EMRFD Message Archive 2684

Message Date From Subject
2684 2009-02-09 11:37:18 timshoppa Relay switching of band filters
I've made really good progress on my homebrew receiver.

It uses a six-pole switch on the front panel and DC wiring to switch
bands. For each band, a diode-switched VCO and a diode-switched
crystal oscillator is turned on in the LO synthesizer, and this way I
can select 3.5-3.6, 7.0-7.1, 10.1-10.2, 14.0-14.1, etc. bands.

Now I want to add switching to the front-end bandpass filters. I've
cobbled together some filters already on the bench and I can swap
them in and out by putting them inline with the coax cable. But I
want to switch these from the front panel too, and DC switching seems
very desirable.

One way is to use SPST relays at the input and output of each filter.
Only one set of relays is closed at a time. Seems straightforward.
But I'm worried about leakage through open contacts into the unused
filters and back out at the end. Would it be better to use a SPDT
relay at the input and output of each filter so that each filter is
grounded when not in use? Or is there something even more clever I
could do?

Tim N3QE
2685 2009-02-09 12:38:27 bobtbobbo Re: Relay switching of band filters
2686 2009-02-10 08:38:11 ha5rxz Re: Relay switching of band filters
Good results depend on the way you connect your relays (this applies
to the low pass filters as well). Connect the filter to the moving
contact on the relay and the normally closed contact to ground. Your
connection to the filter/relay goes to the normally open contact.

The reas
2687 2009-02-10 09:34:08 WB6TPU Re: Relay switching of band filters
2688 2009-02-10 11:26:19 timshoppa Re: Relay switching of band filters
2689 2009-02-10 11:43:19 WB6TPU Re: Relay switching of band filters
2690 2009-02-10 12:01:16 WB6TPU Re: Relay switching of band filters
2691 2009-02-10 12:31:32 k5nwa Re: Relay switching of band filters
2693 2009-02-12 16:27:57 jwharding3 Contacts of Relays
This would seem a very appropriate time for an expert to enlighten
the group as to the need for contact wetting and how to accomplish it
(how to keep the contact surfaces clean when switching practically
zero current)
A little lecture here would be very well received.
Best Wishes
John> >
> >
2695 2009-02-13 07:34:43 Alan Melia Re: Contacts of Relays
Hi, that is a good idea......I am not expert but as an ex telephone company
empolyee I certainly heard of work done in that quarter. I believe for the
application you mention, a few mAs was what was suggested to me (like 1 or 2
!) I suspect that the "driving voltage" may also be important to be able to
punch through the slowly forming oxide, though with these very thin films,
12v is probably quite adequate. I will probe a few old comrades to see if
any more definite information is available.

A apocryphal tale goes that the rules had to ban the use of the new super
"silicone" polishes in the mid 1960s on the polished wood-block floors of
Stowger exchanges, because it quickly formed an insulating layer on the
selector contacts. A return was made to good old Beeswax and the problem
went away.

I believe the wetting current was found to be beneficial even with sealed
contacts like reed relay units (which I used in data-logging switch

Alan G3NYK
2701 2009-02-14 13:29:29 livingj2 Re: Contacts of Relays