Designing a usb/battery electronic load. Need help with choice of opamp.

Discussion in 'The Projects Forum' started by dumle29, Jan 3, 2013.

  1. dumle29

    Thread Starter Member

    Jun 26, 2011
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    0
    Hello. As the title says, i'm trying to make an electronical load. It's not gonna be anything big and expensive, rather something cheap, and portable. depending on the mosfets, and the cooling options I choose to go with. I'm thinking 30-60W?

    Now, since I have never touched anything like this (current sensing - opamps) this is going to be me learning how to use op-amps, to measure low currents.

    This is what I'm after:

    • Very low burden voltage
    • 1µA resulution (maybe on four scales µA, nA, mA, A manually selected)
    • make use of a 3,3v supply source

    Now, what I have been looking at is Dave Jones' µcurrent

    It makes use of the MAX4239ASA+ chip. I could go and copy his schematic, it is open hardware after all, but what would I learn from that?

    Now, before looking at the µcurrent, I had a look at the OP177

    That also looks like it would do the job. Now to make a choice between those two?

    The OP177 is a tad bit more expensive, but it is also available in a lot greater quantity.


    Also a general question. To make this work as inteded, I would want the op-amp to output a voltage, that would divide nicely in 1.1v (where 1.1v would be full scale on my adc(ATMega328p)). Say I used the select-able 4 scales like on the µcurrent, i'd have the four scales like this:
    • 1.1mV/µA
    • 1.1mV/nA
    • 1.1mv/mA
    • 110mV/A (I'd probably use a normal shunt here)

    Any articles I could read up on, to learn how to configure an op-amp as such, or is it simple enough for you to explain if ya want? :p

    Then there is the question of powering the op-amp. As I've understood, an op-amp needs a negative and positive supply? In the µcurrent, Dave uses a LMV321AS5X I suppose I could do the same, but I would have to read up on what its doing, to fully understand, and so that I'm able to build one myself in the future.

    I know these are a lot of questions at once, so let me compress them a bit :p

    • What traps should I be aware off?
    • Would the OP177 works for my purpose, or would I be better off going with the MAX4239ASA+?
    • Am I babbleing, and if I am where?
    • Thank you :)
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,153
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    Not too badly, but I think it would help if you explain what you want your "load" to accomplish. Why are you doing this?
     
  3. dumle29

    Thread Starter Member

    Jun 26, 2011
    45
    0
    It's meant as a piece of test equipment. You can pick from I think 2 or 3 modes:

    • constant current
    • constant resistance
    • constant power (W)

    It's meant for testing power supplies, I'm also baseing it on an ATMega328p to make it super hackable, as I will be loading Arduino firmware onto it. There will also be solderpoints, for all the relevant pins of the arduino, to make it extra hackable.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,059
    All DC, right?

    And when you say "constant", do you mean averaged over some time interval?

    I'm thinking about ripple, for instance, or any other noise riding along on top of the DC. If you want constant resistance, then the voltage and current will have to have noise to mirror the input, just as they would for a resistor. That's different than a device which averages 100 ohms when the voltage averages 5V and the current averages 50mA, with all averages over a time of, say, 0.5 second.
     
  5. dumle29

    Thread Starter Member

    Jun 26, 2011
    45
    0
    When I say constant resistance, I mean that the mosfets will act as a resistor, thay means you dial in the resistance of your choice, say 0.56 ohms, and the load sets itself to that. say it was an average, I think that would work fine, when testing stuff? question is, how low should that average be?
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Constant resistance means you have a circuit that regulates both constant current and constant voltage.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Look at the specifications for this B&K Electronic Load

    Then post what features you wouldn't want from it. That is a 150W variable load with good precision for the price.
     
  8. dumle29

    Thread Starter Member

    Jun 26, 2011
    45
    0
    Well I've thought about it again and I think for a load, 1mA resolution would be more than enough. So I could do away with the µcurrent measuring.

    I'd go for 1mA to 10A range, and a 0-40V range. If constant resistance is too difficult to do, I guess i'll scrap that.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    For that amount of power, I'd suggest buying a digital load system. You would probably end up spending less money than you are looking at building a DIY precision 400W electronic load. That is a LOT of power to dissipate. Basic soldering irons run on about 20-50 Watts, if that gives you an idea of the amount of heat generated, let alone 10 times that amount of power. :eek:

    I'd suggest limiting the upper end of current sink to 1 or 2 Amps, and 20V to start with. Then the max dissipation could be done at a lower cost with a few CPU type heatsinks with fans on MOSFETs for dissipating 40 Watts of power. There isn't much else you can do with a load besides converting it into heat.

    Incandescent light bulbs are also a common DIY load, but the control MOSFETs will dissipate an equal amount, thus the giant heatsink and fan needs.
     
  10. dumle29

    Thread Starter Member

    Jun 26, 2011
    45
    0
    Ah yeah sorry, I was shooting for a 60W load, that would accept those voltages and amps. I don't know if that is the right way to spec it tho.

    So at 40v, it would be able to pull 1,5A while at 6v or lower, it could pull the full 10A. It is supposed to be portable, so a 400W load would be insane :p

    Anyways, I'm heading off to bed, as it is way past the time I should have done so :p

    A little question. What do you guys do with PCB's? Previously I've made my own, and with some pretty good precision, but I want to make this one SMD and compact, so I will have to have the board manufactured. What I've found so far, is an insane 300 eur for 2 prototype boards. Is this really the price-class?
     
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