I am working on a redesign of some circuit boards and am looking for a way to save space and potentially switch to surface mount parts. Some of the largest through hole parts in use are for an RC snubber across a relay contact like so


simulate this circuit – Schematic created using CircuitLab

The R2 is a resistive heater. There are some 8-30 foot wires between the switch and the load. The values of R1 and C1 were chosen before my time and persist because "don't fix what ain't broke". C1 is usually WIMA MKS4J031004C00KSSD and R1 is a standard 0.5W 5% resistor.

First, is an RC snubber appropriate for a circuit like this? Is there a better option? We have seen switches fail, with loads of much lower current and power than the 10 Ohm R2 shown, due to what appears to be carbon build up on the contacts. I would like to minimize these kinds of failures. I'm assuming that the carbon build up is caused by arcing of the contacts on open or close, but I'm not positive on this.

I would like to save space by changing R1 and C1 to equivalent surface mount parts. Is this possible or advisable given that C1 is rated for 400 VAC and 630 VDC?

Edit: Would, e.g., Kemet C2225C104KFRACAUTO be a suitable replacement for C1? It has a much higher DC voltage rating, but no AC rating. Are there other differences between film and ceramic capacitors that are relevant to this question that I should be aware of?

  • 1
    \$\begingroup\$ In the USA UL requires all line connected capacitors to have an 'X' rating or both a AC and DC voltage rating. SMD is not a good idea as their breakdown voltage is often just 200 volts. You would need several caps and resistors in series to meet arcing requirements and heat dissipation. A MOV is better because they do not leak enough current to be a shock hazard if the load is removed, but MOV's can be very large. If it works, don't fix it... \$\endgroup\$ – Sparky256 Feb 20 at 15:20

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