Blown reverse voltage protection diode

hack3rpt · Dec 29, 2020

So I was replacing the charging port on a tablet today, and when I was testing it, I forgot that the PSU probes were reversed and I ended up blowing the reverse voltage protection diode.

The charging circuit takes 5v and the component size is around a 0603 resistor.

Can I just replace it with any schottky
diode with a higher voltage rating? Or does it need to be something more specific?

Thanks in advance.

Pommie · Dec 29, 2020

If the diode is connected from negative to positive then it is there so it blows a fuse if connected wrong. If you didn't have a fuse then you may have damaged other stuff. Just remove it and connect the power supply the correct way to see if it still works. If it's a series diode then it shouldn't have blown.

Mike.

hack3rpt · Dec 30, 2020

Pommie said:
If the diode is connected from negative to positive then it is there so it blows a fuse if connected wrong. If you didn't have a fuse then you may have damaged other stuff. Just remove it and connect the power supply the correct way to see if it still works. If it's a series diode then it shouldn't have blown.

Mike.

The side of the diode with the stripe was connected to positive and the other side to negative/ground.

It does not work without it. The diode itself has a burn mark and is giving out incorrect readings. I can't see any more damaged components, but since there's a soldered shield on top of some, might've burnt something there.

Pommie · Dec 30, 2020

The diode will have conducted a large current from negative to positive until it burnt out and then the internal circuit will have been supplied with the reverse voltage. My guess is that you've burnt out most of the circuit.

Mike.

hack3rpt · Dec 30, 2020

Pommie said:
The diode will have conducted a large current from negative to positive until it burnt out and then the internal circuit will have been supplied with the reverse voltage. My guess is that you've burnt out most of the circuit.

Mike.

I remember seeing the diode getting a tiny bit red and then I instantly disconnected the circuit. Does this help the chances of it still working or is it still very probably dead?

Pommie · Dec 30, 2020

If you removed the diode and it still didn't work then it's probably dead.

Mike.

hack3rpt · Dec 30, 2020

Pommie said:
If you removed the diode and it still didn't work then it's probably dead.

Mike.

Oh well, maybe I really burnt the circuit. The tablet was drawing 0.066 amps, so it probably would be hard to fix anyway.
I ended up ordering a new charging board (diode is not on this board for some reason, it's on the mainboard power connector) because the old one was destroyed. When it arrives I'll remove the diode again and try it. Might just be that the charging board is faulty, the previous repair store did an awful job removing the usb port and repairing the missing traces.

Pommie · Dec 30, 2020

Why would you remove the diode?

Mike.

hack3rpt · Dec 30, 2020

Pommie said:
Why would you remove the diode?

Mike.

To see if it works without it? As I said, that diode is completely fried, you can see a big burn mark on the top and the readings are all over the place.

crutschow · Dec 30, 2020

A low Rds(on) P-MOSFET connected in series with the plus input is a better reverse-polarity protector as it has a low forward-drop and blocks any reverse polarity input..
You connect the MOSFET in reverse with the drain to the plus power source, the source to the charger circuit, and the gate to ground.

hack3rpt · Dec 30, 2020

crutschow said:
A low Rds(on) P-MOSFET connected in series with the plus input is a better reverse-polarity protector as it has a low forward-drop and blocks any reverse polarity input..
You connect the MOSFET in reverse with the drain to the plus power source, the source to the charger circuit, and the gate to ground.

Thanks for the information! Will for sure be useful in the future!
I've looked through all of my donor boards, but I didn't find any suitable P-MOSFET. Can I follow the advice from the person that commented before?
I know that the circuit is probably burnt, but can I just use a regular schottky diode, or maybe don't even use one at all? In this particular case, all I wanna do is just repair the device and make it charge again.

crutschow · Dec 30, 2020

hack3rpt said:
can I just use a regular schottky diode, or maybe don't even use one at all?

If the diode is in parallel with the input you can use a standard diode, as the low forward drop of a Schottky is only helpful when it is in series with the input.
You don't need the diode if you make sure you don't connect the input in reverse again.

hack3rpt · Dec 31, 2020

crutschow said:
If the diode is in parallel with the input you can use a standard diode, as the low forward drop of a Schottky is only helpful when it is in series with the input.
You don't need the diode if you make sure you don't connect the input in reverse again.

I think I won't connect it in reverse again, but just to be safe, I'll find a diode from one of the donor boards and put it there. Thanks for the help everyone!

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