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Triacs, optocouplers, snubbers and small motors.

Discussion in 'Sensors and Actuators' started by (*steve*), Apr 4, 2013.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
    2,832
    Jan 21, 2010
    I decided to make a small board to slow the motor in my laminator (See here).

    In my case, 0.25 seconds ON / 3 seconds OFF.

    A 555 drives a triac output optocoupler which turns on a triac, which turns on the motor. The triac is snubbed using a 22 ohm resistor and a 0.1uF capacitor (in series).

    Unfortunately for me, the motor draws so small a current, the snubber itself provides sufficient current to make the motor turn!

    Doh!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
    2,832
    Jan 21, 2010
    The motor is rated at <4W. It has a DC resistance of around 10k (I'm going to check that again) from memory, so the maximum current has to be under 40mA.

    It seems possible to use the optocoupler to drive the motor.

    It seems like a bad idea, but does it seem OK in this case?

    If so, what value resistor and capacitor seem reasonable for a snubber?
     
  3. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    I did not have time to look at the circuit, but I like the STM Snubberless Triacs: BTB24...

    EDIT: Oh, I didn't realize using BTB24 would be to shoot mosquitos with canons!
    Better part: ACS102-6T
     
    Last edited: Apr 4, 2013
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm using a BTA208X-600F, also well in excess of what is required, but it has an isolated tab which I prefer for this application (shoe-horning into an existing device)

    My main problem turned out to be an extremely faithful construction of a schematic with a mis-wiring! Fixing that makes it all work -- albeit without the snubber.

    I can't measure the inductance of the motor -- my inductance meter claims it's over-range, but I suspect that is caused by the 10.3k DC resistance.

    Due to that resistance, the Q is going to be *very* low. There's probably not a huge risk of significant ringing as long as I absorb any transient.
     
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