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serial in-rush current limiter?

Discussion in 'Electronic Design' started by justin, Jul 10, 2004.

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  1. justin

    justin Guest

    I need to limit 7.5V DC motor current to approx 4 A during startup.
    I'm thinking of a single transistor with serial sense resistor, but not
    exclusively. Any scematic I can "borrow" ?


  2. Robert Baer

    Robert Baer Guest

    Use a passive part, called a thermistor; NTC (Negative Thermal
  3. Robert Baer wrote...
    A simple two-transistor circuit with a p-channel FET can do the job.

    | Q1 IRFZ34 (60V, 0.08 ohms,
    | R1 $1.15 from DigiKey, qty 10)
    | 0.15 3W
    | in o---+--/\/\--+--- S D ----o out 4A current limit
    | | | G 0.22v drop at 1A
    | | B |
    | '----- E C ---+---/\/\--- gnd
    | Q2 2n4403 R2 10k
    | pnp trannie

    This circuit limits the output current to about 4A, at the expense of
    a modest 0.65V drop near the 4A current limit. At normal operating
    currents, like 1A, the circuit delivers nearly the full 7.5V input,
    less 150mV for the sense resistor plus another 80mV for the FET.

    Normally the MOSFET won't need a heat sink (it dissipates under 0.1W
    at 1A), but if a 4A short-circuit current-limit condition can last
    for long (more than 0.35 sec, see the IRFZ34 datasheet's Effective
    Transient Thermal Impedance curves), the FET will need a hefty heat
    sink. The heat-sink size can be eased if you add two resistors to
    create a foldback current limit (see AoE page 316), although then a
    slower motor startup may be an issue.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  4. Winfield Hill wrote...
    I have corrected the p-channel FET's part number in my post above.
    The IRFZ34 is an n-channel FET; the IRF9Z34 is its p-channel mate.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  5. justin

    justin Guest

    Tried it. It is battery powered, to much of a voltage drop.
  6. justin

    justin Guest

    Thanks, that will work.
    In case I need to change I limit: Vgate is a function of V drop over
    R1 so when I need to "open" the FET I need .65 V over R1 (Vbe) to
    close Q2, correct?
    What is the purpose of R2, to pull the gate to the ground? If I leave
    R2 floating, could I use it as an off/on switch?

  7. Bob Wilson

    Bob Wilson Guest

    It is definitely not good practice to put a resistance in series with a DC
    motor. The result will be absolutely lousing starting under load. Worse is a
    thermistor, since its cold resistance will be too high to allow the motor to
    start properly, and the motor will slowly struggle to get up to speed.

    What is needed is a simple constant current source set to limit at 4A. Once the
    motor is up to speed, and its current draw drops below 4 A, the current source
    will effectively not even be there and the motor will run essentially as if it
    were directly connected to a voltage source.

  8. justin wrote...
    If a FET gate floats it's likely to float to a voltage that turns
    on the FET, so a pullup resistor has to be added to keep it off.

    | Q1 IRF9Z34 (60V, 0.08 ohms,
    | R1 $1.15 from DigiKey, qty 10)
    | 0.15 3W
    | in o---+--/\/\--+--- S D ----o out power switch with
    | | | G 4A current limit,
    | | B | R2 10k 0.22v drop at 1A
    | +----- E C ---+---/\/\--,
    | | Q2 2n4403 | on/off switch
    | | pnp trannie R3 10k | /
    | '----------------/\/\-----+---o/ o--- gnd

    If the on/off switch is replaced with a small transistor or FET
    (2n7000), the high-current power switch can be logic controlled.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  9. Ken Smith

    Ken Smith Guest

    This is exactly what a industrial "motor starter" does. On large motors
    not having the resistance in series when power is applied, results in
    disasterous failure perhaps involving the motor ripping its self apart and
    rolling across the floor in flames.

    A correctly sized thermistor would make a good motor starter circuit. It
    starts off with a higher resistance just when you need it and lowers its
    resistance once the motor is running.
  10. I read in that Ken Smith <>

    Ooh, goody! Can I watch? Beats exploding op-amps any day. (;-)
  11. Hal Murray

    Hal Murray Guest

    On large motors
    Back in the '50s, I went to a "technical" high school. The curriculum
    needed updating. We worked on motors and generators.

