OT TVS Diodes - Transorb diodes

[BryanG]

Having a thick headed morning. I am looking at protecting some 24V d.c. circuits with TVS diodes and am getting myself confused. For a 24V d.c. circuit I think I need a Stand Off voltage of 22V with a Breakdown voltage of 24.4V. That would seem to give a close protection. Otherwise maybe a Stand Off voltage of 27V with a breakdown of 30V, but that brings me close to the maximum allowable voltage for various HMIs and devices.

Then I am getting really confused with UniDirectional and BiDirectional. Normally with a diode you get high resistance in one direction and low resistance in the other. So does a UniDirectional breakdown at a voltage in one direction but always have low resistance in the other? And what does a BiDirectional do?

 

[MikeW]

I use the 27V size to clip the output when required on 24 V circuit, you are trying to stop large transients that will fry the silicon. I always try to get bidirectional as there is no chance for them to get installed backwards, even the best panel builder can have occasional pilot error and this is hard to test for other than looking at the marking stripe on the TVS. Bidirectionals are basically two diode place back to back in series. http://en.wikipedia.org/wiki/Transient-voltage-suppression_diode

 

[BryanG]

I hadn't thought of that advantage of using the BiDirectional type. I calmed down a bit after writing the post and had done a bit of web searching, though I still think that most electronic type data sheets are written to be as confusing as possible. Thanks for the help.