You do not want to use a transorb for a repetitive application like this. Transorbs have a limited life. Every spike they see shortens this life. When they fail, and they will, they usually fail shorted causing even more problems.
Larry


---- Bob Axtell <engineer@cotse.net> wrote: 
> Spehro Pefhany wrote:
> 
> <respectful snip>
> 
> >A diode across the coil has been suggested, and is the simplest way of
> >handling the coil inductance. A 1N4148 is fine for any relay you'd be
> >likely to use.
> >  
> >
> Alas, sorry,  it is NOT fine and is no longer considered good 
> engineering practice. I see
> this idea all over the place, and this is bad information. Even relay 
> manufacturers include
> a diode, but it is not good practice.
> 
> The 1N4148 is a "faster" general purpose silicon diode, used at one time 
> because nothing
> any better was available. The 1N4148 cannot switch fast enough to 
> suppress the edges
> of an inductive spike, so was most successful when a cap in series with 
> a resistor is in parallel with
> the coil as well as the 1N4148. Moreover, it cannot handle the high 
> current pulse of the inductive
> collapse. I've seen wholesale device failures with this setup, even with 
> tiny 5V relays.
> 
> Use a Transorber, Varistor,  or TVS device, with a voltage rating just 
> above the voltage being
> applied to the relay or motor. If need be, just trust me on this one, 
> its the truth. Go to the
> technical documents page of any transient voltage suppressor maker and 
> you will understand
> what people are doing. These were originally invented by GE.
> 
> Slow oscilloscopes won't pickup the unsupressed spike, so this can't be 
> easily proven, but if you
> own a small portable radio, you can pick up the unsuppressed spike  on 
> the AM spectrum. With a
> 1N4148 it will still be present, with little improvement. If you then 
> use a transorber or TVS placed
> AT THE RELAY COIL, the spike will be so suppressed that you can't pick 
> it up (but if the suppressor
> is not placed as close to the relay as possible, the RF will still be 
> generated by radiating through the
> conductors going from the relay coil to the suppressor).   Try it.
> 
> --Bob
> 
> >Best regards,
> >
> >Spehro Pefhany --"it's the network..."            "The Journey is the reward"
> >speff@interlog.com             Info for manufacturers: http://www.trexon.com
> >Embedded software/hardware/analog  Info for designers:  http://www.speff.com
> >->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff
> >
> >
> >  
> >
> 
> 
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