protecting electric clocks

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    My Pierce/Geo. Borg electric rewind clocks have a typical failure mode: the contacts to activate the solenoid coil that rewinds the clock every minute or so are burned and the solder joint to one of the coil leads melted and disconnected. I think this is a typical failure for electric wind automobile clocks. When the car gets parked and not used the battery loses charge until the voltage on the coil doesn’t have enough pull to rewind the clock. The contacts stay partly connected and just buzz, arc and spark and overheat the coil leads until the battery finally goes flat.

    I try to keep my cars driving once a month or disconnect the battery, but many times I have screwed up and let a battery go flat.

    It occurs to me that a really simple safeguard would be to interpose a zener diode in the battery lead to the clock. A zener blocks current flow unless its breakdown voltage is exceeded. Thus a zener with perhaps 5.5-6 volt breakdown voltage would stop current to the clock when the battery loses charge and drops below the voltage needed to reliably rewind the clock.

    I know automobile clocks never keep time, and maybe not worth the effort to keep running, but nice to have them ticking away anyway.



    I don’t want chords strung all over the garage to keep all the cars plugged in. I’d be tripping on the extension chords just getting around the garage. I’ve got enough stupid injuries. One car is always on the lift and easy to get messed up raising, lowering, and swapping. I do try to drive them often enough that it isn’t a problem, and batteries going flat isn’t that common a problem for me, I just screw up once every two years or so. The clocks are supposed to have their own fuse, probably for this reason, but the evidence seems to be they aren’t adequate protection for this problem. I don’t know at the moment how much voltage it takes to get the coil to open the contacts, but if one is out on tour with a weak battery that lets the thing start buzzing it could drain the battery in a hurry.

    The zener seems like a very simple fail-safe with or without a trickle charger. Its a little disconcerting from a fire standpoint to watch those breaker contacts arcing and sparking continuously and realizing the wires get hot enough to melt the solder. The solder joint melted on one of mine was a big glob, not a tiny spot on a 22 gauge wire. The good news on these Pierce clocks is that they are contained in a metal case – unlike the cheesy plastic of later years-so unlikely it would start a fire, but it does fry the contacts and wires.



    I think they make a timed delay 1-2 amp fuse that might work. I’m not familiar with the P-A elec clock but if it only needs contact for an instant to wind it the fuse would handle the current for a couple of second then if the points stuck the fuse would blow. Jim L


    Jim, that is a good idea, it might need 4 to 5 amp as I measured the resistance of the coil at 1.5 ohms which would put the steady state current at 4-5 amps. It occurred to me that the zener might have a problem letting the contacts buzz as the inrush current to the coil drops the voltage. I was also thinking about adding a capacitor+resistor suppressor to keep the contacts from arcing.


    Some notes on the Geo. Borg electric clocks, I have finished opening mine up, cleaning and lubricating. I made a new facia for the front clock and transplanted the rewind solenoid coil from the ’36 rear clock I obtained years ago. The ’36 rear also donated its beveled lens and facia to the rear ’35 clock. Both are running and keeping time.

    Some notes for anyone else interested. I experimented with zener diodes, zener diodes controlling a transistor, and the slow blow fuse to try and cut the power to the clock when the voltage drops before the contact points started buzzing. None of them worked well. It also makes sense that there is enough wear on these clocks even when cleaned and lubed that they shouldn’t be left running continuously for long periods and a switch to turn them off or a master disconnect for the whole electric system would be best when the car sits.

    It seems innate resistance of all of the protective electronic elements tried is too great. I measured the internal resistance of the rewind coil at 1.5 ohms, which means steady state the current would be about 4 amps. The internal resistance about 1 ohm more or less of the zener diode and MOSFET transistor was high enough to keep the rewind from working properly. The coil only snaps for less than a second, so a slow blow fuse of 1 amp did not blow despite the high instantaneous current. The resistance of a slow blow 500 milliamp fuse was too much and prevented the coil from getting enough current to rewind and separate the contact points, so it just let current pass continuously at least before I disconnected it.

    The original fuse was 5 amps and would only protect for an outright short. I am going to use a 1 amp slow blow in mine. It won’t keep the contacts from buzzing and burning but might prevent the coil wire ends and solder joints from melting.

    The rewind would work as low as 3 volts, started to buzz and get inconsistent at 2.75 volts, so the battery has to be pretty flat before it stops working and starts to buzz and burn the points.


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