# High frequency AC voltage pulse keeps latching relay

#### HighVoltage!

Joined Apr 28, 2014
152
Can I put a capacitor in parallel to the relay coil so that is shorts out this high frequency AC pulse I occasionally get which is latching relay? If so, without knowing "in air" transmitted frequency/voltage value, how do I size the capacitor without affecting circuitry?

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#### Alec_t

Joined Sep 17, 2013
12,320
Show us a schematic of the mystery circuitry and we may be able to say.

#### Kermit2

Joined Feb 5, 2010
4,162
A series inductance may work as well or better.

#### R!f@@

Joined Apr 2, 2009
9,751
@HighVoltage!
Provide the schematic so we may understand your problem

#### HighVoltage!

Joined Apr 28, 2014
152
@HighVoltage!
Provide the schematic so we may understand your problem
Its actually the same exact circuit. Thanks.

#### AlbertHall

Joined Jun 4, 2014
11,610
Can I put a capacitor in parallel to the relay coil so that is shorts out this high frequency AC pulse I occasionally get which is latching relay? If so, without knowing "in air" transmitted frequency/voltage value, how do I size the capacitor without affecting circuitry?
Do you have any evidence that it is a "high frequency AC pulse" which is latching the relay?

#### Kermit2

Joined Feb 5, 2010
4,162
Inductance is a very common method of blocking high freq.

Those "ferrite beads" at the end of comm cables are an inductance.

You asked for help and lots of us are trying to do just that. Lead a horse to water but....

#### AlbertHall

Joined Jun 4, 2014
11,610
Is the relay or push button subject to vibration?
Under vibration the contacts could touch and cause the relay to latch.

#### HighVoltage!

Joined Apr 28, 2014
152
Is the relay or push button subject to vibration?
Under vibration the contacts could touch and cause the relay to latch.
It is a push button.

There are very high voltages (kV range) all around.

#### HighVoltage!

Joined Apr 28, 2014
152
Occasionally, when something "shuts off quickly", I guess that "surge" causes the relay to close and latch.

#### HighVoltage!

Joined Apr 28, 2014
152
Inductance is a very common method of blocking high freq.

Those "ferrite beads" at the end of comm cables are an inductance.

You asked for help and lots of us are trying to do just that. Lead a horse to water but....
So put an inductor in series with the coil? What value should I use?

#### ebeowulf17

Joined Aug 12, 2014
3,282
What relay are you using? I'm not used to mechanical relays being sensitive enough to trigger off of stray noise the way you're describing. Makes me wonder if something else is going wrong (like Albert's vibration idea for example.)

Either way, the more details you provide, the better the odds you'll get meaningful help. How can someone pick an inductor for you without knowing what load needs to be powered through it and what kind of noise it's meant to be filtering? What's the environmental noise? You mentioned high voltage, but what are those voltages driving? Are the loads mostly resistive or mostly inductive? Are they being switched with mechanical relays? If so, are snubbers being employed?

I might be overshooting and asking too many questions here, but surely you can at least give us a part number or two!

#### strantor

Joined Oct 3, 2010
5,695
What relay are you using? I'm not used to mechanical relays being sensitive enough to trigger off of stray noise the way you're describing. Makes me wonder if something else is going wrong (like Albert's vibration idea for example.)
Yeah this sounds too crazy. I've never seen this happen and it doesn't sound plausible.
The "noise" would have to drive a lot (relative) of current, long enough for the slow (relative) relay to close and energize itself.
I think something else is causing the relay to latch.

I have seen something like this happen before when an operator's console (pushbuttons) was located a ways from a machine's control panel (relays) and the wires ran in a conduit under the floor. The conduit was full of water as it turns out, and a wire had a nick in it.