I have a 2NO 2NC 120V coil, up to 400V contacts contactor. (Electrodepot 934022) I need to power both the coil at 120V and the contacts at 120V from the same source. Manufacturer says this is not possible without contactor malfunction because of feedback. I observe this to be true in real world tests with the device when a load is connected.

So what are the common ways to prevent feedback if one 120V AC single phase source is used to energize both the coil and an NO contact, which is held closed with 120V to pass through to loads?

Shading loops were suggested as a resolution to feedback, but they appear to be something an AC coil manufacturer already builds into AC contactors just to make them work in the first place by preventing the coil from de-energizing as the cycle crosses zero. Shading loops do not appear to address feedback.

The feedback I'm referring to is between the contacts and the coil when they are both powered from a single 120V AC source, where one NO contact is held closed with 120V passing through it. Any attached loads are presumably therefore connected to both the coil and the 120V source, and the coil can therefore de-energize into those loads.

Again, the manufacturer says you can not have both coil and contact powered this way without "doorbell" behavior, and device malfunction. Real world tests confirm this to be true. I'd like to prevent that if possible.

Update: I've added a drawing. Please understand my diagram skills are still limited, and I was trying to draw the device using the only available objects in EasyEDA's library. I also wanted the orientation to match the Contactor's diagram and my wiring. EasyEDA does not indicate when a wire jumps over another, so near R2 you'll see I tried to make the typical jump shape, but if there is no red node, then it means there is no connection. Please also note the Vehicle Chassis Ground to Neutral connection via R1 and R2 is mandatory code for RV power when not running on shore/main utility power. There is no debate over this. The only thing not pictured here is the much larger and more complex overall inverter, charge controller, AC switchgear, DC switchgear, and load devices. That's unnecessary to depict the basic wiring and the feedback I am attempting to resolve. All of those devices were not yet connected during this test, so they are irrelevant. Thank you for any useful feedback.

enter image description here

Here is also a drawing of the front of the Contactor. You'll note its 4 pole, but I couldn't find a 4 pole in EasyEDA, so I used the next best thing.

enter image description here

enter image description here

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    \$\begingroup\$ Could you please add a reference to the manufacturer statement? \$\endgroup\$ May 19, 2021 at 2:28
  • \$\begingroup\$ It was in a phone call today. You can Google Electrodepot's website and call them up. They connected me with a very stern but knowledgeable man named Walter who did not hesitate when answering. He was adamant. \$\endgroup\$ May 19, 2021 at 2:35
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    \$\begingroup\$ You would do best listening to the manufacturer. They know their product. \$\endgroup\$
    – TimB
    May 19, 2021 at 2:42
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    \$\begingroup\$ Yes I saw the other thread after this. IMHO I would not even entertain solid state. The 'violating code' bit I can't say is true or not...I don't see an inherent safety risk (assuming you build it in a proper enclosure and use a reasonable fuse). The relays coil doesn't give a darn about whether or not there are diodes in its path. Somebody else here more knowledgeable about trailer wiring may have some idea why it could be an issue. \$\endgroup\$
    – Kyle B
    May 19, 2021 at 4:56
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    \$\begingroup\$ I’m voting to close this question because the question requires details and clarity. (The schematic provided is either inaccurate, or the circuit is misconceived.) \$\endgroup\$ May 21, 2021 at 12:03

4 Answers 4


You can of course power both the coil and the load from the same supply, but for a NC contact, you must obviously power the coil from the supply side not the load side of the contactor.

There may be a bit of a trap if the supply impedance is high and the load has a large inrush current (large motor or something) as the sag due to the load can potentially drop the relay back out, never seen it happen, but it is I suppose possible.

What exactly are you actually trying to do?

Thinking about it, I am almost sure you are way over thinking this.... Why not just use three pole interlocked switches for the manual changeover? That way the G-N bond is permanently in place at the inverter output (As it should be, that is a separately derived supply), but that is disconnected when the inverter switch is opened, and the shore power feed is fully isolated (including its ground) when that switch is opened.

Still leaves you exposed to an open upstream neutral while running on shore power I suppose, but nothing apart from something like my RCD PEN protection trick really helps there.

There will almost certainly be rules about this sort of thing, but you have not said which country, you are in so it is a little hard to be specific. The US in particular is often very prone to specifying HOW something shall be achieved rather then WHAT shall be achieved in their 'codes'.

  • \$\begingroup\$ Thank you, Dan. Coil is definitely powered from supply side with a direct jumper from contact input to coil input. High impedance supply and large inrush is a possible issue. I tested initially with a shop-vac left in the on position since it's the largest in-rush likely to ever (accidentally) be present in the RV system when shore power is connected. I wanted to ensure the G=N bond would be broken by the contactor even with that load. Later I tested with no load. I have a 10 gauge 100' line, but was intentionally testing a 14 gauge 100' line to simulate using a friends/relative's cord. \$\endgroup\$ May 19, 2021 at 13:16
  • \$\begingroup\$ please note I've added a drawing. \$\endgroup\$ May 19, 2021 at 14:32
  • \$\begingroup\$ thanks again for your help. Between this and another post you and Jonk gave me enough to solve it. It looks like a manual solution is becoming more interesting due to the inherent lower risk of failure vs. a coil of any kind. But I'm also exploring staggering contactors because Electrodepot decided it was possible after all I had a faulty contactor, so they sent a new one, and to my happy surprise all is well except that the G+N bond isn't broken fast enough for upstream GFCI breakers. So staggering breakers, as suggested in another post I did, may be be an automated solution. Thanks again! \$\endgroup\$ May 22, 2021 at 0:36

I've read your question several times, but yet no idea if the nature of the feedback you are experiencing.

