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