How to pick a tvs diode for 24vac

Thread Starter

thedoc8

Joined Nov 28, 2012
162
Have a 24vac transformer that was rectified and filtered and ran the rest of the circuit. The transformer had a tvs diode across the 24volt output before the bridge rectifier and it was shorted. Repaired everything sept the tvs because I could not find the part number. This is my first encounter with a tvs. My question is how to select a new one without knowing the part number. The circuit works fine with out it, but I want to put something back. I know almost 0 on how to select one. Would this be similar to selecting a zener...?
 

dl324

Joined Mar 30, 2015
16,846
It doesn't make sense for a TVS diode to be across the transformer secondary. On the half cycle when it's forward biased, it'll essentially short the secondary.
 

Thread Starter

thedoc8

Joined Nov 28, 2012
162
It doesn't make sense for a TVS diode to be across the transformer secondary. On the half cycle when it's forward biased, it'll essentially short the secondary.
Thanks, I guess I really don't know what the device is. This looks like a diode but it can't be because of the short on the half cycle. Any guess what it could be,
 

Thread Starter

thedoc8

Joined Nov 28, 2012
162
Hello,

There are double diodes, that will have a breaking voltage at positive and negative.
The ZZ-series is an example of them.

Bertus
Thanks Bertus looking now. This was a simple 24 volt actuator for a hot water valve. Simple 24 volt supply rectified and runs the rest of the circuit. Looks like a normal 24vac to bridge to filter and to the rest of the circuit but with this unknown device across the 24volt output. I has been running for a month now without it. I know it is some kind of suppression but don't know.
 
A 1.5KE45CA should work.

See: https://www.littelfuse.com/~/media/.../littelfuse_tvs_diode_1_5ke_datasheet.pdf.pdf

Why. I upped 24 to 28V. 24 VAC transformer has at least 10% regulation, the the line has a 10% variation.

You need the peak voltage to size, so multiply 28 * sqrt(2) and you get 39.2

You don't wan't it to conduct, so size by Vr and you want UNI-directional.

I put a TVS diode on a furnace because the control board is way expensive and so is the thermostat. I can look and see what I picked.
 

Thread Starter

thedoc8

Joined Nov 28, 2012
162
A 1.5KE45CA should work.

See: https://www.littelfuse.com/~/media/.../littelfuse_tvs_diode_1_5ke_datasheet.pdf.pdf

Why. I upped 24 to 28V. 24 VAC transformer has at least 10% regulation, the the line has a 10% variation.

You need the peak voltage to size, so multiply 28 * sqrt(2) and you get 39.2

You don't wan't it to conduct, so size by Vr and you want UNI-directional.


I put a TVS diode on a furnace because the control board is way expensive and so is the thermostat. I can look and see what I picked.
Thanks a bunch for the info, just been out of the repair business too long.
 
Top