TVS diode shorted on HP motherboard

[Askic]

Hello guys,
I have seen a few videos on youtube about repiring HP 15 pavilion laptops that has TVS (Transient Voltage Suppressor) diode shorted to the ground.
Please have a look at the attached picture.
The diode is P4SMAJ20A and according to the datasheet its breakdown voltage (at 1 mA is 22.2V or 24.6V at10 mA).
THis means that if voltage goes higher that 23V this diode will start conducting and short to ground thus protecting rest of the circuits from overvoltage.

What I don't understand if is this diode reversible, for example if voltage goes below 20 V, will it return to its non conducting state, or this diode is some kind of a "one-shot" device which means once voltage is over threshold, the diode is permanently shorted to the ground?

How this protection is supposed to work. I know in case of zener diode, when voltage is above some threshold, it starts conduction and when voltage falls below certain threshold it block again.

I have read in Wikipedia "it automatically resets when the overvoltage goes away", but if this is a common issue on some laptops, I wonder how big overvoltage was in order this diode to stay permanently shorted.

Thank you.

[mon2]

Re: TVS diode shorted on HP motherboard

Hi.

If the threshold of this transient suppression diode (zener) is reached, the diode will conduct to suppress the excessive voltage. If under this threshold value, the diode is "invisible" to the circuit.

Think of it as a relay that is open unless a dangerous peak voltage is reached, then the relay is closed to shunt the higher voltage to ground.

https://cdn.badcaps-static.com/pdfs/...760b8110c1.pdf

Unless the diode has had to deal with heavy current, it should be ok after such an event.

If in doubt, remove this 2 legged part from the PCB and just be sure to use a safe voltage value for this circuit board.

Once the diode is out of the board, you can test it using the diode mode of your multimeter.

The diode should conduct if the RED (+) of your meter is on the ANODE (back side of the arrow head); BLACK (-) of your meter on the CATHODE (the bar / top of the arrow head).

Respectively, the meter should NOT conduct if the meter leads are reversed from the above condition.