Testing A TVS Diode

[Sadlercomfort]

Hi Guys,

Is it appropriate to test a TVS diode using a diode check, and simply testing both ways.

I have a smbj33 TVS Diode which is used between the +/- of a motor in a H-Bridge Circuit.

I'm getting 0.121V and 0.194V each way.

Thanks,
Ash

 

[Harald Kapp]

Consider a TVS as a special kind of zener diode which is designed to absorb comparatively high energy pulses within a short time. Therefore:

• yes, you can check the forward direction (conducting) with the diode check of a multimeter.
• no, you can't check the backward direction completely. You can check that the TVS is not conducting at low voltages (therefore I'd say your measurement indicates a defec TVS). To check the breakdown voltage you need a higher voltage source with appropriate current limiting.

Did you check the diode in circuit? Remember to de-solder one leg for this kind of test, otherwise you will measure other circuit elements, too (e.g. the motor's resistance, the body diodes in MOSFET transistors etc.)

 

[Sadlercomfort]

Right, the TVS diode is SMT so I tested it in-ciruit.

It's also showing less than 200Ω one side and around 50Ω the other.
If i test a known good one, also in-circuit i get like 0.65MΩ each side.

So shall i continue to desolder with a hot-air gun.. and re-test?

 

[Sadlercomfort]

Okay, the low resistance is due to the faulty mosfets which were damaged during a fault, they have shorted.

I removed the TVS diode and tested using a diode check and got nothing, I tested resistance and got an infinite resistance.

I removed two known good TVS diodes and tested them.. i got the same thing.

 

[Harald Kapp]

So shall i continue to desolder with a hot-air gun.. and re-test?

That's the most meaningful approach, although the comparative measurements youmade indicate that the low-Ω TVS is very probably defect.

 

[Harald Kapp]

So the culprit is obviously the MOSFET, not the TVS.

 

[Sadlercomfort]

Is it too late to mention.. I was trying to find what caused the MOSFETS to short?

I had already tested two of the FETs and found they had shorted.. the cause of this is important.
I also tested several other components in the H-Bridge Circuit and found no faults.

I assumed the TVS diode was the cause of the fault, but didn't know how to test this.

 

[Sadlercomfort]

So I've tested the TVS diode and two other known good ones, and have
the same measurements so how do I know if the diode is the faulty?

You mentioned testing the breakdown voltage of the diode with a high voltage source.. How would i setup this test?

I have access to power supplies e.t.c

 

[Harald Kapp]

Here's a description of a test setup.

 

[Sadlercomfort]

Quite straight forward, thanks Harald