Re: Master Clock contact spark quenching

Darren Conway


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.


Darren Conway
Lower Hutt
New Zealand

On 7.01.21 5:12 am, John Hubert wrote:
Can anyone explain why we stick with the Synchronome resistor?

I can make a few guesses;
  1. In a Synchronome, the ‘ON period’ is very short - a few hundred milliseconds per minute, so the losses in a resistor (which only occur in the ON period) are similarly low.  In a car it is MUCH higher.
  2. In the pre WW1 days, resistors were easy.  Larger value capacitors were not.
  3. Many higher value capacitors were then both bulky and relatively expensive - and many (all electrolytic types) were polarity sensitive.
  4. The inductance across a car system is near enough fixed - whereas a clock system will vary from system to system (number of dials) - and so ‘one size fits all’ may not be optimised.
  5. The increase in power consumption for a Synchronome was approx 10%, which is not very significant.

Synchronome and G&J used a resistor across each coil (both master movement and slave dials) whereas Gents only fitted a (capacitor based) system in the master - not the slave dials.  Gents do however have very heavy duty contacts.

One manufacturer whom used an interesting ‘different’ approach was the original Princeps system.  A very light contact based on the pendulum rod ‘made’ the contact (there is no spark on the ‘make’) - but the current break was done by a much heavier duty ‘break’ contact - driven by an electromagnet giving a fast break.  The light contact set only opened after the current had already stopped - so never arcs.


On 6 Jan 2021, at 15:22, Andrew Nahum <andrew.nahum@...> wrote:

Can anyone explain why we stick with the Synchronome resistor?


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