Thread: Spotlight wiring

Originally Posted by SteveFarmer

Hi,

Earlier in the thread it was pretty much agreed it was cheap insurance to use a diode protected relay on a P38, since all the other relays are diode protected.

Someone pointed out the headlights are at least connected to the CANBUS system, since you get bulb failure messages on the dash.

I agree there is no standard wiring, but a diode protected relay should be wired with the correct polarity.

BTW I have been reading about your SC-80 Isolator with great interest, but that is for another thread

Regards
Steve

Steve

I am still very much in favour of diode protection.

Gary

Hi

Attached is the Narva diode protected relay, showing the circuit diagram on its base. Looking at the wiring diagram below (positive switched headlights-pink background in thread #50) can I use this diode protected relay? The relay diagram sort of indicates that the current flow is from 85>86 (unless I am reading it wrong).
This new wiring diagram(pink below in thread #50) shows the switch between the main beam and the relay - all the indications before were that it should be on the earth side of the relay.
I have an inhibitor switch and it appears to work as per a normal switch (creates a circuit according to the meter) not like the fog lamp switch.
Hoping to get it sorted.
Regards
Kev

Hi Kev and there is a misunderstanding on how the diode works as a spike suppressor and as such, how the diode has to be connected.

I can not blow up the picture enough to be able to read the pin connections.

Now as Steve has posted, when using a diode protected relay, PIN 85 MUST GO TO EARTH.

And power ( +12v ) MUST GO TO PIN 86.

This may look to be the wrong way round for the diode but the diode is NOT intended to be used to pass voltage/current through it when powering the relay.

The diode is actually connected up in revers AND ONLY FUNCTIONS FOR A SPLIT SECOND AFTER THE RELAY IS TURNED OFF.

WHAT HAPPENS, put very crudely!

When the coil of a relay ( or any electrical coil ) has powered applied to it, it very quickly builds ups up energy and becomes a magnet. No electrical spike problems at this stage!

But when either one of the coil’s connections is disconnected, there is a large amount of energy still in the coil and this energy basically converts from current energy to voltage energy, and a voltage spike is created.

This spike can be thousands of volts but has no current to it.

The diode, wired in reverse, literally shorts the spike either to earth if the positive connection is disconnected, or to positive if the earth connection is disconnected.

Now if you are worried about the potential of a spike harming your electronics and you have an non spike protected relay, you can wire a diode in reverse to the polarity of power to be connected to the relay.

This would mean, you get a diode and connect the end of the diode with the band around it’s edge, to the positive connection of the relay and the unbanded end to the negative or earth connection to the relay.

So Kev, if you want to add a diode to a non diode protected horn or headlight relay, connections as follow.

PIN 86 goes to positive, usually via a switch and then to the high beam wire. You also connect the banded end of the diode to this pin.

PIN 85 goes to earth and the unbanded end of the diode goes to this pin.

But once again, there is absolutely no reason to need to use spike protected relays in any vehicle, old or new. The spike protection is not there to protect your vehicle’s electronic, the spike protecting is there to make sure the relay meats the EU’s EMF requirements.

Again, all vehicle electronics is and has always had spike protection built into every electronic device and for at least the last 25 years, every input pin, not just the power supply pins, but every input pin on IC’s has also been individually protected against voltage spikes.

Hi
Thanks for the very detailed reply. Firstly, I have reloaded the diagram of the Narva diode relay in a different format so that hopefully you may be able to read it. Let me know if this relay is alright to use.
Your explanation of the diode is starting to make sense.
Does it make any difference if the switch is place in line between the main beam supply to terminal 86 of the relay or between terminal 85 of the relay and the earth, as Steve advocated.

Hi again Kev and that picture is much easier for my failing eye sight.

That relay is fine and should be wired up as covered in my last post ( and Steve's post ) and the CE is certification that the relay meets the EMF requirements.

Also, yes, you can fit the switch in either wire and all will work as required.

Thanks for all your assistance, I will give it a go now with confidence.
Will let you know how it goes.
Regards
Kev

Kev

Good luck mate, I think you now have enough info to sink a battleship

Steve

Hi

It is the battleship here again. I have finally started to wire it up but have noticed that the wires on the back of the Inhibit switch are only 0.8mm thick. I am placing the switch between terminal 85 and the earth. On the pink wiring diagram above, it shows these wires as 2mm. Seeing that it is only a trigger does it have to be 2mm or is it ok to run 0.8mm?
I do not think the inhibit switch is designed to take higm amps anyhow.
Regards
Kev