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Superaquarama

Does the overdrive have a relay ?

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Just looking at ordering a loom from Autosparks, where you can specify one for different options such as alternator conversion.

 

It also mentions wiring for the overdrive relay - is there really one ? The current taken is surely negligible, isn't it ?

 

Thanks,

 

Anthony

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Yes, there is.

 

(FYI: Triumph Spitfires with a J type overdrive came without a relay. The button on the gear knob was always hot when the OD was engaged. I even changed the layout of the wiring because I considered a relay a safer option).

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Anthony, the solenoid for the overdrive has two fields, the small one that holds the overdrive in has a negligible current of about 1 amp, but the bigger field is the one that snaps the solenoid plunger into place before it is held by the small field and draws a much bigger current albeit for only a short period - something in the region of 15 amps, hence the need for the relay.

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The A type overdrive requires a relay. As Menno indicates, the J type does not although I specified one from Autosparks for mine in case I change to A type in the future.

Edited by peejay4A

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The worst thing about the A-type overdrive electrics is the high current pull-in winding.

 

Not only a high current but lots of inductance. This causes arcing when the switch opens.

 

Relays have better contacts, than the column switch, to handle this. Actually you don't want any old relay really.

Ideally one with the right contact material for inductive loads.

 

Of course the original had this.

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Original type still available, I presume the contact material would be correct ?

 

Bob.

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50 years ago, we used to put a Zener diode across a coil in order to quench the back e.m.f.

This arcing does damage the relay - my car has eaten at least 3 in the last 23 years!

I ought to follow my own advice!

Ian Cornish

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You could put a capacitor across the contacts to damp the spark - similar to the contact breakers in the dizzy.

 

Roger

Edited by RogerH

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50 years ago, we used to put a Zener diode across a coil in order to quench the back e.m.f.

This arcing does damage the relay - my car has eaten at least 3 in the last 23 years!

I ought to follow my own advice!

Ian Cornish

No need for a Zenner, any silicon rectifier type diode will do as long as it's reverse voltage spec exceeds the back emf. a 1N4004 etc would do nicely.

 

Bob.

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Thanks, Bob. I knew that someone would be able to tell me of a cheap device that would do the business. Shall pop in to Maplin later this week.

Roger: Capacitor not a good idea as it increases the inrush current when the coil is energised and doesn't help quench on de-energisation.

Ian Cornish

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It was quite common to use a resistor-capacitor network for quenching Ian and the condenser in the distributor does just that for the points. You can actually buy dedicated quench modules too.

 

http://www.okayaelec.co.jp/catalog/english/eg_p52-56_introduction.pdf

 

Having said that, I would use a diode as its just as good for the purpose of saving the relay contacts and more readily avialable. Just make sure you connect it reverse-biassed !

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Honest remark here: Guys, you've lost me after posting #2: diodes and all that sort of difficult words...

 

Menno

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The 1N4004 that Bob suggests is 400V piv and 30A peak surge so it ought to do. I usually use the 1N4007 for this sort of thing though, as that is 1kV piv.

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Thanks Menno,

 

reassuring to know I'm not the only one baffled by all this clever stuff !! :rolleyes:

 

Cheers

 

Alec

Me too!

I have no idea what a 1K piv looks like or what I would do with one.

(however can tell a prophylactic odontotomy when I see one.)

Edited by littlejim

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Sorry for all the geek-talk- I'll try to explain.

When a coil like the overdrive solenoid is energised, there is a magnetic field set up in the core. As the energising voltage is switched off that field immediately collapses and generates a big reverse voltage spike called a 'back emf' - ( emf is a fancy name for voltage). Its the same thing that happens in the ignition coil when the points open but of course there we want it to happen.

 

The voltage spike can be big enough to cause a spark as the switch or relay contacts open and that burns the contacts away after time. You need to stop that by 'quenching' the arc using a capacitor or by stopping the back-emf in the first place. The latter can be done by putting a diode across the coil, oriented so that it does not conduct on the normal applied voltage but will conduct when the back-emf happens and so effectively shorts it out. The diode has to have sufficient rating so that it is not 'killed' by the voltage - piv is peak inverse voltage - and must be able to handle the large peak current (surge) which will flow.

 

On the other hand I have no idea what a prophylactic odontotomy is and probably wouldn't like to meet one on a dark night!

Edited by RobH

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Trouble is that if the diode gets blown, it will probably go open-circuit and you won't know.

 

This is a common problem with electronic prophylaxis.

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Sorry for all the geek-talk- I'll try to explain.

When a coil like the overdrive solenoid is energised, there is a magnetic field set up in the core. As the energising voltage is switched off that field immediately collapses and generates a big reverse voltage spike called a 'back emf' - ( emf is a fancy name for voltage). Its the same thing that happens in the ignition coil when the points open but of course there we want it to happen.

 

The voltage spike can be big enough to cause a spark as the switch or relay contacts open and that burns the contacts away after time. You need to stop that by 'quenching' the arc using a capacitor or by stopping the back-emf in the first place. The latter can be done by putting a diode across the coil, oriented so that it does not conduct on the normal applied voltage but will conduct when the back-emf happens and so effectively shorts it out. The diode has to have sufficient rating so that it is not 'killed' by the voltage - piv is peak inverse voltage - and must be able to handle the large peak current (surge) which will flow.

 

On the other hand I have no idea what a prophylactic odontotomy is and probably wouldn't like to meet one on a dark night!

 

Thanks Rob - it's a lot clearer now (seriously, no tonque in cheek here)

 

But I think that LittleJim, Alec and I will keep it to the Supercalifragilisticexpialidocious version of the good old relay. :D:D That's good enough for me (must sound strange from a guy who has updated his TR with modern day electrics, modern brakes etc...)

 

Menno

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Yep, I have not bothered with a diode either. The main victim of the high current disconnect arc is the contacts inside the solenoid which open when the solenoid is fully in & you can't get to these. The relay contacts only see the low current coil disconnect.

 

Bob.

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Not quite. When a relay closes you don't get one simple "make". The contacts bounce, sometimes quite a lot.

 

Even the snappy lightweight keys on a keyboard do this. So you can get erosion on "making" not just "breaking" because the make actually consists a a burst of make-breaks when end up with a make. Typically this goes on a several milliseconds.

 

It's this kind of electrical spike environment that makes it hard to build reliable electronics to fit in classic cars.

Lots of heavy current switching goes on.

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