A reader comments on the practice of putting diodes across the coil
of a contactor . .
> A diode connected in this way is usually suggested to clamp the inductive
> voltage spike resulting from de-energizing the relay coil. The problem is
> that it provides a path for the current caused by the collapsing magnetic
> field. The net result is that the amount of time it takes to open the
> relay *increases*. This exacerbates the issue of a sticking relay in that
> a slower release time causes more arcing at the contacts.
Yeeeaaahhh BUT . . . I've never been able to document much change
in the contact spreading velocity of the contactors we sell and
recommend when a diode is included in the contactor's coil circuit.
I have been able to document wear and tear on the switch that controls
the contactor and it's much worse if the diode is left off.
On starter contactors, the spring tension that opens contacts
is MUCH larger than for the continuous duty contacts. Further,
given the higher coil current, it's more important that this
stored energy be calmly dealt with than with battery contactors.
Hence, our starter contactors come with the diode BUILT IN. I
have to believe that the folks who make these by the millions
for ground based vehicles find this a useful thing to do.
Battery contactors are generally opened up with VERY mild loads
on the main terminals . . . so again, it's more useful to tame
the contactor's stored coil energy than to be concerned with
contact opening velocity.
> A better way is to dump the current of the collapsing field into the
> battery. You can do this by connecting the diode across the starter
> switch (cathode band to the battery side of the switch) rather than the
> coil. This is a higher impedance path, and it allows the coil magnetic
> field to collapse faster while still clamping the voltage.
Not so. I've done an article on spike catching diodes and posted
it to:
http://www.aeroelectric.com/articles/spikecatcher.pdf
It's rather large (500K .pdf file) and I appologize for the
size but it has 4 pictures of oscilloscope traces taken from
test setups on my bench this morning. If you want to get the
straight skinny on this topic, I'll suggest it's worth the
download time.
Bob . . .
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( Knowing about a thing is different from )
( understanding it. One can know a lot )
( and still understand nothing. )
( C.F. Kettering )
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http://www.aeroelectric.com
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