Clock for a 52-53 Mercury

Post Number 15
How to stop arcing on contact points

Jan 25, 2023

Use a 1N4007 general purpose diode.

Note the diode has a band on one end. This is the cathode, the other end is the anode. A diode will conduct when the Anode is more positive than the Cathode by about 0.5 volts. And will pass a maximum of 1Amp.

The idea is to connect the diode across the inductive load (the coils of the relay) with the cathode connected to the positive supply of the relay's coil and the anode connected to the negative side. The diode will NOT CONDUCT normally, but when the coil of the relay is disconnected, then energy stored in the relay's coil will release in the opposite direction to which it was energized. This is called a CEMF voltage spike. CEMF stands for Counter Electro Motive Force. Electro Motive Force is just another name for voltage and Counter means opposite polarity.

This CEMF is proportional to how much current was flowing in the coil and is inversely proportional to how fast the current is removed. CEMF can be quite high and disruptive as you can imagine .

The purpose of the diode is to limit the CEMF spike to less than 1volt, thus avoiding the problem of a high voltage negative voltage spike damaging the activating component (transistor or relay or ??) with a negative voltage overload, and/or arcing)
 
rjlester said:
Found a discussion on the diode clock fix. Here is what the guy said for which diode to use: Use a 1N4007 general purpose diode.

Go here and look at post number 15 for more details: https://forum.allaboutcircuits.com/threads/how-to-stop-arcing-on-contact-points.191611/
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Thanks for this. I've been running a Borg clock from a '50 on a power supply for a few weeks now after servicing, and there is a tiny spark when the points make their momentary contact, causing some points discoloration. Maybe I'll give this a try.
 
Merc Cruzer said:
If I am not mistaken you live just South of Switzerland correct?:)
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Yes, famous land of clock and watch makers :D


I gave the clock to a friend who has a clock repair shop. He told me the loading spring, on which the electrical contact acts, has been repaired in the past. They made it "harder" and according to him it has difficulty working at 6 volts. Anyway I got another one on ebay (there are many and they are cheap). Let's see what comes...Stay tuned.
 
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