# Contactor Diode, Why is it needed?



## etischer (Jun 16, 2008)

The coil inside the contactor is a big inductor. An inductors purpose in life is to keep current the same, similar to how a capacitor keeps voltage the same. So when the coil is energized, it may have 250mA going though it. When you open the contacts, the coil wants to keep the 250mA going but there is no path for current to flow. 

The air then becomes the current path, and a high voltage arc is produced, just like an ignition coil. 

Volts= Amps * Resistance. 
12V = .250A * 48 ohm when coil is energized
250kV = .250A * 1,000,000 ohm when coil is disconnected. 1Mohm is the resistance of the air gap. 

When you put the diode in, the current will flow though the diode instead of creating this high voltage arc. You can probably see the high voltage arc by connecting the contactor coil up to a battery with loose wires. 

The diode will help save the micro relay that is driving your contactor, or whatever that device might be. The high voltage arc tends to pit the relay contacts. Many contactors come with a built in diode/mov/rc network/surge supressor.



imaspaceguy90 said:


> Hey guys just a quick question. I understand the theory of a contactor. To switch on high current witha small current source as the switch. Since i am running 12v in order to switch on the contactor, what is the point of of the diode? I know a diode keeps current flow in one direction but what kind of damage does it prevent on a contactor. I Appreciate your help guys.


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## imaspaceguy90 (Dec 14, 2008)

Thanks Etischer , i kinda understand the concept now, I just pictured the contactor coil and the switch relay to be isolated from each other to keep this from happening. I know relays can switch currents without the need for diodes but maybe with super high voltage this can pose a problem witha contactor.


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## frodus (Apr 12, 2008)

relays and contactors should always be driven with a circuit that contains a diode, I haven't seen many designs that don't include it.


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## engineer_Bill (Jun 24, 2008)

Just use the diode. If you are using a mechanical switch you can get by with a small cap across the coil, but the diode will work better. Also if you are using a solid state device instead of a relay or switch the inductive surge will eventually kill it.


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## imaspaceguy90 (Dec 14, 2008)

so on a normal automobile, lets say a headlight relay. Where is the diode located normaly on that cuircuit.


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## etischer (Jun 16, 2008)

imaspaceguy90 said:


> so on a normal automobile, lets say a headlight relay. Where is the diode located normaly on that cuircuit.


It probably doesn't have one. I have seen diodes in the cars wire harness for high inductive loads such as horn and starter solenoid. The headlight switch is probably made to withstand the miniscule current from the relay. If the switch is part of a circuit board, then the circuit board may contain some devices to quench the arc.

Make sure your diode is in the correct direction. If you put it in backwards, the diode will always be trying to be a short circuit and it will fry.


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## etischer (Jun 16, 2008)

Current only flows in the direction of the arrow on the diode.


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## ZenDaddy (Jul 22, 2008)

I thought I would over simplify it for the laymen (me)

The coil in the contactor is essentially an electromagnet, when it is energized by the control voltage (typically 12v) it brings the higher voltage contacts together, closing the "switch". When it is not energised the "switch" is open. As long as the precharge circuit is in place there is no danger of an arc across the high voltage contacts in the contactor.

When you suddenly remove the 12v control voltage and the coil deenergizes, it acts like a high voltage generator for a millisecond, sending a voltage spike _back_ towards the source of the control voltage along the _12v control voltage_ path. It is refered to as "Collapse Voltage".

The diode (correctly installed) protects adjacent circuits from that collapse voltage spike.


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## drdhdmd (Aug 5, 2007)

Also remember that for high power applications like our EVs, using a regular diode can slow the opening of the contactor since it is allowing a path for the cllapsing voltage through the coil of the contactor, the magnetic field will collapse slowly and the contacts will slowly open. This is not good if you are breaking a heavy flow of current through the contacts. It is better to use something like an NTE-4933


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## yarross (Jan 7, 2009)

drdhdmd said:


> It is better to use something like an NTE-4933


Or RC snubber across coil.


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## CPLTECH (Sep 14, 2007)

ZenDaddy said:


> When you suddenly remove the 12v control voltage and the coil deenergizes, it acts like a high voltage generator for a millisecond, sending a voltage spike _back_ towards the source of the control voltage along the _12v control voltage_ path. It is refered to as "Collapse Voltage".
> 
> The diode (correctly installed) protects adjacent circuits from that collapse voltage spike.


I agree. It seems that you must have an electronix background. Thanks for giving the correct response/explanation.


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## ranman (Oct 7, 2009)

frodus said:


> relays and contactors should always be driven with a circuit that contains a diode, I haven't seen many designs that don't include it.


 Tell that to Electro Automotive they told me diodes were not needed in their kits. I was going to put them in but was assured they weren't needed. I now have some arcing in one of my relays and have to tap it to open the arc. Diodes are cheaper than relays.


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## edsammy (Jun 15, 2009)

is a diode necessary across a relay that is switching a contactor?


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