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Accurate Variable Current Source

Discussion in 'General Electronics' started by Tom, Jul 24, 2004.

  1. Tom

    Tom Guest

    Hi all,

    Ive been trying to design an accurate variable current source for some
    time now and I am struggling with certain aspects.

    I have developed a current source using two op amps, a number of
    transistors which forms a variable current source.

    I am looking for the following spec:

    current from 0-3amps over a load range 0.1R - 10R

    I have a supply voltage of 40v so achieving this spec is possible but
    I am faced with a big problem with my current design, keeping the
    current source accurate to within 20ma.

    I will explain my circuit...

    I have a current sence resistor on the ground supply rail, at 0.1R.
    The voltage drop over this resistor is multiplied by the first op amp
    to a useable level.

    The output from this opamp goes to the negative input of the second
    opamp, the positive input is sourced from a voltage reference
    (variable resistor voltage divider) to set the required output
    current.

    This opamp output is then sent to the base of a transistor, which
    drains the base of the main drive transistor (when the current load
    gets too high).

    Esentially the design does work, it does limit the current output but
    at lower load levels the current increases.

    I think the problem is due to the gain of the op amps, and transistors
    but Im not sure how I can drive the op-amp in a manner that will cause
    accurate current limiting.

    This is of course a PSpice simulated circuit, as I dont want to try
    anything for real until I can get a decent simulation.

    I would appreciate any advice.

    Cheers,
    Tom
     
  2. Robert Baer

    Robert Baer Guest

    Be advised, that whatever design methodology is used, that the *output
    pass transistor(s)* MUST be able to handle (1) the maximum current (with
    some "headroom"), and (2) the *power* as determined by the voltage drop
    and the current.
    Three amps times 40 volts is 120 watts, way beyond the practical
    capabilities of any ONE pass transistor, even with an "infinite"
    heatsink (do *not* believe spec sheets in this area!!!).

    That said, i have found that FETs make for extremely good current
    sources; they have an almost perfect immunity to S-D voltage changes,
    from "pinchoff" to breakdown.
    Now, the thing to either compensate for, or make small relative to a
    reference, is the change of Vgs with temperature and current (since you
    did say "variable").
    Using an ordinary MOSFET, the Vgs at room temp is roughly 3.5V at
    100mA, so if a 30V reference from gate to ground is used, a 100mV change
    in Vgs due to temperature or current changes is less than 1%.
    The problems with this is that the source to ground resistor at 3 amps
    would dissipate about 78 watts.
    An alternate way is to use the FET as a series controller, and a
    current monitoring resistor (i make one end of the resistor to ground;
    reasons later).
    The voltage developed across the monitor resistor goes to one input of
    an op-amp, and an adjustable reference (from ground) goes to the other
    input. Alternately, a switchable monitoring resistor and a fixed
    reference could be used.
    At high currents, it is a good idea to use Kelvin connections.
    The output of the op-amp drives the FET gate (use a 100K series
    resistor to the gate and a double-backed zener for Vgs over voltage
    protection).
    As long as you do not push more than 500mA thru a given FET, it
    appears that one can reliably create currents from microamps to that
    limit (per FET).
     
  3. Anton

    Anton Guest

    Hi!

    Why not use a LM 338? which as far as I can see can do what you want.

    / Anton
     
  4. SolarWind

    SolarWind Guest


    Hi Tom,

    Your ckt didn't conside the high-low limit conditions, my suggestion
    is:

    1. At low load condition, the sensing resistor 0.1 ohm is too low to
    pick up voltage=IL(load current)x0.1 ohm. Increase 0.1ohm to 10 at low
    load condition or increasing the 1st voltage follow OP-Amp gain.

    2. From your design, at low load level, the transistor that was
    connected in serial to your load will fall into non-linear operation
    range, that means, the transistor(TR1) that connected to the 2nd
    Op-Amp output can't control another transistor(TR2)(this transistor
    it's collect in serial to your load)collector current, the problem
    come from the TR1 in full ON condition didn't drain all base current
    of TR2.
    The solution is, modify your TR2 Base circuitry, connect R1(1 KOhm) in
    serial to R2(1 KOhm) and connect R2 to TR2 Base, connect R1 to +5V
    source, the join point of R1 and R2 connected to your TR1 Collect.
    This change will cut off whole base current of TR2 at low load
    condition.

    Trim R1,R2 value, check the TR1 collect voltage at low load condition
    if some thing goes wrong.

    Good Luck!
     
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