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@cmgate.com
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|