Relay holding circuit, motor control

Discussion in 'General Electronics Chat' started by hspalm, Mar 23, 2012.

  1. hspalm

    hspalm Thread Starter Member

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    Hi,
    I'm need a motor control circuit that has these features:
    *3 inputs, forward, backward, stop. All inputs are one-shot 12v pulses.
    *With a pulse on either dir input, the motor must keep running until the stop signal is pulsed.

    I was thinking to do this with relays. It will be a battery powered application, and so I like that the relays have absolutely no standby current.

    Was hoping someone could take a quick look at my schematic and maybe point out to me if this will work? I don't have three DPDT relays at hand to prototype right now... My inputs are actually 5v digital pins, which will switch a transistor which will switch a relay. But I was thinking to deal with this later.

    The three relays are color coded and marked, so I hope you can see which coil belongs to what switches etc.

    [​IMG]
  2. #12

    #12 AAC Fanatic!

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    In general, that will work but, it has no safety lockout so forward and backward can never happen at the same time. The way this is wired, it won't let the smoke out, the motor will just stop, but you might be in an undefined condition as far as the controller is concerned. A pulse to "stop" will clear the fault.

    In case this helps,
    When using relays, you need at least 5 milliseconds of start pulse, and 10 milliseconds won't hurt. You can do that with a capacitor through a momentary pushbutton instead of making a timing circuit.
  3. hspalm

    hspalm Thread Starter Member

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    Yes, I've been careful not to wire it so a short can occur (so I think at least).

    The digital signals will come from an RF receiver, and will stay high for as long as the human is pushing the remote control button. I'm guessing they will never manage to press and release a button in less than 10ms? Most probably they will push the button long and hard as with a TV remote with low battery.
  4. #12

    #12 AAC Fanatic!

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    Here's a nice 5 volt relay if you can muster 28ma.
    That allows you to use a resistor to waste 7 volts of the 12 volts instead of using a transistor to change the 5V activation pulse to 12V capable.

    and yes, a human is not going to get off the button in less than 10 milliseconds.

    edit: Fwd and Backward has to fire 2 relay coils! That would probably require a transistor anyway. No help with this suggestion :(

    Attached Files:

    Last edited: Mar 23, 2012
  5. pilko

    pilko Active Member

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    If you apply a forward pulse and a backward pulse in any order, then both sides of the motor become + and the motor stops. If you then apply a stop pulse, and depending on the different speeds of deenergisation of the forward and backward relays, the motor will receive a restart pulse in a direction dependent of the relay dropout speeds.

    pilko
  6. hspalm

    hspalm Thread Starter Member

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    I just prototyped it, and it works brilliant, thanks guys! Found some relays in my junk box.

    Thanks, I'm looking for relays right now, so I accept suggestions like this!
    It's not yet given that the relay application will be 12v, only in the finished circuit it will have to switch a 12V motor. Why must the fwd/bckwd switch two relay coils?

    Good one! I tested it now, and the pulse doesn't seem to hit the motor very much, at least it's not visible.

    Found this one though:
    If I first start the motor in one direction, then hit the other direction button, both forward and backward are latched (as you said). The problem with this is that the motor now seems properly halted to the user, but in fact it's drawing 70mA over the relay coils "invisibly". I must think this one out.
  7. #12

    #12 AAC Fanatic!

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    I saw the stop switch as a coil. Wrong again!
  8. ifixit

    ifixit Well-Known Member

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    Here is a re-arrangement that locks out changing directions until the stop is pressed first. The control signals are, positive going, 10mS, 5 volt pulses.

    Needs review and testing.

    have fun,
    Ifixit

    Attached Files:

  9. #12

    #12 AAC Fanatic!

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    Looks good to me.
  10. hspalm

    hspalm Thread Starter Member

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    Brilliant, thanks! If I read this correct, it will also put my motor to stop when hitting the other direction button. Which is great, I like that feature.
  11. ifixit

    ifixit Well-Known Member

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    Pressing the other direction button, or even the same button again will not cause any change. You have to press the stop button first, than press a direction button.

    With the motor in the off state, pressing both direction buttons at the same time will cause unpredictable results, or maybe oscillations. I think it will be a race condition as to which direction would win... or maybe the motor would be left off. You'll have to test this. I would be interested to know what happens. This is a non-issue if you have a micro-controller generating the pulses

    Regards,
    Ifixit
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