SOLVED Are 78L09s susceptible to damage from reverse polarity?

[Harry Klippton]

Yesterday, I brought my brother in law the Leprechaun I made for him, and it immediately didn't work for him. I had played with it for a week with no issues. I believe he plugged it into reverse polarity. He had an unlabeled wall wart with a red tip on the barrel jack and said he didn't know polarity mattered. The intelligent bypass on the regeneration footswitch seems to still be working, but the basic relay bypass on the main bypass footswitch isn't working.

All 3 boards are still getting 9v. The regulator on the intelligent bypass is outputting 4.5v, but the regulator on the basic relay bypass is only putting out 1.2v, not enough to trigger the relay it seems. Anything else I should investigate? Is the relay also sensitive to reverse polarity?

 

[steviejr92]

Im pretty sure the Voltage Regulator is messed up just did a quick search and found this....

https://electronics.stackexchange.com/questions/611485/lm7805-reverse-polarity-protection

Now thats relating to an L78L05 but im sure since its a voltage regulator nonetheless it would behave the same.
Im sure someone else here will have a better answer for you but i hope that helps...

 

[Harry Klippton]

Confirming that the main board and the intelligent relay bypass board both function correctly still, so I've isolated this to just the basic relay bypass board. Gonna order a new regulator now

 

[Harry Klippton]

Indeed swapping the voltage regulator was the solution. If I were to use the relay bypass boards again, I think I'd want to add a protection diode from the DC jack

 

[steviejr92]

I think I'd want to add a protection diode from the DC jack

Would this be a secondary diode to the one that’s already on the board? If so that would definitely protect against reverse polarity?

 

[Harry Klippton]

Would this be a secondary diode to the one that’s already on the board? If so that would definitely protect against reverse polarity?

There's no polarity protection on the relay bypass boards afaik, just in the effect board

 

[steviejr92]

There's no polarity protection on the relay bypass boards afaik, just in the effect board

Got it.

 

[Harry Klippton]

The two diodes might be protecting the relay? But they're after the voltage regulator. Paging @Robert

 

[steviejr92]

So your original idea of putting it on the dc Jack might be the way to go…

 

[Robert]

Those are flyback diodes to prevent issues from kickback voltage from the relay coil. The best plan would be to tap voltage from an area of the circuit that is already protected from reverse polarity (like the cathode of the reverse polarity protection diode of the other relay bypass module).

I'm looking into adding polarity protection to the basic modules. The non-latching is no problem but the latching module doesn't have much free space for an additional diode.

 

[Diynot]

Necromancing here, but @Robert, just populating the latching basic bypass boards and the build doc calls out 2x 5817 and no place to put them that I see. Interestingly, the screen print on the back shows a spot for a diode of some sort right under the 3904 atop the ne555. Am I to try and double stuff the base and emitter holes, or tack solder to the leads, or do I need to put one 5817 in line with the 9v supply AND tack solder the other 5817 where indicated?

 

[Robert]

I think that parts list need to be corrected. Follow the silkscreen on your board.

For polarity protection you either want to use an inline 5817 with the 9V supply or tap the 9V supply from a point that is already protected against reverse polarity. (eg: the cathode of the polarity protection diode on the main PCB)


The LED pad on the back is for mounting an indicator LED directly to the relay bypass board rather than having it control the LED on your main effect PCB.

To do that you omit Q4 and install the LED according to the symbol on the back. R5 becomes the current limiting resistor for your LED so adjust that value accordingly.

 

[Diynot]

I think that parts list need to be corrected. Follow the silkscreen on your board.

For polarity protection you either want to use an inline 5817 with the 9V supply or tap the 9V supply from a point that is already protected against reverse polarity. (eg: the cathode of the polarity protection diode on the main PCB)


The LED pad on the back is for mounting an indicator LED directly to the relay bypass board rather than having it control the LED on your main effect PCB.

To do that you omit Q4 and install the LED according to the symbol on the back. R5 becomes the current limiting resistor for your LED so adjust that value accordingly.

Thank you sir