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Snubber circuit across relay contact.

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Xavier Pacheco Paulino

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Hello,

I attach a fragment of a circuit that I found. 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 really needed and can be removed, right? I would like to know your viewpoints.

If I leave the snubber, how is its behavior in that circuit? I mean, AC current won't go into rectifier until relay contact is closed. When the relay contact is open, the AC current charges the capacitor fastly and current can't go into rectifier, right?
 

Hi,

Generally the snubber reduces/avoids contact arcing.
Contact arcing is caused by overvoltage.

Overvoltage usually is generated by (series) inductor in combination with switched (OFF) currents.
The inductance may be in the source path (left side) or load path (right side).
There is no need for a dedicated inductor part (that you find on the schematic), it may also caused by lengthy wiring or similar situations.

Klaus
 

Can anyone help me understand how it works? I mean, for example, AC signal won't go to rectfier until relay contact is closed. When the relay contact is open, AC input charges snubber capacitor through resistor and can't go to rectifier, is that right? Or does it go to rectifier for a brief period of time until capacitor charges?
 

Neither right. You get a small continuous AC current through the capacitor according to Xc = 1/(jωC) when the contact is open. Some loads may have problems with the small current, e.g. a fluorescent or LED lamp can flicker.
 

When the contact is closed the capacitor holds zero voltage.
When opened, the capacitor's zero voltage holds contact
voltage down momentarily, and you hope that the contact
moves away faster than the voltage rises so that arcing
will not happen.

This comes down somewhat to capacitor and resistor
values along with the electromechanical realities of
the moving arm.

Old-timey relay mfrs used to publish "arc suppression
nomographs" to let you visually get the values based
on key variables (inductance, current, ???).
 

Advantage of this snubber
The voltage peak, which occurs at the instant the contacts open, will be safely short circuited by the RC network
Extending the life of relay

Disadvantage
Small amount of leakage current flow even the relay closed, it is hazardous

UL Never approve this circuit because of human safety concern
 
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    d123

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Hi,

Small amount of leakage current flow even the relay closed, it is hazardous
The current is limited to about 1mA. In case you touch it you may feel it, but it's not dangerous to life.

UL Never approve this circuit because of human safety concern
If all is enclosed and in a box so you can't touch the wires there is no problem for the UL.
If it is not in a box, so you can touch the live wires, then the snubber current is the smallest problem.

In this circuit it seems the relay is meant as a functional switch, not a safety switch.

Klaus
 

Nearly 10mA current will flow, 110V @ 10mA is more dangerous to life :lol:
 

Hi,

Nearly 10mA current will flow, 110V
You are correct. My mind calculation was wrong.

Klaus
 

Disadvantage
Small amount of leakage current flow even the relay closed, it is hazardous

UL Never approve this circuit because of human safety concern
That's off-topic for a single pole contactor, I think. The load is still connected to the mains, with open or closed contact. Capacitor must have nevertheless X2 rating.

Safety considerations won't allow the snubber for a double-pole disconnecting relays.
 

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