Diode across a relay

The cathode (-) of the diode is soldered to the +12VDC side of the coil and the anode (+) is soldered to the ground side of the coil.  You want the diode to be reversed biased (off).  The diode is used to protect the circuits that are controlling the relay.  When a relay is de-energized, a reverse voltage spike can feedback and damage sensistive electronic devices.  When the spike is generated, the diode will be forward biased and short the spike out to ground.  Warning:  If you connect the cathode to ground and the anode to +12VDC, the diode will be forward biased and withing a few seconds the diode with start smoking and burn up.  Many electronic manufacturers install diodes (reversed biased) across the +12VDC power lines that drive their equipment to protect them should someone swap the+12VDC & ground lines.  When that  happens, the diode shorts out the power feeding the device and taking out the fuse, thereby preventing damage to the device. Remember cathode to +12VDC, anode to Ground (GND).

- Jim

I'm not sure how you are wiring the relay.  Either you will tie one side of the coil to ground and apply +12VDC to the other side to energize it, or you will have +12VDC tied all the time to one side of the coil and apply a ground or pull down command to the other side of the coil.  In either case,  the best place to put the diode is right across the coil terminals.  Also make sure you use a hefty diode like 1N4000 series.  Don't use a small signal diode like a 1N914. Follow instructions above for polarity of diode.

- Jim