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current measurement

Discussion in 'Electronic Design' started by Aman Singh Bindra, Sep 22, 2004.

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  1. I have a circuit which has a 48V supply and a water conductivity cell
    having a resistance. The current in the circuit is of the range 0-2
    mA. I want to measure the current in this circuit accurately. The
    easiest solution is to measure the current across a resistor by adding
    a shunt in series with the 48V battery. But the resistor may change a
    little bit with time and temperature. Can somebody recommend a
    solution for measuring current at this high voltage of 48 V. I have
    gone through some off the shelf IC's but they are not for this voltage
    range. I need a reliable solution for the same. Can somebody help.
  2. Mike Monett

    Mike Monett Guest

    Normally, you would avoid the problem and place the sense circuit in the
    negative lead of the cell. But if you want to measure water conductivity
    accurately, you can't use DC.

    The trace cations and anions in the solution will migrate to their
    respective electrodes and form a Nernst diffusion layer. The readings
    will be highly erratic and useless for anaysis. Use an instrument
    designed for this, such as a Hanna

    The conductivity seems strange. At 48V and 2mA, it is about 42
    microSiemens. The actual concentration of ions depends on the wetted area
    of the elecrodes, which you didn't state. But this seems very high for
    pure water applications, and very low for normal water processing.

    What is your application? Are you doing electrolysis, and if so, what is
    the electrode material?

    Best Wishes,

    Mike Monett
  3. CFoley1064

    CFoley1064 Guest

    Subject: current measurement
    The high voltage is not developed across the resistor if it's a shunt. Your
    voltage will be proportional to the current across it (Ohm's Law, V = I * R).
    Almost all the 48V will be across your load circuit.

    The most precise way to measure DC current is with a wirewound shunt resistor.
    If you want to minimize self-heating, you just make the resistor a large
    wattage one. You're right in saying the resistance will change with
    self-heating and changes in ambient temperature, but the change really won't be
    that much (less than 1 mW power dissipation at 2mA), and it will be far less
    than other methods. If you use an Ohmite 45F100 100 ohm, 5 Watt, 1% resistor
    (Mouser 588-45F-100, $1.14 ea. in single quantities), you're guaranteed a 20ppm
    per degree C temperature coefficient. That's pretty good. You'll have 0 -
    200mV developed across the resistor, ready to go.

    If there's a potential for a short circuit, that could damage your resistor.
    Use a fast blow 1/8A fuse in series with the load.

    Good luck
  4. Tweetldee

    Tweetldee Guest

    The problem is not the voltage across the resistor, but the common-mode
    voltage seen by the opamps that handle the voltage drop across the shunt.
    At 48 volts, that's way out of the range of available opamps without
    resorting to more complex design.

    A previous post had a more sensible approach.. put the sense resistor in
    the low side of the loop. That has its own set of problems, but are more
    easily solved than sensing in the high side.
    Dave M
    MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
    the address)

    Never take a laxative and a sleeping pill at the same time!!
  5. CFoley1064

    CFoley1064 Guest

    Subject: Re: current measurement
    Hi, Dave. I thought that's what I meant (view in fixed font or M$ Notepad):

    | |
    | .------o-----.
    | | |
    --- 48VDC| Load |
    - | |
    | '------o-----'
    | |
    | ,-.
    | 3AG |||
    | 1/8A|||
    | '-'
    | | ___
    | o-|___|-- >
    | | To
    | .-. OpAmp
    | 100R| |
    | 1% | |
    | '-'
    | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    I guess I could have been a little more clear. Thanks.

  6. Something like this maybe.

    | D1 D2 D3 R1,270 |
    | | | 0.1% |
    \_|_ \ | |
    12V /_\ /R2,270,0.1% | |
    | \ | \
    | +------------+--------+ /1k
    | | | | | \
    | |s === | _ | |
    | +--| | |/-|--+ |
    | ptype||--/\/\--+--< | |
    | +--| |\+|-----+
    | |d | TL061,
    +-----|---------------+ c/w offset adjust pot.
    | |
    \ \
    3k3,1W / /8k2
    \ \
    | |
    | | |
    | | +--------+-----> Vout, 1V/mA
    \_|_ | | _ | |
    5v6 /_\ | +--|-\| |
    | | | >--+
    | +------|+/| |
    | | | |
    | \ | \
    | /R3,1k | /1k
    | \0.1% | \
    | | | |

    As discussed in a previous thread, the TL061/071 CMV
    includes the +ve supply rail. So it can be used to
    sense supply rail current and send a matching current
    down to a resistor sitting on 0v. The voltage across
    that resistor is buffered by a 1:1 single rail opamp.
  7. Tom Seim

    Tom Seim Guest

    You didn't say how accurately (makes a big difference). There are
    op-amps that handle a high common mode voltage (TI/Burr-Brown). If
    that isn't good enough you can have your circuitry common elevated to
    35-40V, convert the signal with an ADC and couple the output with an
    opto (serial ADCs are ideal for this).

  8. Jeff

    Jeff Guest

    How about a switched capacitor OP amp?
  9. Tony

    Tony Guest

    Is there a new switched capacitor opamp topology that extends input
    common mode range?

    Tony (remove the "_" to reply by email)
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