Flyback diode for relay coil?

Need help with this circuit:
I put together a board to convert a constant 12VDC trigger into a pulse signal to turn on and off an old Classe amp with a RJ12 connection and the circuit worked as promised.

However, I'm wondering if it's necessary to have a flyback diode across the SPST relay coil to protect the electrolytic capacitor and the SPDT contacts.

If the answer is yes, what type of diode should be used? Is it okay to mount two diodes facing each other on the relay coil?

Screenshot 2023-07-31 at 18-03-26 Scheme-it Free Online Schematic and Diagramming Tool DigiKey.png
 
I think the intention is that the pulses on the 12V trigger are long enough that the 100µF capacitor has fully charged and there is no current in the SPST relay when the pulse ends. If that's not the case a free-wheel diode can be added.

Single diode where you've indicated with cathode to the capacitor end. Diodes for free-wheel use are not critical in any way, just make sure they can handle the 12V and the current of the relay coil.

It protects the SPDT NO contact from arcing and the coil from insulation breakdown, the capacitor doesn't see any stress as its a short circuit at short timescales and no appreciably voltage spike will appear across it.
 
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I think the intention is that the pulses on the 12V trigger are long enough that the 100µF capacitor has fully charged and there is no current in the SPST relay when the pulse ends.
Yes, the Classe amp uses a momentary push button switch in its front panel to turn it on and off.
When the constant 12VDC voltage ceases, the SPDT relay returns to its NC contacts, the capacitor discharges through the SPST relay coil, and provides an "off pulse" to the Classe amplifier.
Is there any disadvantage if I install two freewheeling diodes facing each other across the SPST relay coil to protect the SPDT NO contact from arcing?
 
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If you take two diodes facing each other it is the same as no diode, they will never conduct.
Use a single diode pointing to the highest voltage point, just like in the lower portion of the drawing you posted.

Edit: I think it is not necessary. If you follow the circuit, there's already one (a schottly?) in the circuit!

Jan
 
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I drew the circuit.
When a constant voltage of 12V is applied, the SPDT relay turns to the "NO" position, the capacitor is charged through the SPST relay coil, and the SPST relay contacts are closed for about 0.1 seconds until the capacitor is fully charged. This will provide a momentary pulse to turn on the Classe amplifier.
relayNO.png


When the constant 12VDC voltage ceases, the SPDT relay returns to its NC contacts, the capacitor discharges through the SPST relay coil, the SPST relay contacts closed again until the capacitor discharged to the relay must release voltage, and this will provides another pulse to turn off the Classe amplifier.
relayNC.png


Looks like I can't add a freewheel diode to the SPST relay coil!
Perhaps I could simply add a small value capacitor to the SPDT relay "NO" contact to keep it from arcing?
 
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Actually we are not talking about arcing on the SPST, right? You talk about arcing of the SPDT contacts?
That is something different then what you normally do with a diode across the coil to prevent damaging the drive circuit when you turn off the coil current. But that's not the issue here, correct?

You do not need a diode on the SPST coil as there is a current path for the coil current in both SPDT contact positions.
And the SPST coil current is zero when the SPST switches, when the C is charged or discharged.

Jan
 
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