Topband: Relay Engineering Answers (final)

[Ford Peterson]

I figured it out.  And can provide some good engineering info regarding relay selection.

Potter & Brumfield (Tyco) provides an excellent app note about relay selection.

The application dictates what contactor material to specify.  Silver, and silver alloys will "sulfidate" and collect dirt/dust over time.  Three methods can be used to keep a contactor clean.  1) Contact pressures must be great enough to to break the sulfidation film. Only the relay manufacturer can apply this method.  2) Controlled arcing can burn off the sulfidation. This is controlled in the application design.  And 3) contact overtravel can wipe away the residue.  It may be possible to modify some relays to provide overtravel (wiping).  But generally, either a contactor wipes or it doesn't.  Wiping action also shortens the contactor's life, and in the case of some soft materials (like gold), very short life expectancy.

Using method 2 requires a minimum arc voltage and a recommended current to sustain an arc.

Material      V       I
----------  ----  ------
Cadmium   10   0.50A
Copper      13   0.43A
Gold          15   0.38A
Nickel       14   0.50A
Palladium   15  0.50A
Silver,fine   12  0.40A
Tungsten    15  1.00A

I noted that my relays use a 'silver alloy.'  The table indicates 0.4A (4.8W) is needed form a decent connection in Silver (ouch!).  However, many of the Tyco relays with a "silver alloy" have a recommended minimum current of 100ma (1.2W still ouch!).  So a continuous idle current is going to require a heat sink--not an appealing solution.

Tyco recommends a method of "quenching the arc" when closing the contacts.  It's very simple.  Use a charged capacitor/series resistor to provide the needed voltage/current to burn clean the contactor while limiting the amperage to the contactor's maximum rating.  Once closed on a clean contact, the connection will remain viable with little or even no idle current.

An app note on "Relay Contact Life" can be found here (third from last):

http://relays.tycoelectronics.com/application.asp

There is excellent information regarding the use of various material selection for various applications, including small signal, high power, hot switched, applications.  Combining the need for small signal and power (high dynamic range applications) renders method 2 almost manditory for ANY material selected.  Some guidance in the form of capacitor design is provided on the third page.  In my application, which is very low speed, a 4.7uFd capacitor should do the trick.  

Thanks to all for the help.

Ford-N0FP
ford at cmgate.com