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What is this potted block?

CATPart

Well Known Member
Just behind my starter switch and master switch there is a potted block mounted to a piece of support structure. It is about .5" x .5" x 2" and is some sort of composite. 4 wires are in the block. The wires from the master switch split off and enter this block. the other two wires go from the block to the back of the starter switch. I don't see an item like this on any of my wiring diagrams. In the one image below the master switch was pulled through the panel, and you can see the wires going to the block. Anybody know what it might be? Aircraft is 1996 ish, an RV4.

Thanks


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My scientific WAG is that it's a dropping resistor or voltage divider for your starter relay. By any chance does your airplane has a 28v power system?

My $0.02...

Dave
 
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Thanks Thermos, No not 28V. Just a super simple 12V system. I need figure which tabs on the starter switch go to the block, that might help.

Hmm something about this seems familiar....
2001_monolith.jpg
 
You might gain some clues if you disconnect the device and take ohm readings between the leads.
 
If all the wires go to the master switch (and/or ground) then I would guess it is a crowbar overvoltage protection module.
 
Metal Oxide Varistor?

Looking through my builder logs again I found a wiring diagram from aeroelectric for my plane, where it shows something going between the suspect terminals, i highlighted it in yellow, called a "MOV".

Searching aeroelectric for "MOV" I found the article in the other image. So it is a metal oxide varistor i guess?

Is this typical? Not sure if the builder made this or bought it as I see no reference to it. Mburch is this similar in function to the crowbar you mention?



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Looking through my builder logs again I found a wiring diagram from aeroelectric for my plane, where it shows something going between the suspect terminals, i highlighted it in yellow, called a "MOV".

Searching aeroelectric for "MOV" I found the article in the other image. So it is a metal oxide varistor i guess?

Is this typical? Not sure if the builder made this or bought it as I see no reference to it. Mburch is this similar in function to the crowbar you mention?



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The MOV is where most put a diode. It's purpose is to stop arcing on the switch from the coil's magnetic breakdown. Nver used an MOV so can't say how it compares to a diode. I was goint to reply ealier but couldn't understand how something that big could be a diode.
 
The MOV is where most put a diode. It's purpose is to stop arcing on the switch from the coil's magnetic breakdown. Nver used an MOV so can't say how it compares to a diode. I was goint to reply ealier but couldn't understand how something that big could be a diode.

MOV’s are basically a capacitor and resistor in parallel. They function as a spark reducer or arc quencher. In fact “Quencharc” is a common brand or trade name. A bit more sophisticated than a diode but similar function in this application. They should be sized appropriately for the voltage and current in the application to be most effective.
 
If all the wires go to the master switch (and/or ground) then I would guess it is a crowbar overvoltage protection module.
Winner winner chicken dinner…I think :).

MOVs are commonly used for overvoltage/surge protection. In our applications diodes protect against back voltage from the opening of a relay coil contact. MOVs break down and conduct at a specified forward voltage.

I suppose you could use an MOV for coil suppression, but diodes are cheaper, easier and probably act faster.

Dave
 
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Difference between a MOV and a diode is that the MOV clamps any spike and a diode only clamps the negative spike (if installed correctly) produced when relay coil is powered off.

MOVs are typically found in AC applications like surge protectors.

https://www.webstaurantstore.com/images/products/extra_large/408208/1848899.jpg

I think in our application I would use a combination of a MOV and a diode in parallel. The diode to clamp negative spikes to -0.7V and the MOV to clamp damaging high voltage spikes from the relay coil, maybe to 16 or 20 volts -- or whatever the highest allowed voltage spike for your avionics is (30V?).

Finn
 
diode across coil is recommended

Bob Nuckolls has a document dated October 2000 that recommends putting the diode across the coil.

Z schematics show diode 1N5400 across contactor coils and 1N4001 across relay coils.

Bob Nuckolls confirmed the schematic in post #7 came from him. Bob has since changed from MOVs across switch contacts to diodes across contactor/relay coils.

Ref the OPs query and Bob's responses on the Aeroelectric List:
http://forums.matronics.com/viewtopic.php?t=16780715
http://forums.matronics.com/viewtopic.php?t=16780718
.
 
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I thought most put the diode on the *relay*, not on the switch?

e.g.

http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z13-8T.pdf

That is true, but it really doesn't matter whether it is installed on the switch side or the relay side of the wire, as long as it is on the correct wires with the correct polarity. With a diode, you are creating a one way loop (opposite from normal electron flow) to dissipate the coils energy instead of an arc on the contact. As long as that energy is not allowed to get to the switch contacts and arc, it is a success.
 
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