Clock pulse arc eliminator capacitor

Hi all, this is my first post. I have an old '67 Mustang clock that is a mechanical movement that depends upon a spring for kinetic energy to run the clock. About every minute and a half, when the spring is almost "empty", a set of electrical points come together and energize an electromagnet coil which kicks the spring pivot out a ways and essentially "winds" the clock. That cycle repeats endlessly keeping the clock moving - as long as there is electricity present when the contact points meet. One problem that has plagued these clocks is that the contact points will sometimes arc and degrade to the point where they quit making sufficient contact. I was wondering if the arcing could be eliminated by putting a capacitor somewhere in the circuit? I took a multimeter and measured 2 amps if I were to energize the coil continuously (I had a hard time getting any reading with my digital multimeter over the brief instant that the coil actuates). The car is obviously 12 volts. I was wondering if that is enough info to calculate the size of the capacitor needed. Also would be interested to know the best place for the capacitor in the circuit. (Sorry, I don't know how to create a circuit diagram to insert)

by notny41
October 08, 2021

2 Answers

Answer by Foxx

Put a diode across the electromagnet coil. Almost any diode will do, say one of the 1N4000 series. Just be sure to get the polarity right

+2 votes
by Foxx
October 09, 2021

When current flows through an inductance there is energy stored in the magnetic field. If you interrupt the current the energy must be removed, in most cases, converted to heat and the inductor will develop whatever voltage is necessary to do this. If there is no current path available the voltage will rise until something breaks down and in this case an arc is produced which burns the contacts. The diode provides a path which does not require high voltage so no damage is done.

by Foxx
October 10, 2021

Thanks Foxx. I did a schematic diagram that looked almost identical to yours without the diode but the website wouldn't let me post it. thanks for adding that. I haven't tried the diode yet, but what I did do was to put a 3mF electrolytic capacitor in series between the voltage and the switch. That worked for two "clicks" and then it quit energizing the coil sufficiently. So then I added a resistor across both leads of the capacitor to drain it between "clicks". That seems to have done the trick. I tested this setup in the dark and without the cap/res I could see arcs and after putting the cap/resistor combo in, no arc and getting consistant actuations now. A diode would probably be more elegant and efficient. I may have to play around with that too. Thank you for your responses.

by notny41
October 11, 2021

The general industry practice when I made a living at it is to use the diode for dc coils and an RC snubber of say 0.5 mfd and 100 ohms in series across an ac coil. I strongly recommend suppressing transients at the source rather than letting them fly and trying to protect individual devices. That way you will get much fewer pesky strange and puzzling bugs caused by noise!!

by Foxx
October 12, 2021

Once I get the clock dialed in on its time adjustment, I will remove the cap/res combo and solder on a diode across the coil leads. I have a 1N4004. I'm assuming that would work. The silver stripe side goes towards the + side right?

by notny41
October 12, 2021

Sorry - I'm mistaken - my diode is 1N4007. Guessing that should work. I think it is good for 1000v Maybe a little overkill. LOL

by notny41
October 12, 2021

1N4007 is fine and yes the silver stripe is + .

I've run into a somewhat similar clock winder in the distant past but it used a weight which was lifted by the magnet.

by Foxx
October 13, 2021

Diode worked like a charm - no arc, much less space taken up - and it all fit inside the original housing. Thanks so much Foxx!

by notny41
October 13, 2021

Answer by notny41

Diode worked like a charm - no arc, much less space taken up - and it all fit inside the original housing. Thanks so much Foxx!

+1 vote
by notny41
October 13, 2021

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