Topband: Relays in RX array
n2ic at arrl.net
Wed Mar 22 09:47:15 EST 2006
> Tom wrote:
>> I use regular 1-2 amp sealed DIP relays, and never once have had a
>> problem despite having dozens of relays in the signal path. This is
>> with about 20 years or so of use in some of the switches. I
>> recently just pulled a box I built in 1983 out of service, and the
>> relays were still fine.
>> My suggestion would be to buy a low current sealed relay and not
>> worry about it. If you start to have problems, then it's a simple
>> matter to add contact bias.
>> Where I think you will get in trouble is using higher current open
>> frame relays, even those in plastic enclosures or covers. They are
>> often problematic when operated without contact bias current.
>> 73 Tom
> Ford wrote:
> In a Rx array, I agree. Sealed switches are the way to go. But how
> much RF power do you dare run through a contact rated for 2amps at
> 30v or 60v? My assumption has always been that you needed to go to a
> relay that had at least 5-10 amp ratings to survive even modest QRO.
> Pete didn't indicate in his regular query as to whether this was a Rx
> array or some sort of Tx antenna. So rather than rambling on with
> guesses, maybe Pete can describe his application.
> When I was studying the App notes from various sources. My
> conclusion was that a flash over was needed to burn through the gunk
> that naturally forms on contacts exposed to air. The materials
> involved dictated the voltage needed, but the worst case was at least
> a 10.5v arc. The current required was typically no more than 100ma
> rated by the manufacturer, which usually stated it on a data sheet
> as: "Contact rating 0.10A to 30A" or whatever. Some styles and
> materials required considerably less whetting, even though the
> manufacturer may specify quite a bit more current to be conservative.
> If the relay contacts were designed to do a mechanical wiping action,
> as in meshing sideways during closure, then the benefits of running
> DC are minimized if not eliminated. On very low current and low
> voltage relays, this is very easy to design since the contactor arm
> is quite flexible--low current relays can still have a long life.
> On heavy gauge relays, the contactor arm is pretty ri gid and
> provides little, if any, flexing. Sliding contacts under load means
> a short life, so you rarely find wiping action on a relay designed
> for current handling.
Ford and Tom,
Can you provide some examples of "good" relays you have found, say, from
the Mouser or Digikey catalog ? Frankly, it's a little daunting to go
through 65 pages of relays, trying to surmise which ones might be good
for topband and then study the spec sheets.
More information about the Topband