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July 2001

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From:
Chris Robertson <[log in to unmask]>
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Date:
Fri, 27 Jul 2001 10:08:49 -0500
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David,
From personal experience, you are able to place relays
in parallel and they will divide the current equally.

One concern lies in relay failure. Each path should
have current limiting, incase one relay fails, the
others will have to carry the current, possible overpowering
them. If current limiting isn't possible in your application,
attempt fusing of the trace. Trouble shooting a fails
relay of that current is sometimes difficult, because in
many cases it wont be the coil failing, but the contact
carboning up. They will continue to show continuity, but
wont carry the current. This would result in the one
(or more) failed relay, not having a blown fuse.

Chris Robertson
[log in to unmask] <mailto:[log in to unmask]>

Author "Designer's Reference" for
"Printed Circuit Design" magazine
Author "PCB Designer's Reference"
to be released in Spring 2002








> Dear Designer Council,
>
> I'm working on a project where I need to switch a little over
> 60 A of current.
>
> I know I could just put lots of MOSFET transistors in
> parallel, and they will
> share the current. Just like light bulbs in parallel, if one
> is getting more
> than its share, it heats up, *increases* resistance, which
> redirects more
> corrent to the other items.
>
> It looks like mechanical relays are much cheaper, although
> they are only rated
> up to 40 A (something about UL rating ?).
> If I just put 2 relays in parallel to switch that 60 A, will
> they share the
> current properly (like MOSFETs) ?
>
> Is there any tech notes talking about putting relays in
> parallel ? I didn't find
> any at
>   http://siemens.com/
>   http://www.azettler.com/
>   http://www.pandbrelays.com/
> .
>
> I hear that if I put lots of (BJT) npn transistors in
> parallel, when one gets
> more than its share, it heats up, *decreases* in resistance,
> then the positive
> feedback (``thermal run-away'') makes that transistor blow
> up, soon followed by
> its neighbors. That doesn't happen with relays, does it ?
>
> --
> David Cary
>
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