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MMD > Archives > August 1996 > 1996.08.16 > 09Prev  Next

Re: Switches and Safety - Vendors
By John Grant

Hi Doug and AMD List Readers,

On Fri, 16 Aug 1996 07:18:29 +0000 Doug Helfman asked of John Grant:

> John,
> With all this discussion on the digest regarding old switches,
> I'm wondering if I should be worried about my Welte. I assume I
> still have the original switch, even though you replaced the motor.
> What do you think?
> Doug


I took the liberty of forwarding your message on to the AMD List as I believe my response will be of general relevance there as well.

As strongly implied by Craig Brougher, Terry Smythe, and perhaps others, the question of electrical switch longevity is not a particularly difficult one to answer. Switches usually fail mechanically simply because most were never designed or engineered for more than a few thousand cycles, particularly when they are asked to break the highly inductive loading of the typical fractional hp motors that are found in reproducers. The electrical arc which is generated under these conditions is HIGHLY destructive of the not-very-robust internal components typically found in these switches.

My recommendation to you, and others similarly situated, (i.e., whose switches are currently functional), would be the addition of a solid state relay (SSR) to take over the switching of the main motor current. The relatively small (typically a few milliamps) of current required to oper se devices will ensure that no further destructive arcing takes place at the contacts. Of course, the switch will STILL be wearing mechanically, but its ultimate failure should be delayed considerably. Solid state relays typically operate on an input control voltage anywhere between 3-32 VDC. A simple wall transformer with DC output will work for this application. (Some suggested sources for these components is included at the end of this message.)

Even if the original switch has failed and you are forced to engineer a replacement, it would still be advisable to have it operate an SSR since it will still last longer, and will allow you to use a lower-rated (hence smaller) switch in the limited mounting spaces available for them. Addition of these components can always be done in a sensitive and reversible way so as to protect (and respect) the historical design of the apparatus. You should make sure, of course, that the relay contacts are in series with the "hot" side of the line, not the neutral. This in turn necessitates a polarized plug on the line cord or (preferably) a plug with integral safety ground for reasons previously discussed here. SSR's with a rated capacity of 20-25 amps should be selected to allow for the start-up surge of the motors.

Catalogs from the following five companies are "must haves" for the confirmed gadgeteer. All have large selections of electrical switches of various designs, ratings, and mounting arrangements. They also have solid state relays and the wall transformer/DC power supplys to operate them. Most have a selection of obsolete, end-of-production run, surplus, and just plain hard-to-find stuff. The American Science & Surplus catalog also makes entertaining reading as most of the item descriptions have clever, humorous write-ups.

R & D Electronics
Electronic Surplus Inc.
1224 Prospect Ave.
Cleveland, OH 44115
(Can't find phone number right now, write for catalog or call
directory assistance in Cleveland)

All Electronics Corp.
14928 Oxnard Street
Van Nuys, CA 91411
(800) 826-5432

American Science and Surplus
3605 Howard Street
Skokie, IL 60076
(708) 982-0870

Herbach and Rademan Co.
18 Canal Street
Bristol, PA 19007
(215) 788-5583

Hosfelt Electronics, Inc.
2700 Sunset Blvd.
Steubenville, OH 43952
(800) 524-6464

Doug, I will get some current prices on recommended components and get back to you.

-- John Grant

(Message sent Sat 17 Aug 1996, 05:55:05 GMT, from time zone GMT-0700.)

Key Words in Subject:  Safety, Switches, Vendors

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