>Hi to all.  Too busy to read the list much lately, but good to see you
>are all working hard ;-)
>
>My Question:
>Quenching diodes for relays.  I've put 4 relays on a PCB, and currently
>use 1n4004 diodes to quench the reverse emf when the relay is shut down.
>I can't even find any advice in H&H's "Art of Electronics".
>
>The relays draw 100mA at 12V.  Can I use a 1n914?
>
>Any pointers to heuristics, books etc appreciated.
>
>regards
>
>
>Gary Gaskell                         Cooperative Research Centre for
>Research Scientist                   Distributed Systems Technology
>DSTC                                 Ph:   61 7 3864 1051
>Level 12, ITE Building               Fax:  61 7 3864 1282
>Queensland University of Technology  Email:  gaskell@dstc.edu.au
>Brisbane, Australia.                 Ph (A/H) (07) 3857 7912
>_________________________________________________________________
>
>Hi,
       Relays can be considered approximately as inductors. When the
'driver' stops conducting then, by lenz's law, the current trys to keep
flowing and the voltage across the inductor increases - E = L di/dt etc. A
quenching diode provides a path for the current. To a first approximation,
until the armature begins to open and the magnetic path changes, the current
will decay exponentially from it's orignal value annd the energy in the
magnetic field will be dissipated as heat in the coil and the diode. Hence,
to size the diode, ensure that the current carrying capacity is greater than
the standing current immediately before the drive is removed and that the
power dissipation of the diode is adequate. Usually, the power dissipated in
the diode is not a problem as most of the energy goes as heat in the coil.
        The diodes do have another side effect. As they keep the current
flowing they slow (or slug) the release of the relay.
         There are other more complicated networks using capacitors and
resistors that modify the response.

        A reference is:-

                "Switchmode Power Supply Handbook"
                Keith Billings
                McGraw-Hill 1989
                Chapter 18 - Snubber Networks, pp 1.134-143

        also check power engineering text books and manuals rather than
electronic stuff.

        Give my regards to Oz - it's been a long time since I lived there.

Gordon


Gordon A. Moyle                     E-mail: gordonm@teleport.com
Sunset Micro Systems                WWW: http://www.teleport.com/~gordon
1255 SW Huntington Ave              Phone: (503)-644-0358
Portland, Oregon  97225             Fax:   (503)-644-0358
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        To Err is Human, to really foul things up requires a Computer!