About Back EMF Protection Diodes
Our circuits consist of a relay coils being switched by an output from your SJB (The original car light you are tapping from.)
When a coil is switched off, a large BACK EMF appears across the coil. This back EMF may be several thousand volts in value, enough to destroy the SJB over time.
To deal with this we fit a diode at the relay, its low resistance short circuits the back EMF voltage and protects the SJB. You can get the diodes from any diode retailer, and they look like this:
They are referred to as 4001 diode. You may find them listed as 628-9546 or 1N4007-E3 depending where you are looking.
Observe that one end has a silver stripe on it, that is to ensure you fit it the correct way around. It is imperative they are fitted the correct way round.
The diode goes between 1 and 5 on your Omron relay bases, bridging between the only feed from the SJB (being the light you have tapped from) and ground – with the black end of the diode going to the Nr 1 ground terminal. Put another way, black end to earth, silver striped end to switching live from the light you tapped from.
Cover the exposed parts with some sheathing of some kind. I use household earth sheathing (like you might find in a plug socket) to cover the whole exposed diode. I am sure there is a politically correct colour that those who specialise in EU babble will know about – point being, just cover it up somewhow so nothing shorts out on it later on.
