Battery Contactor in RV-10

[rboyer]

I have an interesting issue. I have been wiring my new panel for a couple of months now. I'm in the final phase and have been programming it etc. Yesterday I went to turn off the master and the contactor (solenoid) stayed engaged. At first I though my master switch (a cessna rocker type) went bad. I replaced it with a spare and still had the same problem. So I went to the rear of the aircraft and disconnected the ground (from the master) going to the single pole on the contactor and it stayed engaged. I had to disconnect the battery to get it to dis-engage.

On the bench, the contactor seems to work correctly. I've never seen one fail like this (if it is even failing).

ideas??
Richard Boyer

[Mike S]

Do you have dual batteries??

Is there a second battery somewhere as a backup??

When you disconnected the battery, did the contactor actually "dis-engage"?? did you hear the contactor open?? Did you try re-connecting the battery to see if the contactor was "frozen" in the closed position??

These contractors are pretty reliable, I am suspecting you may have something else going on that is keeping the contactor from disengaging

[Ron B.]

The mounting base needs to be isolated. Is there a crack in the plastic or something at the base that could be grounding the unit? You would not have a bare wire laying over the unit that is making ground?
Good luck

[rboyer]

I don't have dual batteries, but I do have a TCW battery backup in the system. I dont have it on when this is happening and it isn't in the same circuit as the master switch. When I disconnected the battery it did disengage.

As for isolating the mounting base... The case itself is used for ground. If it weren't there would be no way for the contactor to make a circuit. I have seen the kind of contactors you are talking about and this isn't one of them.

There is one post for the master switch. I am using a contactor that came off a cessna right now and it appears to be working correctly. It just doesn't make sense that the old one is working on the bench.

[Mich48041]

Richard,
Can you post a schematic of the contactor circuit?
Does the master switch send positive 12 volts or ground to the contactor coil?
Is there an indicator light or anything else in the contactor circuit?

[rboyer]

[aerhed]

Have you checked that the small post is not grounded to the case (post short)?

[flightlogic]

It may have just "welded" the contactor slightly inside. Sometimes a tap on the case releases them. Sometimes the spring will pop it back in a few seconds after the coil voltage ceases. I have dissected them. It is very crude technology. Pretty much 5th grade science in there.... coil, contactor, spring.
When the high current first surges through, there is often a small spark at the edge of the contacts. Some circuits buffer this, some do not.
With just the right spark, they can weld at times.

[Robert Anglin]

Originally Posted by flightlogic

It may have just "welded" the contactor slightly inside. Sometimes a tap on the case releases them. Sometimes the spring will pop it back in a few seconds after the coil voltage ceases. I have dissected them. It is very crude technology. Pretty much 5th grade science in there.... coil, contactor, spring.
When the high current first surges through, there is often a small spark at the edge of the contacts. Some circuits buffer this, some do not.
With just the right spark, they can weld at times.

This as has bee said is the most common reason for the disks and poles inside the solenoid type units to stick closed. We have seen a lot of them ark together down through the years. If we have one that sticks, we hit it with a hammer handle or so. If they spring back open we know they have ark stuck together. Some times low Voltage will add to this. Most often the only thing you can do is put another one on. Then cut the old one open to find pitting on both the disk and the poles. My two cents. Yours. R.E.A. III # 80888

[Doug]

Do you have a large capacitive load downstream of the contactor? This might take the form of a large filter or may be a high capacity DC/DC converter. The peak inrush current across the initially small contact area during the first millliseconds of contact bounce that creates the heat to weld the contacts.

Under normal conditions the supply impedance is sufficiently high to limit the inrush current - but with a fresh battery (or two) and 2 AWG wiring the impedance is very low.

The solution is to wire a small value resistor in series with the capacitive load to limit the inrush peak. If necessary the resistor can be shorted using a relay or secondary contactor closing a few hundred milliseconds late using a simple delay circuit.

This is not an uncommon problem - most equipment designers will encounter it during the course of their careers.