Switching low level DC current?

[DRWeig]

Hi Group:

I know that when switching low-level current (say, a 4-20 mA signal through relay contacts) that I need to go with gold-flashed contacts and even better a set of bifurcated gold-flashed contacts to avoid troubles due to oxide build-up.

My question is, "How low is low?" I've been taught over the years that 50 mA is the low limit, below which special contact consideration is needed but above which you can use plain old general-purpose silver or silver/cadmium contacts. However, I've heard tell that the limit should actually be higher, maybe even 150 mA.

Does anyone know of a standard, or a manufacturer's recommendation, or even have a strong opinion to share?

I have looked thru P&B, H&K, IDEC, and OMRON tech support and none of them seem to name a threshold.

Thanks loads for your help!

Old Dave

 

[cbarn24050]

Hi, you shouldn't have any problems switching 4-20mA signals, problems normaly occur switching low voltages.

 

[Lewish]

Old style mechanical telephone relays switched about 10 milliamps with no problems. Gold contacts, self wiping design. They lasted 20 years or more.

 

[nbucska]

A very good choice is mercury wetted Reed - switch or relay <nbucska@pcperipherals.com>

 

[tiki]

Hi DRWeig,
what about analog switches, if the loop voltage is not too high?
look at ADI and MAXIM. There will be no contact problem.
tiki

 

[DRWeig]

Thanks all,

This is a super forum!

The typical load in analog controls that use 4-20 mA signals is 500 ohms or less, and the open circuit voltage is almost always less than 30 VDC (usually 24 VDC).

From your input, I'm looking for an open circuit voltage level at which point to go with gold, mercury-wetted, or something solid state. The manufacturers have some data that indicates at what point a good clearing arc will always occur -- similar to an intrinsic safety energy table. Looks like about 150 mA at 24 VDC, or up to maybe 100 VDC at 20 mA.

If I come up with anything different, I'll be sure to post it...

Old Dave