Protection from incorrect power supply?

[Matt]

I've recently sold two different TS type pedals to two different people and let them know what power supply to use.  Both plugged ac adaptors into them which fried a resistor in each pedal.  So my question is, is their a protection circuit that I could incorporate that would prevent parts from frying, take up little pcb real estate and be reasonably inexpensive?

Thanks in advance.

[GrindCustoms]

You're still lucky that just a resistor fried, had that happen and all the JFet, OpAmp and Electro died.

From what i know there's no protection against AC.

[davent]

Seems there is an expensive chip that might do the job, LTC4365. Bit of discussion (started by chromesphere) and links over at diystompboxes a year ago but can't tell whether anything came of it.

Polarity Protection schemes from that diystomp thread.  http://digital-diy.com/general-electronics/419-power-protection-circuits.html

The thread. http://www.diystompboxes.com/smfforum/index.php?topic=101708.0
dave

[alexradium]

On geofex.com there is a good solution,2 transistors and 4 resistors,works with anything.☺

Inviato dal mio Nexus 7 utilizzando Tapatalk

[chromesphere]

Never ended up testing this out.  Could make a good 'add-on' board a bit like the roadrage but sort of a comprehensive input voltage protection board.

[GrindCustoms]

Wow! All good things to know.

[electrosonic]

I just put a 1N5818 schottky diode in series from the power input. It drops about 250mV  when the power supply is connected regularly. If the power supply is reversed is will provide protection as long as less than 30 volts is applied.

If AC power is applied, it will block the negative wave, making a half wave rectifier. This should protect pedal from damage.  The pedal might even still work depending on the size of the filter caps used, though the ripple could cause more hum.

It costs $0.08 from Tayda.

Am I missing something here?

[raulduke]

I use schottky diodes on everything too. You get a small voltage drop, but you also get decent (enough) protection.

Guitarists seem to stand alone in thinking you can plug any supply into any pedal and it will just work.

I think the quicker the demise of the PP3 and battery connections in pedals, the more aware people will become about PSU's and what they can, and can't, use.

[Matt]

First of all, thanks to all who replied.  Lots of good info here! 

You're still lucky that just a resistor fried, had that happen and all the JFet, OpAmp and Electro died.

From what i know there's no protection against AC.

Yeah, very lucky.  Both of these boards were the same and were based off of a paisley drive schematic.  It seems that the 10ohm series resistor took all the punishment.

I just put a 1N5818 schottky diode in series from the power input. It drops about 250mV  when the power supply is connected regularly. If the power supply is reversed is will provide protection as long as less than 30 volts is applied.

If AC power is applied, it will block the negative wave, making a half wave rectifier. This should protect pedal from damage.  The pedal might even still work depending on the size of the filter caps used, though the ripple could cause more hum.

It costs $0.08 from Tayda.

Am I missing something here?

If this will do the trick, it definitely seems like the way to go.  I've seen this method more and more lately in schematics.

[GrindCustoms]

I have plenty of schotky and couple AC and DC adapters, I'll make some tests this weekend. That's pretty cool.

[Matt]

I have plenty of schotky and couple AC and DC adapters, I'll make some tests this weekend. That's pretty cool.

Did you get a a chance to test this out? 

[GrindCustoms]

I have plenty of schotky and couple AC and DC adapters, I'll make some tests this weekend. That's pretty cool.

Did you get a a chance to test this out?

Forgot about it....

Doing it now, give me a few minutes.

[GrindCustoms]

Using a 1N5819 1Watt

9VDC Supply at input, output of diodes gives 8.92VDC

-9VDC at input, output -0.92VDC

No matter how much AC i send through the diodes, tried up to 24VAC, it will pass through and you also get about half DC voltage of the AC supply, wich makes sense since AC is the total of a negative and positive wave and DC being just one side of the 0V.

Both type can flow through... so i'd say that it does'nt protects against AC.

[RobA]

There's some current leakage with Schottky diodes. In some cases, the amount of current that gets through won't be an issue. In others, it could be. One thing I've done is to use a 1N4001 in a crowbar after the 1N5817. I still don't think that this is a perfect solution, but it could help in some situations and it's pretty cheap in cost and complexity.

[electrosonic]

-9VDC at input, output -0.92VDC

Are you measure the open circuit voltage? This seems to me to be the leakage current times your mulimeter impedance (usually 10M) implying 92 microamps of leakage current.

The data sheet  for the Fairchild version shows a maximum of 500 microamps (0.5 mA) at 25 degrees C.

The main question is - will this small amount of current damage anything? I think not, but don't have data to back it up.

No matter how much AC i send through the diodes, tried up to 24VAC, it will pass through and you also get about half DC voltage of the AC supply, wich makes sense since AC is the total of a negative and positive wave and DC being just one side of the 0V.

Both type can flow through... so i'd say that it does'nt protects against AC.

Not sure what you mean by this - can you explain your test setup - I will spend time at the bench to see what I find.

Andrew.

Edit: Just went to the bench found a 12VAC wall wart. I connected it to a 1N5818 diode then to a typical RC power line filter you see in pedals. (a 47 ohm series resistor and a 100uF cap to ground).

At the filter cap I saw the DC rectified voltage (about 20 volts as expected  12VAC (rms) * 1.41.

The over voltage may cause a problem to a circuit which was designed for 9VDC, but I see no issue with reverse voltage in this case.