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neo · Posted: Mon Sep 25, 2006 8:55 pm Post subject: relay with a diode

I've noticed that the relay (S12b) that powers on the motronic ECU on the 1.7 16V engine has a diode in the circuit that activates the relay (pins 85 - 86). What is the diode for? Can I use a normal relay instead of this diode type?

Thanks!
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http://www.quartermile.ws/forum/viewthread.php?tid=889&page=1

tvatavuk · Posted: Tue Sep 26, 2006 2:11 pm Post subject:

Hi Neo,
I don't know which relay is it but there is good reason for that diode there.

Car electronics are sensitive and can be fried easily, ie returning current from relay. For this reason most of car realys have resistor inside housing, which prevents in most cases returning curent.
What happens is while relay is powered current flows in one direction, moment you turn it off, due to magnetic field around coil of relay it will generate/self induce same amout of power but with reverse flow. This is only impuls for short period of time, but usualy will make problems in short or long term.
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Tino Vatavuk
Little black dress which replaced Alfa Romeo 33 S 16v Permanent 4 GMo

Guest · Posted: Tue Sep 26, 2006 2:43 pm Post subject:

So this means that the diode protects only the circuit that powers on the relay ?

Scott Sander · Posted: Tue Sep 26, 2006 3:41 pm Post subject:

Don't remove the diode. It is there for a reason.

I assume you are talking about the protection diode accross the coil.

Have a look at this link. It may help with the explination.

http://www.kpsec.freeuk.com/components/relay.htm#protect
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Scott Sander
'91 Alfa 33 Boxer 16V Monza - Awesome
http://www.sanderfamily.com

tvatavuk · Posted: Tue Sep 26, 2006 10:24 pm Post subject:

Great link Scott.
Cheers
_________________
Tino Vatavuk
Little black dress which replaced Alfa Romeo 33 S 16v Permanent 4 GMo

neo · Posted: Wed Sep 27, 2006 9:36 am Post subject:

Hi!

I've looked at the link. So the diode is used to protect transistors and IC (in case that they activate the coil in the relay) when the relay is powered off.

But the main relay is activated using a switch when you turn the key. There is nothing sensitive in that circuit. But the fuel pump relay is activated by the ECU. Using this logic, why is the fuel pump relay witouth a diode?

Thanks!

p.s. great link Scott
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http://www.quartermile.ws/forum/viewthread.php?tid=889&page=1

tvatavuk · Posted: Wed Sep 27, 2006 5:23 pm Post subject:

Neo all relays I have seen in my AR 33 P4, are either diode type on more sensitive eq or those with resistor inside which does almost same job.
I have to look but AFAIR fuel pump relay is also special one (metal housing).
I learned about that selfinduction problems after I put 2 normal relays 1 for radio, other one for in-out temperature meter. that uses power only to for backlight, lcd matrx and sensors are powered by aaa type battery.
Anyway every time relay would selfinduce, temeraturemeter would go nuts, start beepin, rested redings...
Radio shown more power shock anomalies.

p.s After my key cought fire, and I almost lost whole car few years back, I wouldn't agree with you that there is nothing sensitive.
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Tino Vatavuk
Little black dress which replaced Alfa Romeo 33 S 16v Permanent 4 GMo