Switch Contacts at low voltages

My general rule of thumb for gold contacts is based on

1. Current - under 1mA and I start to consider them.

1. Signal speed. Once I get sub 10nsec edge rates, gold might be the way to go. At < 100 psec edge rates, it's the _only_ way to go [YMMVG] ;)

2. Low level analog signalling. I virtually always use gold plated contacts in these situations.

1. The environmental situation, but even then the signal would have to be somewhat critical.

For low speed, 12V applications at 10mA I wouldn't bother unless you have some other analog signal riding on it. Keep in mind that gold will still get some deposits (used to clean gold fingers with an eraser years ago), so they're not a panacea.

Cheers

PeteS

if it is a slow mechaical switch I would add 0.1uF across each switch. This will give a small surge of current each time the switch is closed that will keep it clean and also filter noise and bounce. Mark

First, thank you to both of you for your responses.

Yes, it would be a slow mechanical switch. I had heard from talking to a few people that they thought that 12V is generally fine as far as breaking through any contact oxide, but I want to be sure.

I already had placed a small capacitor on the circuit for bounce. It would appear, though, that it would also have a positive unintended side effect. I was considering a larger cap with the thought of providing a bleed-off path for any induced voltages as I can't control how Joe Electrician will run the wires in the installation, along the lines of a 4.7uF ceramic (non polarized with low ESR). Based on the capacitor equation, Ic = C*dV/dt, the current could be fairly decent in magnitude, although it would be fairly short in duration. I also found some schematics where the designer placed a relay in parallel with the switch so that when it closes extra current will flow through the contacts that doesn't have to flow through the opto's LED.

Along with the capacitor (as this is a slow signal anyway), I would put a small (< 100 ohm) resistor in series with the cap. That'll still give you the filtering you need but it will significantly reduce the initial surge.

Ceramics are great, but a 4.7uF ceramic driven from 12V will suck over

20A on the initial surge (I know, I've tested it) and I'm not sure you want to blow fuses or otherwise cause distress to the source ;)

Cheers

PeteS

Leach International has a nice articel on this topic @

Regards, Jon

Thank you to you both!

Pete, you have a very good point about the surge currents that would skyrocket and a limiting resistor would be prudent.

Jon, yes that article does explain a lot about why a contact rated for one load level is not suitable for another. Of course, in my application I need to handle the worst case scenario, that being I need low level of steady state current for the opto, with a high surge of current to clean a silver contact switch.

Based on some simulations I ran this morning, yes a small capacitor with a current limiting resistor will supply a surge of current through the contacts while providing a low level load through the opto.

I would put about 5 to 15 ohms in series with that cap. I have seen bare cap across the switch contacts cause erosion and microwelding of the contacts. It was 22 uF tantalum at 28 V(dc), but the excess wear showed up by 10,000 cycles.

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 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
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