Using diodes on a relay for spike protection

Thread Starter


Joined Jul 20, 2009
Hi folks. On the attached schematic, there's a diode on the coil of the relay, to protect the system from the coil spike when the relay opens.

Does it matter which way the 1N4004 diode is "pointing"? i.e., is a clamping diode always stopping positive from reaching negative, or vice-versa, or doesn't it matter?

Would it hurt/help/not matter to also include another diode "pointing" in the opposite direction across the same wires? (Since the diodes are so cheap, and some of the electronic components so expensive.)

Would I need another diode on the Timer Relay? If so, between which wires, and in what direction? ("A1" and "A2" are constant, with a trigger signal coming in on "S" to kick off the timer.)

With those diode(s) in those locations, would all the components of the system be adequately protected?
--- 782XBXM4L-12D Magnecraft Electromechanical Relay, "Ice-Cube" Style, 16A DPDT, 12VDC Coil Specs
--- And the socket for it: 70-782D-1 Magnecraft Relay Socket, 8-pin Cube Style, 2 Pole, 16A, 300V, IP 20 (Finger-safe) Specs
--- Time-Delay Relay to feed 120VAC lighting circuit: 822TD10H-UNI Magnecraft 10 Function Programmable Time Delay Relay, 15A DPDT, Universal Coil Voltage 12 - 240 VAC/VDC Specs
Thanks for your help/thoughts!



Joined Jul 17, 2007
Your diode as shown will short the 12v supply when the access control NO connection closes. This will either fry the diode to a crisp, cause the 12v supply to shut down, blow a fuse in the 12v supply, or cause other undesired events to occur.

The diode must be installed so that the cathode (end with the band) is towards the more positive supply to the coil (reverse biased).

The "spike" occurs when the current to the coil is suddenly turned off. The magnetic field around the coil collapses, which "tries" to keep the current flowing in the same direction as it was when it was powered. This can cause very high peak reverse voltages unless there is a reverse-EMF diode (sometimes called a "flywheel" diode) installed. The diode provides a path for current back through the diode.

A single diode per coil is usually sufficient.

Sometimes a Zener diode is used in series with a standard rectifier diode. This causes the current to decay more quickly. In this case, the standard diode's cathode still goes towards the more positive supply, but the Zener's cathode goes towards the more negative.

There are also TVS diodes:

The timer module is electronic; it doesn't need external diodes.

However, you are switching the current to the 120v/24v transformer's primary. This will cause similar spikes when power is disconnected. You may wish to include TVS suppression across the inputs/outputs of the transformer.

Thread Starter


Joined Jul 20, 2009
Hi Sgt. Thanks for the quick and informative reply.

A few questions --

In my application, do you think I would need the Zener diode you mentioned?

Without a TVS diode on the transformer, what components would be at risk? Would the spike remain on the primary side (and thus feed the spike back to the electrical panel)? (Which should be able to handle it without incident?) Or would the whole transformer or its secondary side (and the door strike) be at risk also?
If I do really need a TVS diode on the transformer, do you know what model # would be a good choice? (Primary is a 120VAC line, transformer's secondary is 24VAC, 20 Watts). I looked at some spec sheets, but swiftly got in way over my head... :)

Thank you!