Re: Master Clock contact spark quenching


Ian Richardson
 

Hi Darren,

You are quite correct, but there are also other ways to minimise the voltage across the contacts as they open.  One is to put a capacitor in series with the power supply such that it is the capacitor charging current which drives the solenoid, then blocks any further current flow.  This approach works well with clocks whose impulse is prolonged, eg Brillié, but may not work for the very short impulse of a Synchronome.  Another is to use make-before-break contacts arranged such that the coil is shunted before the supply contacts open.  This is used on GPO Type 36 clocks. A less familiar approach was used by Matheus Hipp whereby the make-before-break switch actually short circuits both the coils and the supply!  In that case, a resistor was put in series with the power supply to avoid the direct short.

I made full use of the capacitor blocking approach in my electrically reset gravity escapement clock (of which I have written before) wherein the gravity arms form the poles of a reversing switch, but the driving current comes from charging and discharging capacitors so arranged that the operating current is applied via the capacitors on making contact, but when the contact is broken there is no current flowing.

While on this subject, it is worth remembering that Strowger type telephone exchanges used hundreds if not thousands of relays and other electromagnetic switching devices and they nearly always used the R/C type of snubber across the contacts - so they must have reckoned it to be a good solution to the sparking issue!

Best regards,
Ian R
Auvergne,
France



-----Original Message-----
From: Darren Conway <darren.conway@...>
To: synchronome1@groups.io
Sent: Thu, 7 Jan 2021 20:52
Subject: Re: [synchronomeelectricclock] Master Clock contact spark quenching

Hi
There are two parts to this problem.
One is to reduce the back emf from the coil that generates the voltage to arc across the opening contacts.  A diode across the coil will achieve this.
Two is to maintain a near zero voltage across the contacts as they open, for long enough to open far enough to prevent an arc forming.  This is solved with an RC snubber across the contacts. 

Regards

Darren Conway
36 Orr Crescent
Lower Hutt
New Zealand
ph +64  (0)4 569 1963

On 7.01.21 11:33 pm, Ian Richardson via groups.io wrote:
In the pipe organ world, the use of diodes in direct electric actions is standard practice.  I have just fitted up a direct action unit extension organ with about 200 electromagnetic valves, each runs on 15 vdc. and carries a current of about 250mA.  Each coil is shunted with 1N400X diodes or equivalent. There is no sparking - imagine if there was, the organ woiuld break down every day as the coils are activated hundreds of times with each performance!  The diodes cost about €1 per 100.

The point about polarity is, of course, valid but simply solved by using a high current silicon diode in series with the power supply.  If connected the correct way round, all works well; if the wrong way round, it doesn't work at all, so it "fails safe".

Ian R
Auvergne,
France



-----Original Message-----
From: John Hubert <jfphubert@...>
To: synchronome1@groups.io
Sent: Thu, 7 Jan 2021 11:17
Subject: Re: [synchronomeelectricclock] Master Clock contact spark quenching

I do just as you suggest on all my dials and masters.  1N400X series diodes.  The maintaining of current is minimal (looked some years ago with an oscilloscope and it was insignificant).  There is typically some variation of impulse length dependant on the master anyway (Gents tend to be longer and early Synchronome clocks rather short - lighter parts?).  Typically, the coil carries about 300 mA and has a resistance of about 5 Ohms, with a parallel resistor of about 50 Ohms.

Diodes work well - but of course you have to have the right polarity.

John

On 7 Jan 2021, at 10:06, John Haine <john.haine@...> wrote:

Surely now we have cheap reliable silicon diodes the best approach is the now-standard practice of connecting a diode across the coil so that when the contact opens the diode shorts out the inductive kick?  The diode should be reverse biased when the coil is energised.  The only snag with this is that the diode maintains the current when the contact opens for a period which may cause a problem.  That can be made shorter by putting a resistor in series with the diode to dissipate the stored energy more quickly at the expense of a higher voltage.  For example, if the coil carried 1 amp when the contact was closed, and the resistor was 100 ohms, the peak backswing voltage would be 100 volts, quite a lot but probably not enough to cause breakdown in the gap.


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