Contactor coil diode.

Have you made or bought a converted vehicle if so this is for you
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EVguru
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Contactor coil diode.

Postby EVguru » Mon Aug 20, 2007 12:47 pm

Contactors are usually fitted with a freewheel diode across their coil terminals.

This diode clamps the 'back-emf' and prevents over-voltage damage or arcing at whatever is switching the contactor. However it slows the opening of the contactor and makes it more likely for the contactor to arc and/or weld.

On any contactor that you would want to open reliably under load, it's a good idea to put a Zenner diode 'back to back' with the main diode. This allows the current in the coil to drop very fast initially and speed the contactor opening. A 12 volt device is sufficient in most cases.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
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geekygrilli
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Postby geekygrilli » Mon Aug 20, 2007 1:36 pm

Cheers Paul

Can we have something like this everyday...

'EVguru's thought for the day'!?

That'd be cool!

:D

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Postby MalcolmB » Tue Aug 21, 2007 8:52 am

Trying to learn more about back emf suppression, I found this very good explanation on the 4qd site:
http://www.4qdtec.com/catch.html#intro

Paul, when you say 'back to back', do you mean that the Zener should be in series with the main diode, but with opposite polarity?

I know that a Zener diode will allow reverse current when the voltage exceeds its rated value, but I still don't get how it speeds up current decay in the coil. Could you or anyone else explain please?

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Postby Teslas fag packet » Tue Aug 21, 2007 12:20 pm

Well as the current is removed from any inductor the magnetic flux begins to collapse, as this happens it induces an opposite current direction than that which formed it, in other words you end up with a minus voltage, assuming you are trying sink current with say an open collector npn so a voltage below 0. this is often in the order of several thousand volts below 0 and can / will kill the component driving it. So what we do is fit a diode this conducts as the current flow is reversed discharging the coil, however this gives this coil a load to drive into and this slows the decay of flux, like loading up a generator the engine slow's it’s the same the decay of flux slows the contactor opens slower.

The problem is that zeners are SLOW to react and looking at this with a scope may revile a fast negative pulse that COULD cause latent damage to the part driving it
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electricvehicles
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Diode Blocking

Postby electricvehicles » Tue Aug 21, 2007 3:28 pm

I know im going to be in for some flak over this one but --- If a controller is built as it should be ther is no need for blocking diodes.

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Postby Teslas fag packet » Tue Aug 21, 2007 3:45 pm

It should be really from a good design point of view, however if you don’t fit it internally you have the option of switching a say 12V contactor from a controller running at say 72V, I have seen some kit with a clamp terminal and this implies the diode is internal

Just been reading mi last post, I must learn English at some point :wink:
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Postby EVguru » Tue Aug 21, 2007 7:14 pm

I know im going to be in for some flak over this one but --- If a controller is built as it should be ther is no need for blocking diodes.


There is nearly always a need to supress the coil voltage. You can either do this directly at the coil, or remotely by protecting the switch.

The specific case I was thinking of was the main contactor activated by the ignition, where the controller is not directly involved. The ignition switch is not designed to handle the inductive 'kick' of the contactor coil, so some form of suppression is needed to protect it. A plain zener plus rectifier diode is fine in this aplication, but you could equally well use a Transorb and then it might as well be a bi-directional one and you don't need the rectifier diode at all.

You only have to worry about the suppressor slowing the contactor down if it has to open under load, particulary if it's a 'fault' load (like a controller blown short circuit). On the CitiEl, the Curtis controller will only switch the F/R contactors when the current is zero (or close to). They chose the simplest method of voltage supression and specified a diode across the contactor. The trouble there is that if the diode is left off, then the switching circuit in the controller would be damaged. Better to build it in to the controller in the first place and this is what many controllers do.

If you use a Mosfet to switch the coil, then many of these are now 'avalanche rated' which means they can perform a zener function without damage.

Suppressing diectly at the coil does have an advantage as far as radio frequency interference is concerned, becuase it makes the 'broadcast loop' as short as possible.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

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Teslas fag packet
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Postby Teslas fag packet » Tue Aug 21, 2007 7:29 pm

aaaahhhh i didn't realise you were referring to a switch driving a contactor coil

Group Hug :D
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Postby anachrocomputer » Thu Aug 23, 2007 8:26 am

For those who like to see photos of this kind of thing, I have four pictures of the contactors and diodes in Nikki's City-El. Hers is a Series III (mine's a Series II), and therefore has just two contactors, mounted close up to the Curtis controller. One contactor engages for going forwards, and the other is for reverse.

The reverse contactor wouldn't work because the corresponding diode was shorted. The Curtis can detect shorts and shut down accordingly.

Photos here:

http://www.flickr.com/photos/anachrocomputer/sets/72157601606510434/
1994 Suzuki Cappuccino
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geekygrilli
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Postby geekygrilli » Thu Aug 23, 2007 12:15 pm

Erm, OK....

I have a 96v Controller. Start up sequence is:

Turn ignition key
This supplies 12v to the contactor that connects the 96v batteries to the controller. (12v coil, swithes 96v)
It also flicks a small relay that powers up the controller

Pressing the throttle pedal clicks a microswitch which allows another contactor to flick over - supplying power to the motor. (96v coil, switches 96v)

Where do I put my diode(s)?
Does the 12v coil need one?


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