Still some comments:
Controlling AC type relays or contactors is tricky and cannot be done
easily with electronics (particularly not with transistors).
In case an electronic control circuit is necessary, a triac or similar
device could be used.
These should preferably be controlled in "zero current" mode, that is, the
control circuit must wait until the current is zero and not the voltage of
the AC supply. (90 degrees lagging current)
There are (or were), special ICs for this application.
If no zero current mode is possible nor used, then a snubber circuit should
be connected in parallel with the coil, but still it is tricky business.
(the Triac typicaly will not switch off reliably or at all, and also could
be damaged). A snubber must contain a resistor AND a capacitor in series
(rather difficult to design, except by trial and error).
I have never seeen shunt (parallel) resistor with AC contactor coils to
improve the switching, though who knows, I may be getting too old.
Another comment, in case of DC powered coils and wheel diodes:
Circuit theory easily shows that the maximum current that an inductance
will try to pass through an external circuit is equal (and NOT higher) to
the current that was circulating on it immediately before the power is
I don't like analogies, but as an exception: you don't expect your car to
jump into hyperspace like the Enterprise when you clutch suddenly; it will
try to keep its previous speed for a while and (more or less) quickly slow
Inductances are similar. So no risk of overloading the wheel diode if it is
rated at least for the normal operating current of the relay coil.
Note that we a re lucky enough or Nature is so wise, as to have the voltage
on the inductance reverse its polarity (Lenz law) upon switch off, so that
we can connect our diodes without polarity problems.
But the real problem is that if you do not provide an alternative path to
the current the inductors try to maintain after being switched off, the
indutance will try to overcome the resistance by proportionally increasing
the voltage on its terminals (the infamous spike that burns any transistor).
ANY resistance on the circuitt (such as the "shunt" resistance we are
talking about) can only increase the voltage over the original value
applied to the inductance.
That is, simply put, why diodes are the best solution in DC circuits (they
have practically zero resistance in the conducting state).
At 09:25 05/10/2001 -0400, you wrote:
>I wasn't quite "with it" yesterday, and didn't think your question through
>A "clamping" or "wheel" diode across a relay's coil works great *IF* the
>voltage applied to the coil is DC. However, this won't do for AC, as the
>clamping/wheel diode would present a dead short to ground for 1/2 off the
>cycle (when the AC voltage went negative).
>For this reason, in AC circuits only, you must use a shunt resistor across
>the relay's coil (or other inductive load).
>From: Mark Adams [noparse][[/noparse]mailto:madams@a...
>We were looking at some BOSCH relays and they listed an option that
>included a 'shunt resistor' and the ones we currently purchase with an
>included diode. Now, the question is why is the shunt resistor there?
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