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uCurrent for the poor - an adapter for measuring small currents

zgierzman 52707 23
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  • uCurrent for the poor - an adapter for measuring small currents

    I came across a project once David Jones uCurrent .
    All in all, it is a very simple system. Voltage is deposited on the measuring resistor, which is then amplified x100 and fed to the output.
    Unfortunately, the item is not cheap, about 70 EUR excluding shipping costs.

    uCurrent for the poor - an adapter for measuring small currents

    I decided to make my version, slimmer, cheaper, but with similar (or even better in some respects) parameters.
    What did I not like about the original?
    - Power. One CR2032 battery and artificial zero. The circuit is complicated (like one opamp, but always), and the power supply of the output amplifier is only +/- 1.5 V.
    - The use of two expensive and hard to reach opamps in the amplifier. The cascade circuit improves the frequency response a little compared to one amplifier with higher gain, but is megahertz important, for example, when measuring the current consumed by the circuit in the sleep state?

    Changes made by me:
    - Two ranges instead of three. Each multimeter measures the milliamps, and for the oscilloscope you can use a resistor in series.
    - Easily available operational amplifier - you can buy it at TME.
    - Power supply from two 12 V batteries. This gives an output voltage range from 0 to 10,000 mV, so in the nA range I can measure currents from 0 to 10 uA, and in the uV range up to 10 mA. This is 10 times more than the original.
    - Place for a diode protecting the amplifier input. Just in case.
    - I also resigned from the battery voltage monitoring system.

    Thus, the layout was created as in the diagram. Simple, not to say primitive.

    uCurrent for the poor - an adapter for measuring small currents

    The value of the capacitor C1 has been transferred from the original schematic - if necessary, I will try to improve the compensation using a generator and an oscilloscope.

    A tile pattern was created behind the diagram. As the scheme is simple, the board came out in one layer without any problems, without combining with bridges etc. Easy to make at home.

    uCurrent for the poor - an adapter for measuring small currents

    And then the layout itself.

    uCurrent for the poor - an adapter for measuring small currents
    uCurrent for the poor - an adapter for measuring small currents

    0.1% resistors

    Time for measurements:

    The offset voltage is less than 1 mV. With the input shorted, the output voltage fluctuates in the range of 0.3 to 0.8 mV.

    The accuracy of indications is surprisingly high, better than 0.5%. But I assume it will be safe to assume 1%. Calculated using the technical method, because I have nothing to calibrate this device with. Yet the meters I use to measure voltage and resistance have their limitations.
    The voltage source was a 3V battery, a current limiting resistor in series and my adapter. The voltage is measured at the ends of this series circuit, and its total resistance, including the leads, is measured with a multimeter.

    For the uA range:
    At the entrance = 3.1630 V, R = 99 890 ?
    And the theory. = 31.6 uA
    And d. = 31.8 uA
    error = 0.42%

    For the nA range:
    At the entrance = 2.9434 V, R = 5134000 ?
    And the theory. = 573.3 nA
    And d. = 574.4 nA
    error = 0.19%

    Enough for me.

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    About Author
    zgierzman
    Level 28  
    Offline 
    zgierzman wrote 1280 posts with rating 819, helped 59 times. Live in city Zgierz. Been with us since 2005 year.
  • #2
    ptero
    Level 18  
    1. How with the stability with ordinary resistors?
    2. Are these metal plates short-circuiting the metal housing of the battery? - just scratch the paint.
  • #3
    TechEkspert
    Editor
    The original has a near-zero DC offset and drift through the use of autocorrelating zeroing techniques ", how can the effects be affected by using OPA192 instead of MAX4239?

    Maybe you will make a wider production available for purchase, e.g. on auction portals, or maybe it could be introduced to the elektroda.pl gadgets? ;)

    What do you estimate the cost for unit production or the final price for small lot production?
  • #4
    zgierzman
    Level 28  
    TechEkspert wrote:
    The original has a near-zero DC offset and drift through the use of autocorrelating zeroing techniques ", how can the effects be affected by using OPA192 instead of MAX4239?


    OPA192 landed there a bit by accident.

    Now I see that TME has MAX4239 in SO8 housing, but I didn't see it then (or it wasn't on offer). Besides, its power supply is +/- 2.75 V maximum, so I would have to stay with two 1.5 V batteries, or one 3 V with artificial zero. I was looking for something else, so I didn't focus on it.
    Then the OPA189 caught my eye, which has a similar offset to the MAX. But it is also missing from TME. There is OPA192, which statistically has a larger offset, but when I measured what I have, it satisfies me. Maybe I drew a good copy :-)


    uCurrent for the poor - an adapter for measuring small currents uCurrent for the poor - an adapter for measuring small currents uCurrent for the poor - an adapter for measuring small currents

    TechEkspert wrote:
    Maybe you will make a wider production available for purchase, e.g. on auction portals, or maybe it could be introduced to the elektroda.pl gadgets?


