Because this is an area that I'm interested in, I googled this a bit
further and came up with instances of people using a MOV for preventing
this sort of thing. In general, the aim is to prevent the current surge
associated with switching inductive and to a certain extent
capacitative loads. The MOV, diode idea and transient voltage
suppressors are all candidates. Google any of the above, or "relay arc"
for more info.
I'd be curious to know what you come up with.
Chris
I am switching a wire wound resistor using a 24V high current relay.
The relay acts as a high side driver between the battery and grounded
load. Wire wound resistors have some inductance but it is very low (I
believe below 1uH). I do not think this is a contributing factor to my
problem. I may be wrong.
SAE says 24V automotive systems typically sit at 28V while charging.
They can drift up to 32V under normal operating conditions. I am
driving a 1 ohm load. My product seems to have no problem passing 28A
at 28V. A pretty good spark is generated when the relay contacts
separate, but this is manageable. My problem occurs when system
voltage rises to 32A at 32V. When the relay contacts separate a
sustained arc of approx 2-3 seconds is created. This arc is severely
damaging to the relay contacts. It melts the contact acting as anode.
One or two of these arcs kills the relay. The anode contact melts away
and the relay can no longer conduct.
I have tried all recommended approaches. RC network across relay
contacts, RC network across load, diode across load, MOV. No
suppression network seems to work. The only thing I found that works
is to increase the gap between relay contacts. I have not had a chance
to do extensive testing on modified relays yet to see if it is the
cure.
My stock relays have fine grain silver contacts and are gapped 0.4mm
apart. The relays I modified are gapped to approx 0.85mm. They seem
to start arcing at about 36V which is outside the normal operation
range of the vehicle and should be good enough for me.
I'm looking for an expert opinion from someone who has been down this
road before. Am I missing something? What does it take to extinguish
an arc? Am I on the right path, etc....
Thanks for your input Chris. I welcome you, and others, to join in for
more trouble shooting.
Gerb