Making my own electric kettle controller

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I am in the process of coming up with my own controller and came up with a wiring diagram pictured below. The thing that is different here than most controllers is that I want to use a typical welder outlet that is 240V with 3 prongs. In order to get my 120V line into the controller box I am also going to put a 120V plug on the box as well so I can also control my pump. This, in my opinion will give me the Neutral line that I need that you would typically get with a 4 prong 240V outlet like a drier plug. I'm only using one PID and one heating element because I just use one kettle for my HLT and boil kettle.

I think this will work, if you don't think it will give me an explanation as of why. I'm not too concerned about the electrical code because I am just going to use a spa panel and wire it so I can plug it into the welding outlet that I have available so it will be "portable" in a sense. Also, FYI, all the wires will be sized appropriately.

Let me know what you think, if it will work or if it wont. Like I said if it wont work, tell me why.
 

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doug293cz

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The circuit as you have it drawn will trip the GFCI in your spa panel. If you can't figure out why, I can explain it.

Brew on :mug:
 

doug293cz

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You are powering the main power contactor coil from one of the hots of the 240V supply. The current from the coil will return thru the neutral of the 120V power feed. This will mean that a few milliamps of current from one of the 240V hots will not return thru the other 240V hot, leading to a current imbalance. This imbalance is exactly what the GFCI is designed to detect, and trip when detected. It only takes ~5 mA of current imbalance to trip the GFCI.

If you want to use 120V coils on your contactors, you need to use the 120V hot to power them, not a 240V hot. The other option is to use 240V coils on the contactors, but then you need to switch both of the hots that go to the contactor coil.

Brew on :mug:
 
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I was thinking that one of the switches might have been the issue, but I wasn't quite sure. Thank you for clearing the mud!

So for the systems that have the 4 wire plugs (H-H-N-G), is this a non-issue because it has it's own neutral? I'm just curious.
 

doug293cz

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I was thinking that one of the switches might have been the issue, but I wasn't quite sure. Thank you for clearing the mud!

So for the systems that have the 4 wire plugs (H-H-N-G), is this a non-issue because it has it's own neutral? I'm just curious.
In a 120V system, the hot and neutral wires pass thru a current sensing coil. In normal operation, the current in the hot and neutral are equal and in opposite directions. So, the net current sensed by coil is 0. In a 240V system both hots and the neutral run thru the current sensing coil. For 240V loads the current flows out in one hot and back in the other hot, and if there are no 120V loads, there is no current in the neutral. If the 240V hots carry equal and opposite currents, and there is 0 neutral current, again the net current thru the current sensing coil is 0. If 120V loads are present, then current flows out thru one of the hots, and back thru the neutral (or vice versa depending on which part of the AC cycle you are in.) In this case the currents flowing in all three wires are different, but the net total, when accounting for direction of flow, is still zero.

If you mix the neutrals from different GFCI feeds, whether they are all 120V, all 240V, or a mix of of 120V and 240V feeds, the balance in the individual feeds in disturbed, and the GFCIs trip.

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Bobby_M

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Besides tripping the GFCI, it's probably better to just use a 240v coil on the contactor anyway so that the controller can function even without the 120v line.

Personally I think the extra contactor on the input line is redundant. The "element enable" contactor is fine on its own. If you really want the PID's input voltage to be switched, just run one of the legs through a panel switch and switch that milliamp load directly.

Note, you also need to power the PID with 240v to avoid the same problem with the GFCI imbalance on the neutral.
 

jlash630

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Besides tripping the GFCI, it's probably better to just use a 240v coil on the contactor anyway so that the controller can function even without the 120v line.

Personally I think the extra contactor on the input line is redundant. The "element enable" contactor is fine on its own. If you really want the PID's input voltage to be switched, just run one of the legs through a panel switch and switch that milliamp load directly.

Note, you also need to power the PID with 240v to avoid the same problem with the GFCI imbalance on the neutral.
i think you mean, he has to power the PID with one of the hot legs of the 240V supply and use the neutral that is run through the GFCI with those hots... Otherwise, we would never be able to have 120 control power in these panels.
 

doug293cz

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i think you mean, he has to power the PID with one of the hot legs of the 240V supply and use the neutral that is run through the GFCI with those hots... Otherwise, we would never be able to have 120 control power in these panels.
No, most PIDs can be powered with either 120V or 240V (modern switching power supplies are like magic.) The 120V can pass thru to the pump outlet (thru a switch) and does not have to connect to anything else in the panel.

It's also possible to power the contactor coils and PID from the 120V supply.

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doug293cz

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Besides tripping the GFCI, it's probably better to just use a 240v coil on the contactor anyway so that the controller can function even without the 120v line.

Personally I think the extra contactor on the input line is redundant. The "element enable" contactor is fine on its own. If you really want the PID's input voltage to be switched, just run one of the legs through a panel switch and switch that milliamp load directly.

Note, you also need to power the PID with 240v to avoid the same problem with the GFCI imbalance on the neutral.
I agree that the input power contactor is superfluous, unless you use it to implement a safe start interlock that prevents powering up unless the element enable and pump switches are off. The PID draws so little current that it is ok to hot plug it into the outlet.

Brew on :mug:
 
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