Re: Master Clock contact spark quenching

Ian Richardson

A wee bit off-topic, but.......

It should perhaps be remembered that, as Malcom has said, a reverse current is induced in the coil by the collapsing magnetic field when the circuit is broken.  When using a diode to quench this (which it does very effectively) the effective short circuit delays the collapse of the magnetic field in the magnet.  With the electric clocks with which we are all familiar, this is not usually a problem.  However, in the pipe organ field (of which I wrote a couple of days ago) this can be a problem as when a key is released there can be an noticeable lag before the valve closes - one reason why purists insist on purely mechanical actions in organs.

Ian R

-----Original Message-----
From: Malcolm Rix <malcolmrix@...>
Sent: Wed, 6 Jan 2021 16:43
Subject: Re: [synchronomeelectricclock] Master Clock contact spark quenching

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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