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Transistorised clock

Discussion in 'General Electronics Discussion' started by j101, Jan 7, 2010.

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  1. j101

    j101

    3
    0
    Nov 21, 2009
    I found this circuit to modify & repair my smiths car clock. My clock is 6 volt this circuit is for 12 volts could you please tell me how to modify the cicuit for my clock to work on 12 volts Now the transistor is connected temporarily to test its operation & fine tuning – I used some small pots in place of the resistors to confirm my calculations (remember I have limited skills in these fields – the main rule in my shed is to always have a bigger hammer at the ready!). Immediately the balance wheel increased its swing & speed, so to dampen it a bit I added a capacitor/resistor combination to the base side of the transistor, eventually settling on a 0.1µF capacitor & 20 kΩ resistor. My logic was to use similar size resistors so the current through the contacts is still minimal, but this resistor value may vary clock to clock because of Balance/hair spring, and capacitor/resistor tolerance variations. This particular clock likes the balance to oscillate about 540° – too little, it will stall easily, too much and the scroll ends hit the escapement teeth. I tried to get its consistency as close as possible before final assembly so the last bit of fine tuning could be done with its own regulating pins. End result was about +- 2 minutes a week.


    Diagram, components and finished circuit ready to fit. There is not a lot of room in the case, so I chose to put this lot on a small circuit board to fit in a small space off to the side of the coil, but I also built another one with the components layered & wrapped in shrink tubing so it could snake around the gear train under the coil.

    It’s not a bad idea to put an inline fuse in the power lead when refitting the clock back in the dash to protect all that hard work – 250mA is ample.

    Material List

    One 1N4002 (or higher) Rectifier Diode
    One BC 327 Transistor (PNP type as the base is switching to ground)
    One 22 kΩ Resistor (20 kΩ min – 33 kΩ max for the transistor base to switch properly)
    One 0.1 µF Capacitor (Tantalums are compact but also polarity sensitive & expensive @ +$1 ea)
    One 20 kΩ Resistor
     

    Attached Files:

  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    I'd simply put a resistor in series with the collector, having about the same resistance as the coil.
     
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