Hi,

I attach a fragment of a circuit that I found. Some stuff can't be well appreciated, but what I want to note is that there is an snubber network across the relay contact. As far as I know, snubbers are needed when there is inductive load. But that part is just feeding a rectifier bridge. So I think that's not needed. Is it? I could just remove the capacitor and let only the resistor, because it seems to be a pre charge circuit.

 

Keep in mind wiring and source Z can have an inductive component to it

Its a good practice.

Regards, Dana.

 

That network will reduce arcing across the relay contacts when the contacts open while carrying current.

 

In order for a snubber across relay contacts to be more than a space waster it must be able to "absorb" sufficient energy to reduce either the voltage across the contacts or the current in the arc as the contacts open.

If the contacts open during the portion of the waveform when the capacitor is charging from the input supply, the instantaneous voltage on the capacitor will be very nearly equal to the instantaneous input voltage, with any difference resulting from the forward voltage of the diodes in the bridge and circuit resistance. There is no excessive voltage to suppress, but the current can be very high. If the instantaneous AC input voltage is less than the instantaneous capacitor voltage, the current through the relay contacts is zero.

There is no possibility, except under gross failure conditions, for the energy stored in the capacitor to discharge through the opening relay contacts, whereas that is a near certainty with an inductive load. The snubber is unlikely to be able to shunt enough current during the capacitor charging part of the cycle to do any good for the contacts if they open during that time.

The snubber also forms a low-current path that will allow the main filter capacitor to slowly charge. If the load is a switch mode power supply with undervoltage management, as soon as the filter cap voltage rises to the turn-on threshold the switcher will start, then very quickly discharge the cap to the turn-off threshold, and the whole thing makes a very stressful relaxation oscillator.

I consider the snubber to be somewhere between useless and worse than useless.

 

In order for a snubber across relay contacts to be more than a space waster it must be able to "absorb" sufficient energy to reduce either the voltage across the contacts or the current in the arc as the contacts open.

If the contacts open during the portion of the waveform when the capacitor is charging from the input supply, the instantaneous voltage on the capacitor will be very nearly equal to the instantaneous input voltage, with any difference resulting from the forward voltage of the diodes in the bridge and circuit resistance. There is no excessive voltage to suppress, but the current can be very high. If the instantaneous AC input voltage is less than the instantaneous capacitor voltage, the current through the relay contacts is zero.

There is no possibility, except under gross failure conditions, for the energy stored in the capacitor to discharge through the opening relay contacts, whereas that is a near certainty with an inductive load. The snubber is unlikely to be able to shunt enough current during the capacitor charging part of the cycle to do any good for the contacts if they open during that time.

The snubber also forms a low-current path that will allow the main filter capacitor to slowly charge. If the load is a switch mode power supply with undervoltage management, as soon as the filter cap voltage rises to the turn-on threshold the switcher will start, then very quickly discharge the cap to the turn-off threshold, and the whole thing makes a very stressful relaxation oscillator.

I consider the snubber to be somewhere between useless and worse than useless.

So in few words you mean that the snubber in that circuit is useless?

 

Everything you wanted to know about snubbers.

http://rudys.typepad.com/files/snubber-e-book-complete.pdf

Looking around web I see both pro and con on snubbers, I did this because
ebp's prior post, having swallowed years ago "must use snubbers".....

Here is an interesting video on suppression, but no disclosure
as to how its done.

https://www.arcsuppressiontechnologies.com/arc-suppression-facts/nosparc-in-action/

Regards, Dana.

 

Everything you wanted to know about snubbers.

http://rudys.typepad.com/files/snubber-e-book-complete.pdf

Looking around web I see both pro and con on snubbers, I did this because
ebp's prior post, having swallowed years ago "must use snubbers".....

Here is an interesting video on suppression, but no disclosure
as to how its done.

https://www.arcsuppressiontechnologies.com/arc-suppression-facts/nosparc-in-action/

Regards, Dana.

Thanks for the documentation. Anyways, I just want to know as a fast insight if that snubber circuit across the relay is really needed. Some says yes, other says not. Still confused.