5817 as a reverse polarity protection in series - Datasheets vs. the Interwebs!

[midwayfair]

What ought to have been a simple "does it work" has been complicated by the spreadsheet. The 1N5817 and 5818 specifically say that they have reverse polarity protecton applications.

I've read elsewhere that a Schotkey has too much reverse current leakage and that makes it a horrible choice.

So what's the word?

Anecdotal, of course, but I've used the 5817 in multiple pedals before I started giving this stuff any thought and I've never blown one, so I'd really love it if someone told me everything was going to be okay!

[PRR]

> I've read elsewhere that a Schotkey has too much reverse current leakage and that makes it a horrible choice.

http://www.vishay.com/docs/93256/1n5818.pdf
Page 3, figure 2: typical reverse current at 9V is 0.001mA. Not too horrible.

[midwayfair]

> I've read elsewhere that a Schotkey has too much reverse current leakage and that makes it a horrible choice.

http://www.vishay.com/docs/93256/1n5818.pdf
Page 3, figure 2: typical reverse current at 9V is 0.001mA. Not too horrible.

Could just be that the stuff I read (too lazy to go backt o google for links) was for audio equipment run at much higher voltages, and thus more likely to be an issue?

Thanks for the reassurance, PRR!

[R.G.]

Yep. Schottky diodes are the least bad of the series diode treatments commonly available. They only lose you about 0.4-0.5V and the leakage is not a problem in any pedal I can think of.

[lars-musik]

I have to revive this ancient topic because I fear my standard polarity protection scheme is flawed.

Some time ago, I chose the SD103AWS as SMD replacement for my usual 1N5817 (datasheet: https://www.diodes.com/assets/Datasheets/ds30101.pdf) and a use it regularly in serial configuration.

Now I sold one of my pedals (a King of Tone workaliek with a switchable LT1054 voltage doubler) and soon got the feedback that it didn't work. So I had it sent back and quickly realized that the customer must have applied some seriously wrong PSU, because the whole power section was severly damaged. The schottky was blown and was conducting happily away and the LT1054 and some corresponding diodes were also gone. The customer than admitted that he didn't know what kind of PSU he tried, so here's some room for speculation.

BUT I somehow thought that if a wrong power supply hits the schottky and it got damaged it would blow like a fuse and wouldn't conduct. Now I realize it just opens up (although the peak reverse voltage is 40V) and leaves the rest of the circuit unprotected. Is this the rule with Schottkies or an exception?

[R.G.]

[...] The customer than admitted that he didn't know what kind of PSU he tried, so here's some room for speculation. [...]

There's your problem.

There isn't any good way to tell whether a semiconductor will fail shorted or open. If you know the internal construction very well, you might make a good guess, but it will only be for that manufacturer's specific part, not another manufacturer's part.

At some point, you have to decide whether to make 100% of your customers pay for protection circuitry only needed by the 1% or less that will stick unknown power supplies into the pedal.

There is a bit of marketing wisdom that says that there are customers you DON'T want. It can just be tricky to draw the line.

[anotherjim]

So, we don't know if it was a reverse supply that did the damage, but could have been excessive voltage?

Hmmm.... well, there aren't many "wrong" AC-DC adapters that are centre negative plugs. If it was an AC-AC supply, that could have been a double whammy. Your protection and filtering - diodes, caps, LT1054, could have conspired to produce and sustain the peak voltage of an AC input. LT1054 is 15v max & that failed first.

I guess better protection adds a zener (maybe 12v) across the input, but there has to be some series resistor before it to limit the current. If the zener fails, it could go open or short - there is no perfect protection.

[amptramp]

It is long past time to get away from the tyranny of the 9 volt battery.

If you ran a higher voltage, even 12 volts, you could use a 1N4007 in series and have no problem with any DC supply that wasn't a deliberate attempt to destroy it.  If you look at the guaranteed input and output common-mode voltage for a TL07X op amp, the input common mode range can be as low as ±11 volts running from a ±15 volt supply or 4 volts away from the rails.  At that rate with a 9 volt supply, you have a common mode range of 1 volt that disappears entirely if the battery goes down to 8 volts.  Fortunately, typical devices go from -12 to +15 with ± 15 volt rails or 3 to 9 volts operating from a 9 volt battery.  (Note: this means the op amp should be biased for 6 volts and not 4.5 volts for maximum dynamic range of linear operation with a 9 volt supply.)

[bool]

The best polarity + stupidity protection is a 5watter zener and a 3watter resistor. Of course you can inline a 5817 (or rather a 5819 because it can take more of the "wrong voltage").

It's a bit bulky though ... not a problem if you are after "vintage" buld style but nevertheless it's humongous compared to more common and more svelte methods ...