Is a snubber or a diode needed to extend the life of the relay contact?
Yes, it is necessary on inductive loads. The high voltage that arises when an inductive load such as a contactor coil is interrupted produces an arc and burns the relay contact. Using a freewheeling diode connected in reverse parallel with the coil on DC coils, and an RC snubber on AC coils, suppresses the arc energy and noticeably extends contact life.
When the current of an inductive load is suddenly interrupted, a high voltage rise appears across the coil. This voltage produces an arc between the relay contacts; every arc transfers material from the contact surface and over time makes the contact stick or stop conducting.
Suitable suppression methods:
- DC coil: a freewheeling diode connected in reverse parallel with the coil. It is cheap and very effective; just watch the polarity. The diode lengthens the release time somewhat, and if fast release is required a diode plus zener combination is used.
- AC coil: an RC snubber in parallel with the coil. Typically a series resistor and capacitor, selected according to the coil values.
- Varistor: can be used on both the AC and the DC side to clip the peak voltage.
Connect the suppression element as close as possible to the load end, that is, near the contactor coil; the effectiveness falls as the cable length increases.
You must also choose the current rating down according to the load type: when using a contact for which only a resistive load rating is given on an inductive load, common practice in the industry is to apply a significant derating.
The full topic: Wireless remote control installation: pairing, relays, safety