a lesson learned ;)

[pinkjimiphoton]

i always used to use TWO protection diodes in circuits i built, one inline with the B+ and one to ground.
at some point i was told it was redundant and unneccessary, and i could see why, so i just started using a single diode.

so i'm at this <kinda lame> guitar show last week, and i gotta valvecaster kinda dealio plugged in with a 12vac wart @ i think 2 amps.  somebody wanted to try something, which always gets me excited cuz often once they try it, they buy it at these things and i can always use funding for the utility fuzzbox research kitchen <i was honored recently to give one dweezil zappa a monkey balls octave up... i give a lot of shit away> so i was a bit casual and not paying attention and plugged the damn ac into the dc in on the other pedal. LOUD buzz. horrible. i immediately unplugged it, realizing there went the sale.. i was like, shit, sorry, my bad, this one must have a problem!!
and i grabbed another one. did it a couple more times. i was like... wtf!!!!
so by then they'd left unimpressed, and i tried a few other pedals.
i watched a midrange booster i'd built let the magick smoke pour out when i realized that my dumb@#$% ass had been plugging in the 12vac wart into 9vdc jacks.
believe it or not, all but the one pedal survived, that one  cooked the dip ic i had in it... something nice and expensive, can't remember which part, but like 3 bux a piece or so at tayda lol.. or some esoteric oddity i'd bought on ebay for a project i still have to get to or whatever <like jc's killer phaser... got all the parts, but still waiting for a break in the usual shit to get to it>

but here's the thing... the pedals i built the old way? with both diodes? no damage, no HUM, either... i'm talking ge fuzzface ffs!!! i'm not really sure why this would be, other than it being maybe a full wave rectifier instead of half wave?
but i always thought you needed 4 diodes to do a full wave.. sucks having literally slim to nun <g> electrical knowledge, i am STILL a 10 toed freak of a monkey with a breadboard n all..

but i think in the future, i'm gonna include that measley 1 cent part on builds as insurance against people that hopefully are smarter than me  making the same mistake i made, and repeatedly. just the other day. after monkeying with electronics since 1977, and pedals for a decade or so.

so.... is it that extra diode? or was that owsley at berklee all them years ago changing my perspective?


amazingly, one pedal DID smoke but appears to work fine. go figure!!!  i gotta investigate more...

but anyways... if its gonna prevent this from roasting a circuit, and allow the circuit to run on either ac OR dc, is it worth the expense? is this real, imagined, mojo?

love to hear everybodys thoughts.

peace!

[EBK]

12V is not, generally speaking, a huge voltage, but it would be above the limit for some charge pump ICs (blown charge pump is what immediately jumped into my head).
Otherwise, your protection diode essentially would've made your circuit see its power turn on and off repeatedly, which could (maybe) cause damage under circumstances that someone else can describe.

I'd say the extra (second) diode is not what made the difference.

Edit:  Forgot to say the most important part -- I'm sorry that you had to go through that experience.

[reddesert]

For the pedal with a single diode that got fried, was it in series with the V+, or is it a parallel diode, from ground to V+ ?

If a parallel diode conducts and fries itself sacrificially, if it then goes open-circuit instead of being shorted, then your circuit is no longer protected.

[pinkjimiphoton]

For the pedal with a single diode that got fried, was it in series with the V+, or is it a parallel diode, from ground to V+ ?

If a parallel diode conducts and fries itself sacrificially, if it then goes open-circuit instead of being shorted, then your circuit is no longer protected.

it was parallel wired. the ones with the series and parallel didn't even hum, they still worked and appear to have been undamaged.

i stopped using the series diode a couple years ago, so thats what i'm gonna start including again, its worth the small voltage drop i think to make it a little more bulletproof.

thanks for the responses.

i will check the pedal that smoked, i bet the diode is fried in that one, tho it still works. thanks for the advice!!

[Phoenix]

Yeah, the redundant one was the *PARALLEL* reverse biased one, not the series one, you dropped the wrong protection diode.

[pinkjimiphoton]

maybe i'm confused bro, the one i dropped is the one in series, not the one to ground. the one in series is what i think i'm gonna start adding again.

[Vitrolin]

what about, not using the same connector for AC and DC??
EHX for example uses some old speaker connector for 12VAC adapters

[samhay]

>the one in series is what i think i'm gonna start adding again.

Do this. The voltage drop is a price worth paying, particularly if you use a Schottky - the 1N5817 is popular.

[mac]

Do this. The voltage drop is a price worth paying, particularly if you use a Schottky - the 1N5817 is popular.

The small diode on resistance with a big cap to gnd makes a nice noise filter,
and when the diode stops the negative voltage of an AC source, the cap provides a bit of charge to smooth the ripple, like in rectifier.

mac

[anotherjim]

I wouldn't tell anyone to drop the series diode. That's the most important one for me.
Now, both the parallel diode - if its wired after the series, then it should never ever do anything, not even with a bad supply and might as well not be there.
Then if the parallel diode is before the series, it's going to smoke with wrong supply.

...but if you do put in a series diode, the pedal circuit must have a supply bypass capacitors even if a published circuit doesn't show any. Without a series diode, the circuit benefits from the capacitance fitted in the psu. With the series diode the psu capacitor is blocked by the diodes one-way action. If anyone refuses to to use series diode protection - because it made the pedal sound "funny" - you have to ask if they fitted an adequate supply capacitor in the pedal.