    I don't think I have time for production, but it could be added to the gadgets (if they were interested). I'd be happy to assist :-) Then you could bring in from a Mouser or another Farnell OPA189 to improve the parameters :-)

    TechEkspert wrote:
    What do you estimate the cost for unit production or the final price for small lot production?


    Hard to say. PCBs are a penny thing, in the Elecrow 10 x 10 cm is $ 5 for 10, so there would be 20 boards. They don't make a fuss if you prepare your own panel, unlike a JLCPCB where they'd tell you to split it up and pay twice :-) . Including shipping costs, it's less than $ 1 per tile.
    Battery plates are about PLN 5.
    Switches approx. PLN 6.
    OPA189 about PLN 8.
    0.1% resistors are not the cheapest, but they don't cost a fortune either. For the production "for people", you could also look for more precise resistors, 0.05%, 0.02% and even 0.01% are available :-D

    So with a few dozen pieces, it would be about PLN 25 without banana nests. Just like that. Sockets, mounting, housing, etc. on the buyer's side - such a putty.

    ptero wrote:
    1. How is the stability with ordinary resistors?
    2. Are these metal plates short-circuiting the metal housing of the battery? - just scratch the paint.


    I don't know what stability you mean, but I haven't done any research on either thermal stability or over time. :-)
    Gills at my place they do not short circuit, because the batteries have a foil sleeve on them :-)
  • #5
    And!
    Admin of Design group
    If you wanted to share the project with more people, the "kit" would actually be better than the assembled product,
    while banana connectors, as you write, are optional, what is important is PCB, acrylic, a set of components without batteries.
  • #6
    piterek-23
    Level 33  
    A great project and such gadgets I would like to see in the elektroda.pl store ;)

    Where did you get these battery holders? I'm looking for those, but with AAA batteries, and I can't find something.
  • #7
    zgierzman
    Level 28  
    piterek-23 wrote:
    Where did you get these battery holders? I'm looking for those, but with AAA batteries, and I can't find something.


    https://www.tme.eu/pl/details/keys82/baterie-pojemniki-i-uchwyty/keystone/82/

    This particular model fits AAA, N, and A23, which is exactly 12V. The only difference is the "length" spacing.

    uCurrent for the poor - an adapter for measuring small currents

    And! wrote:
    PCB, acrylic set of elements


    The problem with this acrylic is that I cut my tiles for the sockets I had in the drawer. That's why the red socket is different from the black one, because I didn't have the same. But the diameter of the nut may be different on other models, so the top PMMA plate would have to be designed differently - as more universal. Besides, I don't know how much it would cost to cut it - I carved mine myself. But you would have to add a few PLN to the PLN 25.
  • #8
    Błękitny
    Level 12  
    Congratulations on a successful project! I have a question because I am building something similar only on the LF356, namely what is the measurement bandwidth, could a colleague measure with a current generator, say a few nA at a frequency of e.g. 100kHz and 3MHz? I would be grateful for the results :)
  • #9
    Urgon
    Level 37  
    AVE ...

    A very nice alternative, although I personally will not find it useful. Plus, absurdly cheap. I always thought the original uCurrent was a little too expensive compared to its intended use. Autozero or choppers are cool, but not everyone will benefit from their capabilities.

    Regarding the amplifier error, you might have added some compensation. The 100 k? potentiometer, the multiturn, is best connected between V + and V-, and its brush through the 470k-1M resistor to the IN-pin of the amplifier. An even cooler option would be to add an input stage on JFETs with compensation. The cost is 4 NPN transistors, two JFETs, 6 resistors and one potentiometer.
  • #10
    Janusz_kk
    Level 29  
    Urgon wrote:
    An even cooler option would be to add an input stage on JFETs with compensation. The cost is 4 NPN transistors, two JFETs, 6 resistors and one potentiometer.

    But why? Secondly, it will not compensate for it as there are amplifier inputs, it is better to give an amplifier with processing.
  • #11
    zgierzman
    Level 28  
    Błękitny wrote:
    what is the measurement bandwidth


    Here you are.

    uCurrent for the poor - an adapter for measuring small currents

    Bandwidth varies greatly depending on the range. In the uA range it is only 73 kHz.
    The rise time of the impulse response is 4.8 us, so BW = 0.35 / trise = 73 kHz

    uCurrent for the poor - an adapter for measuring small currents

    For the nA range it is better, because the rise time is only 320 ns, which translates into about 1 MHz of bandwidth.

    uCurrent for the poor - an adapter for measuring small currents
    uCurrent for the poor - an adapter for measuring small currents

    As the band of the amplifier is 10 MHz and the gain is 100, I did not expect better results :-)
  • #12
    Urgon
    Level 37  
    AVE ...

