Re: Master Clock contact spark quenching

Malcolm Rix

When the switch breaks, current is still flowing in the magnet coil, as the magnetic field collapses in the iron core, reverse current is induced. Since the circuit is broken the voltage rises very high and a spark may jump the gap. When synchronomes were in service a resistor was used to drain the current and avoid the voltage reaching a level that would spark. The modern way is to use a silicon power diode (e.g. 1N4001) mounted in reverse across the switch. These were not available in the clock’s heyday, hence the resistor. A capacitor might delay the rise of the peak, but either a resistor or diode is really the way to go.


On Jan 6, 2021, at 23:23, Andrew Nahum <andrew.nahum@...> wrote:

I have often wondered why spark quenching can’t be done better with a capacitor as used across the breaker points in all older style automotive ignitions.  Can anyone explain why we stick with the Synchronome resistor?

On 5 Jan 2021, at 09:40, John Haine <john.haine@...> wrote:

As it's now 2021 you could consider using a solid state relay - silent, should last for ever, no inductive kick to make any arcs across the clock contacts.

RS sell 'em in 5s which is inconvenient unless you have a use for the other 4, or could share a pack.  There are probably other suppliers - in fact eBay:
have them at double the price in ones though.

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