This is a bit of a specific request. I have a classic car with a faulty electromechanical clock, which I want to repair. The clock has a solenoid which is used to wind the mechanism. The clock then runs for about two minutes, at which point a pair of contacts close, triggering the solenoid which in turn winds the clock back up and opens the contacts.
This approach has flaws that lead to reliability problems:
- The trigger contacts are closed very slowly, being connected to the clock winder. This leads to a very slow touchdown and I've discovered that some current can flow before the contacts are fully closed, enough to partially trigger the solenoid but not enough to wind the clock properly and reopen the contacts. This leads to an unpleasant buzzing noise.
- If the car's battery gets run down (very common occurrence on a car that is used infrequently), the contacts can close but there won't be enough energy to drive the solenoid to re-open them. Current then flows through the coil and the whole thing heats up. This can cause catastrophic damage to the solenoid or minute hand gear, which is plastic.
The voltage divider is there to provide a low voltage to drive the LED inside the relay. I probably have the LED itself upside down, please ignore.
The relay's datasheet is shown in the pictures of this eBay listing https://www.ebay.co.uk/itm/302831495739.
One thing not shown is a 100 ohm resistor across the solenoid coil which I suppose is acting as a current limiting device.
So the question is, will this work? Or would a different circuit that gives a timed pulse to guarantee the solenoid is energised for long enough to throw the contacts open be better? I would just clean the contacts but I did that when I had the clock apart for its initial repair.
To address the second point, the attached document outlines a control circuit that protects the clock in the event of shorts or other problems. It says the 16uF capacitor has enough energy to run the clock but I think it can actuate the solenoid only once per charge. Perhaps it is acting as a reservoir that is capable of charging when the battery is low, then releasing the charge as a slug of current that does actuate the solenoid..?
The other thing is the spark arrester circuit. Is it worth fitting this instead of, or as well as the relay? Space is very tight inside the clock but I think I could get both in there..