    @Janusz_kk
    I was thinking about something like this:

    uCurrent for the poor - an adapter for measuring small currents

    Q1 and Q2 should be glued together, as well as Q3 and Q4. R1 and R9 should be of the best possible quality. I did not choose the Rs value. Control should be started from R12 at the positive input of the amplifier shorted to ground. Then R10 is regulated when both measuring inputs are shorted to ground. The circuit has a significant input impedance, so you need to take care of good cleaning after soldering and, optionally, painting it. Overall, this should work fairly decently.
  • #13
    Janusz_kk
    Level 29  
    Urgon wrote:
    I was thinking about something like this:

    I guessed :) except that this system does not exist here, this is how the oscilloscope inputs are made, which must be high ohmic, but here the gain from it is negligible, because we do not need such a high resistance, and the flow of the entire system will destroy the good parameters of the operational amplifier, all the more so, that there are amplifiers on fetal inputs on the public and you do not need to organize such guerrillas, moreover, look at e.g. AD8551, input current
  • #14
    zgierzman
    Level 28  
    Janusz_kk wrote:
    there are amplifiers on fetal inputs available to the public and you don't need to organize such guerrillas


    In the diagram @Urgon is the LF356 which has this feature: "JFET Input Operational Amplifier". I am also eager to find out why it is worth adding a handful of discrete elements to add a JFET buffer to the JFET input.

    This is from the LFx5x datasheet including LF155, LF156, LF256, LF257, LF355, LF356, LF357 amplifiers

    uCurrent for the poor - an adapter for measuring small currents
  • #15
    Urgon
    Level 37  
    AVE ...

    There is an error in the feedback circuit in the diagram - wrong resistor values and wrong connection layout.

    You're right, it doesn't make sense a bit to be honest. I used the schematic for a good op-amp simply. But I thought that by adding three relays, converting two potentiometers to digital and adding ADC, you can make a micro / nano / picoammeter with auto-zero offset and temperature drift.

    EDIT:
    One advantage of such a solution, as I showed, over the "naked" op-amp - the polarization current can go below 1pA (LF356 has 30 pA, OPA192 has 5 pA), so the noise would be lower. But you probably need to choose better transistors.
  • #16
    PiotrPitucha
    Level 33  
    Hello
    Today you can buy good amplifiers looking at temperature drifts etc.
    In the past, these inventions did not exist and you had to deal with them :)
    In the Czech Amaterkie Radio there was an excellent solution for a multimeter adapter for measuring very low voltages, with zero measurement flow.
    I will try to find this schematic someday, but the circuit roughly converted the DC voltage to the AC voltage using the CD4066, and then this voltage was amplified in the operational amplifier and went to the perfect rectifier.
    The circuit didn't have a bandwidth to the one presented here, but when it comes to sensitivity, it was probably better. I will try to find the layout in a pile of newspapers and place it for posterity :) .
  • #17
    Urgon
    Level 37  
    AVE ...

    IN one episode of EEVBlog a meter is shown which has a current range of 100 attoamps, or 0.0001 pA. JFET input stage, all spider-soldered on probably polyester supports.
  • #18
    Jawi_P
    Level 35  
    Very interesting. Interesting resistance at the input. But for such ranges it is probably not strange.
  • #19
    Urgon
    Level 37  
    AVE ...

    I will add one more curiosity from the 1960s: a circuit with an input impedance of more than 1 T?, an output impedance of 50 ? with a voltage gain of 0.98.
    uCurrent for the poor - an adapter for measuring small currents
    I found this in the article High Input-impedance Amplifier Circuits using bipolar transistors, integrated circuits and fets in ac amplifiers presenting low loading on the signal source .
  • #20
    TechEkspert
    Editor
    The value of the resistor on the gate of the BFX63 transistor makes it difficult to buy in a local electronics store (there probably still existed in those years).
    I understand that this is not a DC amplifier?
  • #21
    Urgon
    Level 37  
    AVE ...

    I found such a resistor on the ebaj for about four dollars, the cost of shipping is another 15. Apart from the given parameters, I do not know anything else about this degree, because it was only given as an example of a solution - I have to look for a second article from another magazine.
  • #22
    PPK
    Level 27  
    I had about a hundred such resistors (2 W). They were sitting in the rails of industrial ionizers (for paper - printing house). 5% tolerance.
  • #23
    zgierzman
    Level 28  
    Nice, and you still have them and you can share them with interested parties? If not, why this entry?

    You know, I had two good cars, but I gave them to one man in black. I would love to give you one, but it's too late.
  • #24
    PPK
    Level 27  
    What's the mockery for?
    I am writing because such strips are often visited by a guest on the Warsaw Stock Exchange. There are different strips with different resistor values. If you care, I'll look in my basement. I had 100 M from ten years ago. They've gone, but maybe I'll find something else. At 100%, I think I still have about 68 M. Besides, I also saw 100 M from surplus on Allegro. Not to mention online stores.