Re: Master Clock contact spark quenching
The problem with a capacitor across the contacts is that when the contacts first touch, the capacitor charging current is very high. This damages the contacts.
The problem with a resistor is that when the contacts open, full current is flowing through all the coils. The coils try to maintain this flow by raising the voltage (back EMF). This can easily reach 100's of volts. This voltage will arc across the open contacts. The resistor will only reduce the current.
The solution is to use an RC snubber circuit as described here
These reduce the initial capacitor charge current and hold the voltage across the contacts low enough for long enough to allow the contacts separate far enough to prevent an arc forming. When applied to a Synchronome with a 12V supply, these formula limit the initial charge current to "only" 24 Amps with a 1ohm resistor. I would increase the resistor 10x. This initial charge current is overstated because with a 12V supply, I have 54 ohms in circuit to limit the current to 220mA required by the slaves.
The other part of the approach is to reduce the back emf from each coil. This can be effectively done with diodes but that requires making sure each slave coil has the right polarity connected. Not difficult to do.
An alternative is to connect two zener diodes, back to back, in series across each coil. Each zener should be rated to at least twice the supply voltage. These devices were not available when Synchronomes were made.
Another alternative is to use a driver circuit to separate the contacts from the coil/slave circuit. This works except for the coil in the master clock and the coil for the in-case slave.
On 7.01.21 5:12 am, John Hubert wrote: