Switching AC / DC Relays, introducing gaps
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Posts: 46,084
Hi Rich.
Thanks for the information on SSRs. It's good to know.
I' ve had problems in the past using AC contactors with DC power (apart f
romthe obvious coil resistance values).
The problem is that they can get magnetised permanently and may remain
"closed" even after the DC power is off. We solved this (though not much
technically) introducing a small gap in the magnetic circuit (a thin paper
works - for a while).
Regards, Jose Luis
At 20:40 05/10/2001 +0000, you wrote:
>Switching highly inductive AC relay coils (large contactors) is very
>easy to do. Most Solid State Relay modules do employ zero switching.
>An appropriatly sized SSR can easily handle the demands of a
>contactor coil. Also, a transistor can drive a small DC relay, which
>can then switch the contactor. Finally, the difference between an AC
>coil and a DC coil is that the AC coil has a "chatter" ring on the
>coil. A coil is after all a coil. Almost any electromechanical relay
>with an AC coil can be operated by DC. A relay with a DC coil wont
>work well with AC due to the absence of the chatter ring on the coil.
>Over the years I have driven many AC coil relays with DC with no ill
>effects.
>
>Precautions should be taken to assure that the power supplying
>sensitive electronics is well filtered and regulated. I almost always
>use .1uF caps across Vdd and Vss pins
>
>My two cents
>
>Regards
>rich
>
>
>--- In basicstamps@y..., Jose Luis Juarez <jl.ingenieria@i...> wrote:
> > 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
> > switched off.
> >
> > 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
> > down.
> >
> > 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).
> >
> > Regards,
> >
> > Jose Luis
> > At 09:25 05/10/2001 -0400, you wrote:
> > >Mark,
> > >I wasn't quite "with it" yesterday, and didn't think your question
>through
> > >completely.
> > >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).
> > >
> > >
Original Message
> > >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?
> > ><snip>
> > >
> > >
> > >
> > >To UNSUBSCRIBE, just send mail to:
> > > basicstamps-unsubscribe@y...
> > >from the same email address that you subscribed. Text in the
>Subject and
> > >Body of the message will be ignored.
> > >
> > >
> > >Your use of Yahoo! Groups is subject to
>http://docs.yahoo.com/info/terms/
> >
> >
> > [noparse][[/noparse]Non-text portions of this message have been removed]
>
>
>To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
>from the same email address that you subscribed. Text in the Subject and
>Body of the message will be ignored.
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
[noparse][[/noparse]Non-text portions of this message have been removed]
Thanks for the information on SSRs. It's good to know.
I' ve had problems in the past using AC contactors with DC power (apart f
romthe obvious coil resistance values).
The problem is that they can get magnetised permanently and may remain
"closed" even after the DC power is off. We solved this (though not much
technically) introducing a small gap in the magnetic circuit (a thin paper
works - for a while).
Regards, Jose Luis
At 20:40 05/10/2001 +0000, you wrote:
>Switching highly inductive AC relay coils (large contactors) is very
>easy to do. Most Solid State Relay modules do employ zero switching.
>An appropriatly sized SSR can easily handle the demands of a
>contactor coil. Also, a transistor can drive a small DC relay, which
>can then switch the contactor. Finally, the difference between an AC
>coil and a DC coil is that the AC coil has a "chatter" ring on the
>coil. A coil is after all a coil. Almost any electromechanical relay
>with an AC coil can be operated by DC. A relay with a DC coil wont
>work well with AC due to the absence of the chatter ring on the coil.
>Over the years I have driven many AC coil relays with DC with no ill
>effects.
>
>Precautions should be taken to assure that the power supplying
>sensitive electronics is well filtered and regulated. I almost always
>use .1uF caps across Vdd and Vss pins
>
>My two cents
>
>Regards
>rich
>
>
>--- In basicstamps@y..., Jose Luis Juarez <jl.ingenieria@i...> wrote:
> > 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
> > switched off.
> >
> > 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
> > down.
> >
> > 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).
> >
> > Regards,
> >
> > Jose Luis
> > At 09:25 05/10/2001 -0400, you wrote:
> > >Mark,
> > >I wasn't quite "with it" yesterday, and didn't think your question
>through
> > >completely.
> > >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).
> > >
> > >
Original Message
> > >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?
> > ><snip>
> > >
> > >
> > >
> > >To UNSUBSCRIBE, just send mail to:
> > > basicstamps-unsubscribe@y...
> > >from the same email address that you subscribed. Text in the
>Subject and
> > >Body of the message will be ignored.
> > >
> > >
> > >Your use of Yahoo! Groups is subject to
>http://docs.yahoo.com/info/terms/
> >
> >
> > [noparse][[/noparse]Non-text portions of this message have been removed]
>
>
>To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
>from the same email address that you subscribed. Text in the Subject and
>Body of the message will be ignored.
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
[noparse][[/noparse]Non-text portions of this message have been removed]