Pnp test box

[Josemitejam]

Hey everyone,

So I made this npn test box which has been a life saver. But I was wondering, if I wanted to make it pnp, could I just take out the dc jack and install a battery snap (but backwards, and flip the LED)?

Here's a picture:

[mozz]

Does it test them or is it just for substituting?

[Josemitejam]

Oh sorry, it's just for testing a pedal before the off board wiring happens. So what I'm wondering is if I want to switch it from testing npn circuits to testing pnp circuits, would all I need to do is swap out the dc jack for a battery (but the ground becomes positive, etc.) and then swap the LED? I'm pretty sure that's the case, but just double checking.

[mozz]

Is it really testing them or are you using it to swap transistors?

[Josemitejam]

It's not for testing transistors but for testing a circuit..like "rock it before you box it". But this one is specifically set up only to test npn circuits , I'm just wondering if what I need to do for pnp test box is swap out the dc jack for a battery (but the ground becomes positive, etc.) and then flip flop the LED?

[GibsonGM]

To use this for 'true bypass testing' a positive gnd circuit, I'd take a right-sized barrel plug from my junk box, determine which is center/outside, and attach a 9V battery clip to it.  CLEARLY label this plug setup at REVERSED.   Then just plug it in, now reversed from normal negative ground, and use it.    One could place a normally reversed LED in there in parallel to give an indicator.

I wouldn't fuss around with a more complicated system, as that may cause mistakes.  Positive gnd pedals aren't very common, this won't get used much.  Just my 2 cents.

Note: the Boss 'standard' is center negative for the barrel of the plug.   so your battery snap rig would be center POSITIVE for reversed use with the existing jack.     AND - If your PCB has any kind of reverse-polarity protection, say a diode reversed...then using the 1 rig to test both gnd types WON'T work.    TBH, I'd just keep the neg gnd setup, and for a one-off, would be careful about my wiring of switches   

[Josemitejam]

Awesome! I'll give that a shot - thanks!

[GibsonGM]

You're welcome! 

You can (and should) use your meter to verify that the battery snap is set up to feed the barrel plug the 'right' way for this before you start plugging it into the jack feeding your effect board, lol 

Typically if you have the probes backwards the readout will show " - 9V", so you'd want to see " 9V " when the red probe is in the center of the barrel, with black on the outer sleeve, for this setup.  A minus means the opposite is true, of course.   You can verify this with your meter using the 'right' power supply you have now.   You probably know this but it doesn't hurt to be safe.