    The speed control on DC montors was the stator current.
    (I think, it's been a long time.) More current made a stronger
    magnetic field so it didn't have to go as fast to match the
    voltage on the rotor. The standard setup used a huge rheostat.

    One day, our lab exercise was a startup sequencer for a ~10 HP DC motor.
    It was big. Probably leftover from WW II. It was mounted on a
    platform with castors.

    We got it all wired up and pushed the "Start" button. Klunk for
    the first relay. Motor starts spinning. After a few seconds, Klunk
    for the second relay (shorts out some of the starting resistance).
    Motor goes faster. ...

    The last stage shorted out all of the startup resistance and also
    removed the short on the speed control rheostat.

    The problem was that we had missed a wire. The rheostat was open.
    That's the "full speed" mode. It was pretty exciting - amazing
    how fast a big chunk of iron can spin. We got the stop button
    before anything bad happened but there was no question that we
    had screwed up big. I still have a mental picture of that big
    motor slowly walking across the floor with everybody in the lab
    looking in our direction in terror. (inlcuding the instructor)
  12. N. Thornton

    N. Thornton Guest

  13. N. Thornton wrote...
    A large motor takes a while to slow to near zero speed, and
    inrush limiters cool off pretty rapidly. These properties
    help reduce the small window of serious malfunction.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  14. Bob Wilson

    Bob Wilson Guest

    I meant a perminently connected resistor. An example of this is that
    companies that make small cell phone vibrator motors specifically state that
    they do not recommned putting a resistor in series for speed control.

    I agree that some "effective" resistance can be a good thing with larger
    motors, particularly perminent magnet or shunt types, which had massive
    starting curent. But this resistance should be shorted out once the motor
    comes up to speed and the back EMF cuts the current to reasonable values.
    Having a significant value of resistance in series with these types of
    motors once they are up to speed, results is lousy speed regulation under
    Unfortunately, a thermistor is useless if the motor has to be stopped and
    restarted quickly. The thermistor will have no time to cool (and revert to a
    higher resistance) before the motor is restarted. Also, it can be hard to
    find a value that has a high enough resistance when starting, a low enough
    one under running conditions, and a fast enough thermal time constant that
    it reduces its resistance fast enough so it doesn't cause the motor to
    labour when starting.

  15. Bob Wilson

    Bob Wilson Guest

    Years ago we had a young engineer fresh out of school who had a habit of
    installing electrolytics in backwards. He earned the name "Blaster", which
    stuck for years.

  16. Ken Smith

    Ken Smith Guest

    It depends a lot on how the time constants match etc. If you turn the
    motor off and right back on, its no problem because it is still spinning
    near full speed. You have to wait for the motor to spin down before you
    switch on to create the surge. Somehow you have to keep the thermistor
    from cooling back off in that amount of time or you have to stop the motor
    more quickly than it naturally would.

    I really don't remember. I was just responding to someones bogus claims
    about a resistance being bad.
  17. Ken Smith

    Ken Smith Guest

    Or perhaps remove it all together. A DC motor is usually run up to speed
    at more than its rated continuous current. This means that the run up
    "time constant" will be shorter than the stopping "time
    constant". Although the curves are not exactly exp(X) shaped, I suspect
    that with some design time, it wouldn't be too hard to make the surge
    limiter cool fast enough.
  18. Ken Smith

    Ken Smith Guest

    If this is what you mean, then we agree. A resistor in series with a
    perminent magnet motor hardly reduces the running speed at all unless the
    resistor has a large value or the motor is loaded.

    Unless "lousy speed regulation" was part of the goal. At one time it was
    common to have printer imbedded in products to record what happened over
    time. The paper started on one roll and ended up printed and on the
    other. In the take up motor on a printer like this, a series resistor
    limits the stalled torque and the supply voltage limits the top
    speed. Typically the motor would be one intended for 12V operation and
    instead be run from 5 or 6V. This gave it very long life too.

    If the motor is stopped in less than its natural stopping time then yes
    this is a problem.
    They cool quite quickly though if you heat sink them so I don't see a huge
    problem here.

    What do you mean about labor?
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