything was, ultimately, soldered back in place (yes, I'm talking to you sneaky R8 too).The results were worse than ever. The "short" offset varies a lot, always negative and depending on luck it is either around -90mV regardless of range or around -7mV. The same on ON. The problem is that passing around 5mA through the microcurrent nothing gets registered (but the led lights up).Since there's nothing more to do I'm declaring this attempt failed and I'll buy a new microcurrent.Thanks all

author: YetAnotherTechie
author_rank: Regular Contributor
timestamp: May 31, 2021, 08:55:42 pm
content: If you haven't destroyed the pcb, i might be interested in buying the unit.

author: Moriambar
author_rank: Unknown
timestamp: June 01, 2021, 06:12:12 am
content: Quote from: YetAnotherTechie on May 31, 2021, 08:55:42 pmGive me time to get the new one and compare it with what I have. Then I'll dm you, as long as you arrange the shipping (and pay for it) I won't take any extras for the board (with connectors).

author: Moriambar
author_rank: Unknown
timestamp: June 03, 2021, 07:01:33 pm
content: Hi, I had nothing better to do, so I went back to this and found out that the rail is not well split: the virtual ground, with a 3.3V supply, stands at +2.2V w.r.t. negative terminal of the battery.Can we perhaps salvage this still? Do you think that swapping out the lmv can do it or perhaps there is something else?Thanks

author: Moriambar
author_rank: Unknown
timestamp: June 04, 2021, 08:53:25 pm
content: Ok now I replaced the lmv, and the zero is ok, perfect… the problem is that there's no voltage difference from the negative battery terminal to ground (measured at the - output). The LED only comes on briefly but as soon as I connect something (not necessarily a current, just a multimeter probe connected to a dmm) it goes off…

author: Moriambar
author_rank: Unknown
timestamp: June 05, 2021, 03:11:15 pm
content: pin 2 and 5 of the lmv showed a short (!!!) swapped the lmv again, but to no avail: the led comes on, the gadget works but it shows the wrong value (eg: with a -3mA offset shows ~-1mA when ~6mA are applied). I guess this is failed definitely