Reverse Polarity Protection Diode in Tonebender MKII

[fuzzy645]

In planning my next build (Tonebender MKII), so I decided to be proactive and search the forum for any issues reported by others who have also traveled down the same path.

I noticed quite a few posts with the same issue regarding the use of a reverse polarity protection diode. The posters all report that when they remove the diode, the circuit works just fine. It seems as if most folks are using the schematic posted on FuzzCentral (1st diagram at the bottom of my post).  However, the Fuzzcentral author also has posted a modified NPN version of the circuit with the reverse polarity protection diode (the 2nd diagram at bottom of my post).

So I would like to use this polarity protection diode, but I'm puzzled why several people have reported the exact same problem.  I'm wondering if  the problem is the orientation of the diode is  correct for the NPN diagram, which shows cathode to positive, anode to negative located at the top of the physical diagram.  I'm just wondering if someone adapts this to a PNP circuit and leaves the diode in the same location "physically" (in the diagram as depicted at the top), you would actually have cathode to negative and anode to positive, and that would cause a major problem.    Whatcha think?

I guess the 2 summary questions are:

1).  Should the reverse polarity protection diode always be cathode to positive, anode to negative regardless if PNP or NPN circuit?

2).  I guess even before question 1, I should ask if the way this diode is configured in the 2nd diagram below correct, or is there a better way to  do it?

Thanks!

[Kesh]

1. Yes, for diode in parallel with supply systems like this. So long as we remember that for PNP circuits like this the positive is the ground.

2. It's correct, but there may be better ways to protect against reverse polarity.

[fuzzy645]

1. Yes, for diode in parallel with supply systems like this. So long as we remember that for PNP circuits like this the positive is the ground.

2. It's correct, but there may be better ways to protect against reverse polarity.

Thanks for your reply Kesh, much appreciated.  I assume it would then be hooked up as follows:



I've been reading some other threads and it seems the consensus is this approach will only offer moderate protection against incidents that are short in duration, such as if if someone were to accidentally reverse the battery momentarily.  Not sure if something exists that would be better but still pretty simple to implement.

[Kesh]

No. That runs from ground to ground and is useless. Your circuit doesn't have DC-in shown, so it kind of depends. But normally just put it where it usually is but reversed.

[fuzzy645]

No. That runs from ground to ground and is useless. Your circuit doesn't have DC-in shown, so it kind of depends. But normally just put it where it usually is but reversed.

OK, thanks.  So would this be correct instead?

[LucifersTrip]

Slightly OT, but since this relates to power supplies....many of the MKII's I've built do not work well with a 9V adapter.

I do not get oscillations or any type of wailing...I get a degraded tone, lower volume, and many times a small hiss (shhhhh). So, definitely start by powering with a battery.

If you want to use a 9V adapter, bring the power in via a 50-100 ohm resistor. That solved the problem in all cases.