To re-iterate what's been said many times, old circuits like the FF ARE different.
They only ran on battery and the battery is a very good bypass capacitor. So, they did not fit a bypass capacitor. As a psu capacitor may be a long way from the pedal circuit, the circuit ought to have its own capacitor anyway. I mention supply capacitors because it matters in relation to any polarity protection system fitted.
They are simple circuits, usually with resistors in every path. Reverse power, so long as the voltage doesn't exceed the ratings of the Q's does nothing -  no damage at all. However, a supply capacitor in the pedal, since it's probably got to be a polarized type, will be damaged.
With integrated circuit chips -  any kind - there will be a low impedance path between its supply pins if polarity is reverse. The chip is usually destroyed when that happens.

[Rob Strand]

i watched a midrange booster i'd built let the magick smoke pour out when i realized that my dumb@#$% ass had been plugging in the 12vac wart into 9vdc jacks.

Despite the how to do the protection debate.  Whatever protection you use should provide some protection.  I don't see how the booster got fried.  If you use a parallel diode it needs to be big enough like 1N4004 etc not 1N914.

If you have charge pumps in your design then the charge pumps will fail due to over voltage (not reverse voltage) if you plug in 12V supplies.    To protect against this you need *overvoltage* protection.

[thermionix]

AC

[Rob Strand]

AC

That shouldn't cause it to fail with a protection diode.

If you put AC in a circuit with a parallel protection diode the supply will modulate (forcefully) but it will be largely positive.  You might get a lot of hum at the output.

AC with a series diode might not produce any hum as the bypass caps act like a peak detector and keep a relatively constant supply.  So this version sounds least offensive to the user.

[Electron Tornado]

amazingly, one pedal DID smoke but appears to work fine. go figure!!!  i gotta investigate more...

Ceee-ripes! Whatever you do, don't let that get out or people will think it's an essential part of the process!

"Hey, has this pedal been pre-smoked?"
"Yeah, man, pre-smoked for ultimate tone."
"Groovy."

[italianguy63]

>the one in series is what i think i'm gonna start adding again.

Do this. The voltage drop is a price worth paying, particularly if you use a Schottky - the 1N5817 is popular.

I agree.. I usually do this.  1N5817 in series, and a 1N4001 in parallel.

MC

[Phoenix]

>the one in series is what i think i'm gonna start adding again.

Do this. The voltage drop is a price worth paying, particularly if you use a Schottky - the 1N5817 is popular.

I agree.. I usually do this.  1N5817 in series, and a 1N4001 in parallel.

MC

If you've got a series diode, the parallel one is completely redundant. If it comes before the series diode, it provides no protection, but will itself burn up (potentially taking other components with it) under reverse polarity conditions. If it comes after the series diode, it serves no function at all, unless the Vrrm/Piv of the series diode is exceeded (possible with schottky's), in which case it will just burn up anyway.

[italianguy63]

>the one in series is what i think i'm gonna start adding again.

Do this. The voltage drop is a price worth paying, particularly if you use a Schottky - the 1N5817 is popular.

I agree.. I usually do this.  1N5817 in series, and a 1N4001 in parallel.

MC

If you've got a series diode, the parallel one is completely redundant. If it comes before the series diode, it provides no protection, but will itself burn up (potentially taking other components with it) under reverse polarity conditions. If it comes after the series diode, it serves no function at all, unless the Vrrm/Piv of the series diode is exceeded (possible with schottky's), in which case it will just burn up anyway.

Gotcha.

[antonis]

unless the Vrrm/Piv of the series diode is exceeded (possible with schottky's), in which case it will just burn up anyway.

In such a case, Vrrm/Piv of the single series diode (no parallel diode) should also be exceeded in case of reverse polarity power plugging..
(it will actually be exceeded a bit more due to absence of parallel diode forward voltage drop..)

P.S.
Just to say a teasing Hi, Greg..

[mac]

If you apply 9AC to a 9DC circuit, with no protection at all,
What kills the circuit, the -9v, or how fast it goes from 9V to -9v, or both?

In other words, what is more dangereous, -V or dV/dt?

mac

[Phoenix]

If you apply 9AC to a 9DC circuit, with no protection at all,
What kills the circuit, the -9v, or how fast it goes from 9V to -9v, or both?

In other words, what is more dangereous, -V or dV/dt?

mac

The fact that the voltage is changing doesn't matter at all, if for instance you had a 4.5V peak signal riding on a 4.5VDC bias (so the high and low peaks would be 9VDC and 0VDC in absolute terms), nothing would be damaged at all, though the pedal would not operate over much of that range and may not operate correctly for some of it.
It's only the negative voltage that will reverse bias junctions and cause damage, though if it's 9VAC, which has peaks of 12.726V, it's always possible you could damage something from the overvoltage before the reverse voltage did depending what part of the line cycle you plugged in at, but the reverse polarity is what would seal its fate.

Of course, if you're talking really high frequencies, things start to behave differently, even with a DC bias to keep the voltage all the correct polarity, dielectrics need to be derated at high frequency, but that's definitely not something we need to worry about here.

Just to say a teasing Hi, Greg..

Hi Antonis!