Back EMF Protection

Hi guys, I was hoping you could help me with a query regarding using diodes to protect against back EMF in relay circuits. I am using a IMO TY4 12v delay on timer to delay my intruder alarm autodialler from triggering for approx. 20s. The alarm manual states a diode must be connected across the coil , but it doesn't give any values. I would be very grateful for any advice.
Thank you, Shane

It's not very critical, just use the 1N4001 and make sure it's connected in reverse with the power to the coil, anode to - and cathode to +.

Just about any diode or rectifier will work.