Re: Master Clock contact spark quenching

John Hubert

Interesting.  The Silent Electric (closely related to early GPO clocks) uses make before break and resistors (unknown, but very probably just like the GPO ones).  The were designed to work with Silent Electric’s own ’Silectock’ rotary armature mechanism at 250 mA and some systems used many dials.  No mention is made of relays in the contemporary literature (1919 catalogue) which states;

“The contact arrangement consists of two pairs of platinum points - one being for the line circuit for the dials and the other for the shunt or short circuit, according to whether the clocks are operated from battery or from electric light supply.  The motion given to the contact springs in conjunction with this method of cutting out the shunt, except for the instant prior to breaking the circuit, results in sparkless action”.

The terminals in the clocks are marked “Pendulum Battery”, “Dials Battery” and “Dials”.  The same contacts and markings are used in both the 1/2 second and 1 second pendulum versions of the clocks.


On 8 Jan 2021, at 14:12, Simon Taylor <smktaylor1@...> wrote:

The GPO used a 200 ohm resistor in series with a 1 micro farad capacitor across the contacts on later type 36 clocks with simple make contacts. The earlier types with make before break contacts had a 2000 ohm resistor. and worked with the earlier relay 90A which had 2 x 500 ohm coils in parallel.

All of the contacts are made of 10% iridium and 90% platinum. The hipp toggle contacts pass between 400-500mA for the pendulum drive coil. Although running a GPO slave cct direct it is quite possible, I would still recommend using either traditional relay (coil 2000-5000 ohms) or a solid state device or arduino system.
Simon GPO Clocks

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