# Diode across contactor



## frodus (Apr 12, 2008)

It's called a coil supression diode.

When you remove the current source from an inductor (the relay coil), the voltage spikes. The diode gives the current a path to go. If the wiring diagram shows it, then install it on there.


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## Siwastaja (Aug 1, 2012)

Do NOT use a diode across a contactor/relay coil in a serious application unless you are 100% sure what you are doing, especially when switching DC. A diode may slow down the switching too much and shorten the life of the contacts dramatically or in worst case cause instant welding of the contacts by not breaking the arc at all. 

A combination of a diode and a resistor, or a zener, or a zener and a resistor, is used. Special components for suppression exist, too. These suppress the voltage spike enough to protect the drive circuit, but not too much; only a small current goes through the coil, slowing down the actuation only negligibly.


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## frodus (Apr 12, 2008)

Agreed,

Don't use just any diode, use what the manual for your controller recommends. Also, make sure polarity is correct. The band should go towards the positive supply lead of the coil.


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## sabahtom (Mar 1, 2011)

The DMOC manual doesn't say anything about a diode across the contactor. The drawing attached was done by someone else who was going to use the same setup. Drawing is attached, it's low-res, sorry. I don't have the original file.

I have an electrical engineer who's willing to help with the installation but he hasn't done an electric car before. This part of it is way over my head so any pointers you can give will be great.

I'm using one big battery pack of 98 cells, not two as in this drawing. Where's the best place for the contactor? In the dash near the ignition switch?

Thanks


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## Ziggythewiz (May 16, 2010)

As shown in the diagram, your contactor(s) usually goes near the controller. You want it to isolate the controller when powered down. 

There's no reason for it to be near the ignition switch unless that's where your controller is. You want to minimize the lengths of traction wire you need (cost, weight, etc) while smaller wire like that switching your contactor doesn't matter much.


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## frodus (Apr 12, 2008)

Some controllers have all that build in... they limit current on the driver that drives the contactor. I don't know about the DMOC, so you'd have to ask someone who knows more about it.


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## TEV (Nov 25, 2011)

From the DMOC manual : Note, no contactors should be included in either the DC or AC wires in Figure 2.

You should ask your EE friend to read the manual.


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## TEV (Nov 25, 2011)

And if you are afraid of a runaway controller situation, that happens only on DC motors controllers, and the DMOC is an AC one.


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## kennybobby (Aug 10, 2012)

i don't see a diode across the contactor in your diagram--there is a free-wheeling diode across the control coil of the contactor, but not the larger power contacts. The free-wheeling diode provides a path for the control coil current to circulate when the control is turned off and the magnetic field of the coil is collapsing.


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## sabahtom (Mar 1, 2011)

TEV said:


> From the DMOC manual : Note, no contactors should be included in either the DC or AC wires in Figure 2.
> 
> You should ask your EE friend to read the manual.


Hi TEV

It's a bit late here but I can't see that bit of text anywhere in the manual. Could also be I've got a different version of it. I've attached a screenshot of the wiring diagram from that manual.

The screenshot doesn't show a contactor anywhere so I guess there's no need.


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## TEV (Nov 25, 2011)

They recommend to have the High voltage connected to the controller all the time, and disconnect it only when you put the car in storage.


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## sabahtom (Mar 1, 2011)

TEV said:


> They recommend to have the High voltage connected to the controller all the time, and disconnect it only when you put the car in storage.


So if I'm looking to do a short test drive, I need to wire up the battery pack to the controller and the controller to the motor, and connect the pot boxes and ignition? Thought I must be missing something.


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## TEV (Nov 25, 2011)

sabahtom said:


> So if I'm looking to do a short test drive, I need to wire up the battery pack to the controller and the controller to the motor, and connect the pot boxes and ignition? Thought I must be missing something.


I did just that for my test drive, but I did installed a 250A fuse between battery pack and controller.


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## frodus (Apr 12, 2008)

You *always* need a fuse (or DC circuit breaker), but you may not need a contactor (as stated by TEV).


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