SPDT Relays in Parallel

Question

Currently I use inexpensive relays and power supplies to switch in 12Vdc(~200mAdc) via a SPDT relay. Somewhere around 60,000 units we have to replace the relay because of bad contacts. I would like to wire two SPDT relays in parallel. I would like to close one relay, say K1 and 1000ms later close a second relay K2. After finishing with the module, open K2 and then wait 1000ms open K1. This way the contacts of K1 takes the burden of arching, while K2 has little or no arching. Is this an acceptable course of action to extend the life of the primary relay contacts?

Answer 1

A power supply typically has a large capacitive inrush which reduces mechanical life of some rated for 1m hr based on the contact arc energy.

Disconnects rarely have any arc , unless there is energy stored in line filters before the caps, so this method will not save the primary relay.

A PTC with a second relay to short out the PTC or resistor would reduce the current to a resistive load and thus increase MTBF to whatever that life expectancy is. These are more common method to protect the relay contacts that are under-rated for surge currents. A small current of 10% of rated is actually good as that going to increase reliability if they are not gold plated contacts by “wetting” (read , burning through) the thin oxide that forms after prolonged shelf life of periods of silver or other metal alloy oxide, dust, heat corrosion or extended periods not used.

Anecdotal

Back in the day, when I had a box full of 100x 15A and 25A relays with sense contacts in parallel on a remote link, I had new relay sense contact failures with a 2A rating used for TTL logic. When I ran a test to sequence all relays as fast as the sense contact was “0” closed, it would sound like a constipated machine gun. Tapping the box would make it continue until I put a 22uF cap to a pull up resistor to discharge the cap on closure, then it would self-heal and run smooth like a good machine gun rotating thru all the relays. (This was for 2 payload umbilical cables with power to each of the batteries and the experiments)

Answer 2

Whichever relay does the actual switching (usually opening is worse than closing) will wear out first, all other things being equal. So I don't see much advantage over a single relay.

With only 200mA at 12VDC I would expect a lifetime that is quite long from a modest relay, perhaps approaching \$10^6\$ operations, assuming either a resistive load or an inductive load with a catch diode.

One would normally consider a MOSFET for this kind of situation, but maybe you have some other constraints.

Answer 3

Protection of switch and relay contacts is generally done with an RC or RCD network. I would have a look at Chapter 7 of Noise Reduction Techniques in Electronic Systems by Henry W. Ott. It gives a very comprehensive overview of the various protection techniques and when they should be used. Specifically it mentions:

The rest of the chapter gives guidance on selection of the values for the RC or RCD components.