Back EMF Diode On A Strike

I have a situation where I have to wire two different strikes in parallel and my manufacturer recommends I add a back EMF diode.

I have used them before, but I used to remember my techs saying they used to just make them in their trucks or on site with a bit of heat shrink, and a diode and a resistor.

I am not as savy and was looking to purchase them made up, but seeing the prices on them makes me want to reconsider busting out the soldering iron and the heat shrink, but not sure what parts I need or how to wire them.

My distributor recommended the HES Smart Pac at $37.00 a piece, which is a little pricy. What are the other options my fellow access control folks are using?

This is a good question, and I will be following answers too.

A similar situation/application is using Metal Oxide Varistors (mfg example) instead of diodes, although the damage prevention function is similar.

I have heard of using MOV instead of Diodes, and was under the impression Diodes were for strikes, and MOV's were for magnets.

Some manufacturers distinguish between AC = MOVs, DC = diodes.

I do know that using diodes even with DC maglocks can be an issue, causing slow releases though.

Why don't you just use a simple diode? just place it across the leads as shown in the picture.

Could cause a sticky door without a resistor, no?


sticky door syndrome only exists with maglocks.

The idea behind this diode is so that when power is cut from the lock the return voltage has where to go. (Through the diode, and then back through the lock) the power dissipates is milliseconds, so the door won't stay unlocked.

Even if using Failsafe (energized when locked) strike?

At some point the lock has to be de-energized, whether that locks it or unlocks it. The collapsing magnetic field around the coil generates a voltage in the circuit that can damage upstream equipment. The diode essentially "shorts" that path.

A diode is not suitable for an AC strike, which is where you'd NEED to use an MOV.

The diode essentially "shorts" that path.

True, but I wasn't arguing against the diode, I was talking about also using a resistor so that the current sent back thru the solenoid was not enough to momentarily cause the keeper to stick in a DC fail-safe installation.

I just learned about back EMF and MOVs or diodes in the spring access control course. I have a site with a bunch of RCI 8310 maglocks installed on Keyscan 8500 door controllers. Looks like they supply MOV surge protection with the unit though I don't recall the installer wiring them separately so I think it must be internal to the unit. Does anyone wire these on door strikes?

Are you using relay boards with the 8500 controllers?

It does indeed look like the RCI8310 has an internal MOV, so the immediate need is taken care of, although it is not clear how that component is replaced in the field:

For Back EMF in low voltage

DC electric strike applications, a

1N4007 (1A, 1000V) diode will suffice.

You can buy these at a Radio Shack-type store or online through a typical electronic parts supplier. They are very inexpensive ... usually get 20 or more pcs. for under $10.00

Install with the striped end to the + side of the lock connection.

You can get 200 for $10