Short Circuit Protection ;Re-inventing the wheel
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Posts: 46,084
Those diodes in parallel with inductances are also called "Wheel Diodes".
So, why re-invent the Wheel?
I have seen those diodes connected and doing their job nicely for more than
30 years. In all this time nobody (as far as I know) has ever come with a
better solution.
A cheap 1N4007 wll be enough for practically ANY relay coil that can be
controlled by a normal transistor (except probably for big power types).
A snubber circuit (a capacitor in series with a resistor) could also reduce
the spike. But snubbers are complicated to design and they do not protect
in case of fast enough pulses on the coil (the capacitor gets charged). The
resistor is required to damp the circuit and avoid oscillations.
The shunt resistor might be something new... But it is more probably a
device for reducing the current in the relay coil when it is operated
(though it should not be called "shunt" in this case). The systems I've
seen, use an additional contact on the relay to include this resistor in
series with the relay coil (only once it is effectively closed), so that
the permenent current through the coil it is much reduced. This frequently
used on big DC contactors.
Regards, Jose Luis
At 10:53 04/10/2001 -0400, you wrote:
>I'll go out on a limb here and venture that the shunt resistor provides a
>similar function as the clamping diode would. However, a simple shunt
>resistor would, of course, increase the current dissipation of the circuit,
>requiring a more robust power supply. This practice, IMHO, is not
>environmentally friendly, as increased power consumption demands more from
>your local power company (or more gasoline, if an automotive application)
>
>Re-visiting the clamping diode theory: let's say the inductive load (relay
>coil, in your case) is energized, and we open the circuit. The collapsing
>magnetic field attempts to continue the current flow; however the only path
>is through the clamping diode. The current flow may be quite high, which
>may exceed the rating of the clamping diode. To fix this problem, we could
>add a current limiting resistor in series with the clamping diode; a
>properly selected resistor value would prevent voltage "spikes" across the
>inductive load, while limiting the amount of current through the clamping
>diode. Variables to consider here are the impedance of the inductor
>(Henries), the voltage applied across the inductive load, and the max
>current rating of the clamping diode.
>
>Hope this helps...
>Steve
>
>Mark Adams [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=gpX3JnRSoeQfLEQuX_Xv77QtjvkvOdlznX60B7K6K30wZq3QM3enGqpNoZSZBHEYvaYp0WucPROgDV0]madams@a...[/url wrote:
>
>Here is another basic question...
>
>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? It
>is my impression that the shunt is a LOW resistance value for other
>purposes than to protect a relay coil. Am I getting wrapped up in the
>wording and missing the point?
><snip>
>
>
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>
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[noparse][[/noparse]Non-text portions of this message have been removed]
So, why re-invent the Wheel?
I have seen those diodes connected and doing their job nicely for more than
30 years. In all this time nobody (as far as I know) has ever come with a
better solution.
A cheap 1N4007 wll be enough for practically ANY relay coil that can be
controlled by a normal transistor (except probably for big power types).
A snubber circuit (a capacitor in series with a resistor) could also reduce
the spike. But snubbers are complicated to design and they do not protect
in case of fast enough pulses on the coil (the capacitor gets charged). The
resistor is required to damp the circuit and avoid oscillations.
The shunt resistor might be something new... But it is more probably a
device for reducing the current in the relay coil when it is operated
(though it should not be called "shunt" in this case). The systems I've
seen, use an additional contact on the relay to include this resistor in
series with the relay coil (only once it is effectively closed), so that
the permenent current through the coil it is much reduced. This frequently
used on big DC contactors.
Regards, Jose Luis
At 10:53 04/10/2001 -0400, you wrote:
>I'll go out on a limb here and venture that the shunt resistor provides a
>similar function as the clamping diode would. However, a simple shunt
>resistor would, of course, increase the current dissipation of the circuit,
>requiring a more robust power supply. This practice, IMHO, is not
>environmentally friendly, as increased power consumption demands more from
>your local power company (or more gasoline, if an automotive application)
>
>Re-visiting the clamping diode theory: let's say the inductive load (relay
>coil, in your case) is energized, and we open the circuit. The collapsing
>magnetic field attempts to continue the current flow; however the only path
>is through the clamping diode. The current flow may be quite high, which
>may exceed the rating of the clamping diode. To fix this problem, we could
>add a current limiting resistor in series with the clamping diode; a
>properly selected resistor value would prevent voltage "spikes" across the
>inductive load, while limiting the amount of current through the clamping
>diode. Variables to consider here are the impedance of the inductor
>(Henries), the voltage applied across the inductive load, and the max
>current rating of the clamping diode.
>
>Hope this helps...
>Steve
>
>Mark Adams [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=gpX3JnRSoeQfLEQuX_Xv77QtjvkvOdlznX60B7K6K30wZq3QM3enGqpNoZSZBHEYvaYp0WucPROgDV0]madams@a...[/url wrote:
>
>Here is another basic question...
>
>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? It
>is my impression that the shunt is a LOW resistance value for other
>purposes than to protect a relay coil. Am I getting wrapped up in the
>wording and missing the point?
><snip>
>
>
>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]