What I have got you want to do is very commonly done and should give no problems at all. Consider buying a different brand relay.

Two possible ways though, voltage feedback is the first, current the second one.

In case of voltage feedback I believe it could be the load connected is dropping so much on the line feeding both the load and the coil that it fails to put enough pressure onto the contacts. Hard to believe, unless your feeding line is insanely undersized.

In case of current feedback it may happen, with very high current, that the magnetic field from the power connections simply cancels the one from the coil, again loosing correct contact pressure. Hard to believe too, this may happen with pulsed current in the kiloamperes range. If this is the case it may be cured by swapping the the coil connection phase so to add instead of cancel the two fields.

  • \$\begingroup\$ Thank you! I'll check for ways to ensure voltage feedback is not occurring. I do see one possible way that it could, so I'll eliminate that and re-test. I see now possible way current feedback could occur in the manner described. \$\endgroup\$ May 19, 2021 at 13:06

I suspect you have managed to create a misunderstanding through verbal descriptions and (here) written descriptions with no schematic diagram.

You certainly can drive the relay coil off the 120V supply you are using as the INPUT to the switch.

You cannot drive the relay coil off the switch OUTPUT : that way, according to connection, you can either never switch it on or off via the relay (an NO contact gives a latching relay aka NVR switch), or switching it on immediately disconnects the power (NC contact gives an electric buzzer or bell).


simulate this circuit – Schematic created using CircuitLab

The first circuit above drives the coil whenever V1 is present, successfully. The switch contacts disconnect Neutral from GND whenever V1 is present. This is not an electric buzzer.

The second drives the coil FROM the relay switched output (via S3). It can never start the relay ... until you press SW1. Then the relay switches ON and stays on. Until you press S3 when it switches off. As a safety feature, if V2 goes away the relay switches off and your power saw won't restart suddenly when V2 comes back on. This is called a "NVR" switch (or No Volt Release) and you've probably seen it in power tools.

The third circuit is probably what you described to the vendor, and as he says, it will continually oscillate as a buzzer.

This is why you always describe what you are doing with a schematic.

  • \$\begingroup\$ I have added a drawing. I was not powering the coil from the output. \$\endgroup\$ May 19, 2021 at 14:31
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    \$\begingroup\$ Drawing makes no sense. R1 and R2 are shorted out. \$\endgroup\$ May 19, 2021 at 15:16
  • \$\begingroup\$ Please read notes. EasyEDA uses nodes to depict connections so wires just cross one another. There is no short. R1 and R2 are ground and neutral, which are bonded until the coil energizes, then they are open. When on shore power that bond must be broken, otherwise you have a downstream subpanel G+N bond which isn't allowed because ground would then carry return current alongside neutral. This is standard RV wiring, there is no debate about this subject. \$\endgroup\$ May 19, 2021 at 16:39
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    \$\begingroup\$ I can trace from both to G+N so where exactly is the break? \$\endgroup\$ May 19, 2021 at 20:20
  • \$\begingroup\$ As stated, when shore power is connected (the only source in the diagram), that's when the coil activates and breaks the connection. Otherwise there should be no break, because the inverter (not yet in the lower diagram) will be the source. When not on shore power the main utility panel hot, G, and N will all be disconnected. The G+N bond made by the contactor will provide a return path for inverter output. I haven't yet drawn the inverter wiring because it's complex and distracts from the original question. I only need to solve the issue of breaking G+N when on shore, so that's all I've drawn \$\endgroup\$ May 19, 2021 at 20:48

The circuit in the image you provide is probably not what you want.

enter image description here

Consider the wires connected to the AC voltage labeled "-". One wire goes through a node labeled "Main Panel G+N", to a node labeled "Main Panel Earth", to a node labeled "Vehicle chassis ground", then to the relay contact labeled R2. Going in the other direction, the AC labelled "-" connects to the relay terminal labelled R1. Thus, whether the relay coil is energized or not, whether the contact is open or closed, there is a direct connection between R1 and R2. I cannot see any purpose in that arrangement.

  • \$\begingroup\$ That's not how relay drawings work. They show you the normal state. You have to imagine the relay coil activating and pushing out to understand the energized state. I didn't come up with that convention, but every relay I buy is drawn that way, and all of the relays in EasyEDA library are like that, so I believe it's industry standard. I think I got it from this point forward. Thanks. \$\endgroup\$ May 20, 2021 at 18:06

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