Topband: Relays in RX array
ford at cmgate.com
Tue Mar 21 23:18:51 EST 2006
> 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
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 rigid 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.
My thinking is that running 0.1A at 12v is a mighty waste of current and heat (1.2W). My experiments showed me that even the clunkiest relay could work well with some current running through it. Low power equals easier parts procurement too. App notes and data sheets are great. Unfortunately, the local surplus store (and consequently my junque box) is often devoid of either luxury. I just used the bias T and an impedance bridge and started measuring RF contact resistance. Start with a beefy relay with good promise of being suitable for the QRO application. Increase the voltage / current until the contact was stable (i.e. low R). Add a pinch of current for good measure. If this is Rx only, go with sealed relays. They are cheap and tiny. Going with surplus relays requires care. Most surplus has been sitting in a box for years and is likely to be in the worst shape it will ever be in terms of contact gunk. If 10ma to 25ma won't cause it to conduct RF properly, then find a different relay.
ford at cmgate.com
More information about the Topband