Accidental Battery Polarity Reversal?

[Revolver1]

Hi all,

I'm new here and have only recently started building a few pedals assisted by alot of great reading around here.

This may seem like a really silly question but I have tried searching and can't seem to find an answer.

If I have a PNP circuit like a Fuzz or a Rangemaster with the positive battery lead connected across one of the Jack sockets, (as in the positive ground to the battery is disconnected when the Jack plug is unplugged).

If I had the Jack's unplugged and for some reason accidentally reversed the polarity when changing the battery would this damage the circuit or would it be safe with the Jack disconnected?

[antonis]

Hi & Welcome..

How did you manage to reverse polarity of a 9V battery..??
(battery poles shouldn't snap on connector..)

P.S.
As fas as IN plug was out of respective jack, there was no closed circuit..

[PRR]

> battery poles shouldn't snap on connector..

You can make momentary contact. In extreme cases that can be enough. However most small audio circuits will take a long reverse voltage without dying.

[Revolver1]

Hi all and thank you for the speedy response.

No, Thank God I didnt do it. Perhaps I should explain.

As an example one of the pedals I built was a Sola Sound mk1 tonebender. From what I read this is a difficult circuit to get sounding right. I followed the recommended HFE on the layout drawing I was following. When I first fired it up it was making a bit of noise so I swapped out Q1 for another Transistor I had that was pretty close HFE wise and that fixed it.
Gave it a blast and I'm blown away with how good it sounds.   

I generally like to build things like this as close to the original as possible so I don't like to add extra stuff if it's not really needed but I started to think about the reverse polarity issue.  I'd hate to fry this thing as I like it so much and I feel I got lucky as a novice to have it sound exactly as I imagined.
It's not likely I'd find another trio of Transistors like this again.

Although It's not likely to happen there is the chance if I fumbled the battery snap when changing out a battery it could touch down reverse polarity.

Or if I accidentaly tried to connect it and it was reversed. Even though I couldn't permanently connect it in this way the terminals still would have touched.

My thinking was that with the Jack removed as it always would be when I changed out a battery it would be safe because the power couldn't flow.

As I said at the start I'm pretty new to all of this, especially working with a reversed ground so I just wanted to check with some of you more knowledgeable people to make sure I couldn't accidentally damage the circuit.

I've done reverse polarity protection on a Grey Way build I did but that has battery or DC Jack supply options so I thought it necessary.

Because this is a Fuzz I was happy just to run it off a battery.  I don't want to add unnecessary circuitry but more importantly I don't want to fry a great pedal.

Does that make sense?

[Revolver1]

Sorry, that was meant to say Grey Wah.

By the way the power isn't switched by the foot switch as per the original, just the signal.

[Phend]

A diode should help.
Here is one method, see D1:



Another is by putting the diode across the - and + of the battery.
If the battery is connected correct there will be no flow thru the diode.
Connect the battery wrong and you make a momentary short across the battery, hence protecting the circuit.

The first method, the diode will drop the voltage a tad.

I Believe these methods should work for PNP, not being the expert here others could add more. Of course the diode would need to be put in properly.

[Revolver1]

Thanks for the response Phend.

So does that mean the circuit can be damaged by reverse batery polarity even with the Jack disconnected from the Jack socket?

[antonis]

No..

With jack disconnected, there is no closed circuit..
(there is discontinuity between Ring and Sleeve, hence no way to GND..)

[Phend]

For clarity, the diode goes from one side of the battery to the other, usually at the beginning of the circuit.
In other words,  take a 9v battery and put a diode across the terminals. Connect it one way, the correct way, and no current will flow, connect it the other and the battery will go dead fast, of course that is the wrong way to put it in the circuit. So putting the diode in your circuit the correct way no current will flow thru , Unless you try to connect the battery wrong way by mistake current will flow thru the diode and not thru the circuit thus protecting it, momentarily of course because you will quickly know you are trying to connect the battery the wrong way.

[Sooner Boomer]

For clarity, the diode goes from one side of the battery to the other, usually at the beginning of the circuit.
In other words,  take a 9v battery and put a diode across the terminals. Connect it one way, the correct way, and no current will flow, connect it the other and the battery will go dead fast, of course that is the wrong way to put it in the circuit. So putting the diode in your circuit the correct way no current will flow thru , Unless you try to connect the battery wrong way by mistake current will flow thru the diode and not thru the circuit thus protecting it, momentarily of course because you will quickly know you are trying to connect the battery the wrong way.

A great way to do this would be to use an LED as the diode. Reverse the leads, and it lights up.

[antonis]

A great way to do this would be to use an LED as the diode. Reverse the leads, and it lights up.

And blow out almost instantly..
So, no reverse polarity protection anymore..

P.S.
The above can't be happen 'cause LED should already be destroyed by 9V reverse voltage applied to it when polarity is correct..

[Revolver1]

Thanks guys, I think that really answers my question.
For my needs no need for reverse polarity protection as long as I disconnect the jacks before changing out the battery.

I wouldn't go the led route as this would likely introduce switch pop that drives me nuts.

In fact I'm going to remove the leds from my Anologman pedals for this reason.

Thanks all, really appreciate it.

[GibsonGM]

May I ask why you don't want to do at least a reversed diode across the power leads? It isn't in-circuit unless you do the power reversal, and costs pennies...
no big deal, but it COULD happen   

YMMV in the end.

[Phend]

As the saying goes, $hit happens then it dies, or something like that.

[antonis]

If one can't afford even 300mV, or so, voltage drop for a series Schottky diode and doesn't want to mess with more complicated solutions (like Mosfet/Zener configuration), he better watch out his action when powering up..!!

[Revolver1]

Sorry boys, now I'm completely  confused???  I thought the consensus was Jack disconnected = no power can flow???
Therefore no need for protection?

As I said, I've done it before and even have parts laying around but what's the point if power can't flow with the jacks disconnected?

So to be clear let me ask the original question once again.

If the jacks are disconnected and I reverse the polarity of the battery snap can the circuit be damaged?

Cheers

[antonis]

For third time (or so), NO...!!!!

Stereo IN plug OUT of jack -> Power circuit, OPEN -> NO current flow either way..!!

Clear enough..??

[Phend]

..x

[GibsonGM]

protection would be for when you (or a friend!) stop being 100% PERFECT and forget to pull the plug out....or pull the output plug by accident instead, having flipped things upside down...it is 10 cents!     you work hard on your circuit - just take the time!   

[Sooner Boomer]

A great way to do this would be to use an LED as the diode. Reverse the leads, and it lights up.

And blow out almost instantly..
So, no reverse polarity protection anymore..

P.S.
The above can't be happen 'cause LED should already be destroyed by 9V reverse voltage applied to it when polarity is correct..

I guess you've never heard of current limiting resistors?