# Help with contactor, keyswitch and precharge circuit wiring.



## rezirezi12 (May 21, 2020)

Hi guys, So I'm building an electric cafe race and have bought a 72v 8kW conversion kit and a 72V 150A 50aH battery. What I'm not sure about is how to wire the contactor and key switch. I have the basic idea but I'm still not sure. I found a wiring diagram for kelly controller but that controller has a pin for a key switch whereas mine doesn't have that, so I doubt that wiring would work. Can someone help me, please? I know I need a precharge circuit but I just can't figure it out.
Thanks in advance.


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## remy_martian (Feb 4, 2019)

Search the forum. We've discussed contactor wiring...and recently.


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## _GonZo_ (Mar 23, 2009)

rezirezi12 said:


> I found a wiring diagram for kelly controller but that controller has a pin for a key switch whereas mine doesn't have that, so I doubt that wiring would work. Can someone help me, please? I know I need a precharge circuit but I just can't figure it out.


Can you let us know what Kelly controller model do you have, so we can help better?


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## GregShay (Aug 20, 2016)

rezirezi12 said:


> Hi guys, So I'm building an electric cafe race and have bought a 72v 8kW conversion kit and a 72V 150A 50aH battery. What I'm not sure about is how to wire the contactor and key switch. I have the basic idea but I'm still not sure. I found a wiring diagram for kelly controller but that controller has a pin for a key switch whereas mine doesn't have that, so I doubt that wiring would work. Can someone help me, please? I know I need a precharge circuit but I just can't figure it out.
> Thanks in advance.


As you figure out the specifics for your controller, it helps to keep in mind what the precharge is for, why it is there (and what happens if you don't have it.)

The controller high voltage input has some capacitors. Left unconnected, these caps bleed to zero. Turning on the contactor HV DC into empty caps causes a current spike with damaging energy, potentially to the caps themselves and burns the contactor points with an arc. It's rather like a small explosion. I had 72VDC touch with thin 22 guage wire and it went off like a firecracker.

So the precharge uses a resistor (typically 10k) to keep the controller HV input caps charged up when the contactor is open. Not enough current to run, just keeping the caps charged.
Then, when the contactor closes, the resistor is bypassed, and let's high current flow to the controller.

Depending on the details of the setup, a precharge can be as simple as a 10k, one or two watt resistor across the main contactor open when off contacts. Be warned, this means battery HV is applied to your controller even when the main contactor is off. For safety, a precharge relay could be used to make the resistor connection, but this has to be done allowing enough time to charge before the main contactor closes (or you get the current spike). 

A controller with a dedicated precharge pin may have the resistor internally. But you can have it external. The principle is the same.

You will have specific details, but this is an overview.

Greg


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