Question for the electrical enginerds

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  • K_W

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    I added a push button to my car's trunk release motor circuit so that the trunk can be opened with the car still on (Fobs disabled when on).

    The trunk release motor is on a gear system with a return spring. Best I can tell (can't see), the designers used an 87/87a relay that operates the motor when triggered by the fob receiver, but is normally grounded to deal with the voltage generated when the motor winds back.

    I added a button and a capacitor / resistor combo that allows a relay to be tripped for a 1/2 second operating the motor for a similar amount of time to the factory wiring, and I isolated the two from each other with two diodes.

    They both work great, but I am concerned that since there is no longer a path to ground on both legs, when the motor is being wound back, that there may be a feed back issue that may damage something over time... should/can I add a diode or resistor to ground on the (+) leg of the motor... or am I over-thinking it?

    Diagram...
    FXjhf8p.png
     
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    1911ly

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    Motors aren't like relay coils. Once the motor stops the energy is dissipated by the current the motor is drawing. If you added a relay then add a diode across the relay coil. Cathode towards the plus battery voltage. You should do it at the relay.
     

    K_W

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    Motors aren't like relay coils. Once the motor stops the energy is dissipated by the current the motor is drawing. If you added a relay then add a diode across the relay coil. Cathode towards the plus battery voltage. You should do it at the relay.

    The relay I added was design for vehicles and has a resistor across the coil, is that enough?
     

    1911ly

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    The relay I added was design for vehicles and has a resistor across the coil, is that enough?

    Probably, but I am old school. I was taught to always put a diode across a DC relay coil. Just something I have always done.
     

    Mr Evilwrench

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    The diode across the relay coil is there in case you're driving the relay with a transistor, which would include an integrated circuit. The relay coil will generate a high negative voltage spike upon release, which can damage the transistor(s) in the drive circuit. If there's no other semiconductor in the drive circuit, there's probably nothing to damage. The resistor will dissipate the spike as well.

    A motor doesn't mind being disconnected. Just turn it off like a light bulb and it'll stop moving pretty quickly, especially under mechanical load. No ground path necessary. If you need it to stop NOW, you can arrange to short the motor, but that's rarely necessary.
     

    K_W

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    Hey man, go back removing people's gizzards and reattaching arms, or whatever nerdy doctor stuff you do.


    KW, you sure this a motor and not a solenoid? Do you have to reverse it to get it to lock again?

    Yes it's a motor, looks like a common hobby motor. It has a worm gear that turns a larger gear with a pin that pivots a lever which releases the catch on the latch. Then a spring winds the motor back to starting point.

    [video=youtube_share;Tjd22-xBwhQ]https://youtu.be/Tjd22-xBwhQ[/video]
     
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