# Inductive Load - Blown Decoder Output



## NMTrain (Feb 4, 2012)

I expected the outputs of a stationary decoder designed for solenoid snap motors to be protected from the inevitable "inductive kickback", or at least the documentation for the decoder to tell you it isn't and how to externally protect it.

Well, after a couple of "blown" switch machine decoders, I hauled out the DSO and took a look.

Well, they aren't protected.







at least a 40v spike. BOOM!

Drop a couple of diodes across the coils. (Connect the banded side of the diode to the positive side of the coil). Bipolar coils will require an bit more thinking.







and it is good.

If you are connecting any inductive load such as a solenoid, relay or an accessory motor, I'd sure make sure that the function output is protected.


----------



## Greg Elmassian (Jan 3, 2008)

whose decoder blew up?

and I always put a "kickback" diode across anyway... old school I guess

Greg


----------



## NMTrain (Feb 4, 2012)

These were Digitrax DS52's, not that it matters.

I too am old school, but one would think that a device that is primarily used for this purpose is either going to have a proper protection circuit built in or the documentation explicitly stating it's the end users responsibility to furnish it.

I will not make that assumption again.

That's why old school engineers wore belts and suspenders.


----------



## Greg Elmassian (Jan 3, 2008)

I am using DS64's with solenoids to control 24v air valves.

Have had one quit in 10 years.

Greg


----------



## JPCaputo (Jul 26, 2009)

High quality components for running on low power DC typically have a damping diode or small capacitor to clamp the inductive spike. The solenoids marked with polarity usually have a diode built in.


----------

