Electronic assistance request

Discussion in 'Electric Horology' started by Old newbie, Dec 7, 2019.

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  1. Old newbie

    Old newbie Registered User

    Nov 30, 2019
    Country Flag:
    As I build the drive circuit for the E. Howard Schoolhouse clock I have a question concerning the value of the capacitor needed to store the reverse current generated by the collapsing field of the two advance solenoids.

    It is a very simple circuit as the coils trigger at 1.5 VDC. This is supplied by a D cell. A NO reed switch detects the passing second hand of a highly accurate Quartz movement. A diode prevents any back EMF but should I utilize a Capacitor beyond the diode or am I over-engineering this??

  2. praezis

    praezis Registered User

    Feb 11, 2008
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    Region Flag:
    IMHO you do not need a capacitor.
    Even more simple and reliant than the diode is a resistor parallel to the coil. Its value: 10x the coil resistance.

  3. Dan_in_WA

    Dan_in_WA Registered User

    Dec 21, 2019
    Country Flag:
    If you add a capacitor to a circuit with a coil, you’ve made a tank circuit. The coil will discharge, charging the cap, but it doesn’t stop there.

    The cap discharges back into the coil at some point, and you get an oscillation that will take some number of cycles to dissipate.

    I’d stay with just the diode. For more information, try searching Flyback Diode if you’re interested.

    I’m an old radio guy, and came across this site searching for an answer to the clock portion of a 1950’s (?) alarm clock radio.

    This site is impressive, to say the least!

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