Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!hplabs!hp-pcd!hplsla!tomb
From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: additional info on switch problem
Message-ID: <5170043@hplsla.HP.COM>
Date: 30 Jun 89 16:18:33 GMT
References: <4100@tekig4.LEN.TEK.COM>
Organization: HP Lake Stevens, WA
Lines: 65

rusty@cadnetix.COM (Rusty Carruth) writes:
>In article <4100@tekig4.LEN.TEK.COM> jime@tekig3.LEN.TEK.COM (James W. Edwards) writes:
>>I have a problem for you electronics wizards.  I would like to control
>>the DC power being supplied to two loads as shown in the following diagram:
>>
>>  +--[Load 1]-------------[CP1]-------------+-------+
>>  |                                         |       |
>>  |                                         |       DC
>>  +--[Load 2]-------------[CP2]-------------+      POWER
>>  |                                                 |
>>  |                                                 |
>>  |                                                 |
>>  +-------------------------------------------------+
>>						     |
>...etc..., you will need this info to make sense out of my reply
>
>Quick question:  Why not just use a choke and a relay?  Like so:
>
>cp<x> ==  ____()()()()()()()____
>                                |
>                               \~
>                                \
>                                 \o-----------
>                         ______
>                               )
>                               )
>                               )
>                         ______)
>
>where ()()... is the choke, and the other mess is the relay, use
>NO contacts if you want relay fail to leave loads on.
>
>I await all the reasons I know will be forthcoming to explain why this
>won't work :-).

Happy to oblige!  (:-) :-) :-))  Actually, I have nothing against relays,
but wonder why you recommend a _choke_ in series with the contacts!
After all, an inductance tries very hard to maintain a constant current,
and will accomodate large voltage changes if you try to change the
current through it quickly.  So what happens when you open the
relay contacts?  You get an arc.  I learned this lesson the hard
way with relays, many years ago.  The relays were sealed surplus
jobs, and I couldn't see what was going on.  But I took one apart
and found vaporized contact re-plated all over the inside!  Admittedly,
I was controlling current through a rather large inductance, with
120V open-circuit, which is far worse than what you suggest, but still,
you would do well to put some arc-suppression across the contacts.
One way to do this is to put a diode across the inductor, in this case
(assuming the load itself is resistive).

>
>Oh, I suppose we WILL need to know if the load is purely resistive or not,
>won't we?
>
>(and finding the proper value of the choke is "left as an exercise for
>the reader" :-), translated, I'm too lazy right now to find the info
>I'd need to figure it out for you, and besides, I don't have all
>the info I'd need in order to figure it out anyway.... :-).
>
>Good luck.
>---------- 
>Rusty Carruth  UUCP:{uunet,boulder}!cadnetix!rusty  DOMAIN: rusty@cadnetix.com
>Daisy/Cadnetix Corp. (303) 444-8075\  5775 Flatiron Pkwy. \ Boulder, Co 80301
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>----------