Applying RG Keen's PNP Power Switching circuit

[chptunes]

hey guys.. i've been reading up on power switching.  http://www.geofex.com/FX_images/PNP_power_switching.pdf

is this the right way to apply this circuit?

Thanks..


screencast

[R.G.]

It's not a bad way.

You may be able to increase the 1.5K base resistor to the PNP. The PNP acts like a switch when it's saturated gain is about 10.  JFET circuits like this don't pull much current, a few milliamperes usually. So the base current to the PNP doesn't need to be more that a few hundred uA in most cases. 1Ma is plenty. With 9V in, you lose half a volt to the base emitter, so the base resistor has 9-0.5V = 8.5V across it, and you want 1ma (about...) so the resistor needs to be smaller than 8.5V/0.001A = 8.5K. 1.5K certainly works, but you could get by with 4.7K, 5.1K, 6.8K, or 7.5K for most JFET circuits.

If there's more current loading than just the JFET, lower the resistor to keep the base current no less than 1/10 of the load current.

[chptunes]

thanks a bunch..

[Digital Larry]

So, would that PNP transistor also serve as for backwards-power-polarity-protection?

DL

[R.G.]

So, would that PNP transistor also serve as for backwards-power-polarity-protection?

A little, but not too much. The breakover voltage of the base-emitter is a problem. It will probably prevent damage for momentary reversals, but something like a 9Vac power adapter will still kill it eventually.

[chptunes]

^^^
..ah, good to know.

I installed this power switching circuit in my 12vDC amplifier too.. which draws more than 100mA of current.  An Rbase value of 1.5k seems to be working fine.

My 12vDC amp is described here:  http://www.diystompboxes.com/smfforum/index.php?topic=101184.0


-Corey