How to protect relay contact when use inductive load?

Question

Hi friend, I control a solenoid valve coil 24VDC 1.5amps like this circuit. When the relay switching on, I can see the arc at the contact point. But when switching off have no arc. Can I reduce or remove the arc to protect contact of relay?

Answer 1

To protect the relay contacts from the inductive kickback from the solenoid switching off, add a diode like this:

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Unfortunately, this will slow down the de-energizing of the solenoid.

To compromise between protecting the relay contacts and keeping the solenoid snappy, you can add an additional Zener diode like this.

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When the relay switching on, I can see the arc at the contact point.

Are you sure you are seeing the arc when the relay is switching on? That is somewhat unusual. Generally, the high voltage needed to generate an arc through the air is created by interrupting a circuit, and resulting inductive kickback.

Possible causes of arcing are contact bounce and/or already damaged contacts.

To reduce the effects of contact bounce, you could put an RC snubber circuit across the relay (or possibly a transient voltage suppressor diode). This will, unfortunately, also slow down the solenoid.

^{simulate this circuit}

Answer 2

Yes you could create a snubber circuit for it, but relays are specifically made to withstand thousands of switching operations and your current doesn't seem that large to warrant a snubber circuit.

A similar question is posed here. For inductive loads you would need a parallel RC or RCD snubber as the capacitor prevents rapid voltage spikes, which are what sustain the arc.

Additional resources on wikipedia and here as well.

Answer 3

A solid-state solution would eliminate all arcing: turn-off and turn-on. You can either drive the solenoid with a solid-state relay (some already have clamping/protection diodes already built into the module as part of the package). Or, you can achieve similar results with a cheap transistor, base resistor, and a discrete rectifier for the generated EMF.

The advantage of the SSR module is that it's a complete turn-key solution. You also get the added benefit of galvanic isolation in most modules. This solution is relatively expensive.

The transistor solution is cheap, but it's a DIY approach - basically you are making your own solenoid driver at this point. Adding your own isolation adds complexity if needed.

To answer your question, you are going to need to add impedance in series with the solenoid - some amount/combo of series inductance or series resistance to minimize the "pull-in arc". Or, switch the relay to a faster "quench the arc real fast" version or an oversized contactor so the arcing won't matter much.

Answer 4

I tried this circuit, it decrease the arc at the relay contact, sometime i dont see the arc. I put a 0.68uF capacitor parallel the solenoid, near the relay. enter link description here

^{simulate this circuit – Schematic created using CircuitLab}