1N4001 vs 1N5817 TS9-busted

[EGRENIER]

I have a friend with a TS9 that failed after plugging in reverse polarity from a 12v adaptor - smoke, crackle, pop…. After visual inspection it appears that a diode busted, it actually seems to have popped in half.

After research I found this part of the TS9 power circuit:


I didn’t quite understand that as I was expected the diode to be in series, but later read that older circuit used W03C (1N4001) in a “crowbar" formation, where it goes between +9V and ground, with the cathode pointed toward +9V.

From what I understand reverse polarity will generate so much current that the diode bust, thus protecting the circuit.

So I have 2 questions:

Is my understanding right ?

And can this diode be replaced by a 1N4817 in the same “crowbar” configuration and work as a protection diode ?

 

[Robert]

The goal isn't necessarily to destroy the diode, the diode shorts any reverse voltage/current to ground, shunting it away from the rest of the circuit.
Unfortunately that typically results in the diode being damaged, and if your friend didn't have a friend who knew how to replace it the pedal would be just as broken as if it had a shorted opamp.

In fact, if the diode completely opens it is no longer protecting the circuit.

Clip/desolder the remains of the diode out of the circuit then try powering up the pedal (make absolutely sure you use the correct polarity power supply), if it works you can safely replace the diode and everything should be good to go. If it doesn't work then replacing the diode isn't going to help and you'll need to investigate further.

You generally see a standard power rectifier used for parallel polarity protection (1N4001 - 1N4007) but I can't think of any reason a 1N5817 wouldn't work if that's what you have.

 

[EGRENIER]

The goal isn't necessarily to destroy the diode, the diode shorts any reverse voltage/current to ground, shunting it away from the rest of the circuit.
Unfortunately that typically results in the diode being damaged, and if your friend didn't have a friend who knew how to replace it the pedal would be just as broken as if it had a shorted opamp.

In fact, if the diode completely opens it is no longer protecting the circuit.

Clip/desolder the remains of the diode out of the circuit then try powering up the pedal (make absolutely sure you use the correct polarity power supply), if it works you can safely replace the diode and everything should be good to go. If it doesn't work then replacing the diode isn't going to help and you'll need to investigate further.

You generally see a standard power rectifier used for parallel polarity protection (1N4001 - 1N4007) but I can't think of any reason a 1N5817 wouldn't work if that's what you have.

OK so let’s see if I get this right. The design is not to use the diode as a fuse but to open a quick path for the current to ground therefore not sending the reverse current to other component, correct ?

Now if it completely busted in half and didn’t offer that quick path anymore, therefore the “shunting protection” was no longer there and there a high risk that reverse current made it to the circuit. That‘s not a positive outlook…

 

[Robert]

That's correct.

But there's still hope, reverse polarity doesn't always destroy components immediately, and just because the diode looks completely destroyed doesn't necessarily mean it wasn't still shorted across the VCC line. (unless of course you could visibly see it separated into two completely unconnected pieces)

At this point I would remove the diode completely and try the pedal. You don't have much to lose.

Besides, by now you know there isn't a single component in there that costs more than a dollar. You can still save the day.

You could have a bad opamp or electrolytic capacitor.... reverse polarity didn't harm any of the resistors or ceramic/film caps so you can eliminate 90% of the circuit right off the bat.

 

[EGRENIER]

That's correct.

But there's still hope, reverse polarity doesn't always destroy components immediately, and just because the diode looks completely destroyed doesn't necessarily mean it wasn't still shorted across the VCC line. (unless of course you could visibly see it separated into two completely unconnected pieces)

At this point I would remove the diode completely and try the pedal. You don't have much to lose.

Besides, by now you know there isn't a single component in there that costs more than a dollar. You can still save the day.

You could have a bad opamp or electrolytic capacitor.... reverse polarity didn't harm any of the resistors or ceramic/film caps so you can eliminate 90% of the circuit right off the bat.

Thanks Robert, greatly appreciated.