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NPN to PNP control? Answered

So as I get into this project the circuit keeps building and now I once again have another problem that I can't figure out. As the title hopefully suggests I am using a NPN PN2222A trans. to control a PNP 2N3906 The idea was to use the 5v from a Ardinuo UNO to control a 12v. Currently I am testing with a 9v I had on hand. I thought it would pretty straight forward but every time I supply power to the NPN it creates the short needed to turn on the LED, but once I cut the power the LED stays on until I create another short (using a probe) uphill of the NPN. The only thing I can think of is that the LED doesn't the resistance needed for the power to re-saturated the PNP. Any thoughts?
I will admit  haven't done any of the math needed yet for the resistors.

Also can someone send me a link to the programs you use for drawing the layouts so I can move past MSpaint?



Best Answer 2 years ago

The way to start if you don't know the relay current is to turn the PNP as much as it can be. The spec sheet indicates 100 ma with a gain of 30. That means the base current needs 100/30 =10/3 = 3.34 ma I'm going to, round up to 5 ma. Then find a Resistor that can allow 5 ma at 12-o.3-o.3 = 11.4v

The formula for R = Vr / I = 11.4V / .005A = 2280 ohms round to a real value = 2.2K


Now lets turn the NPN fully on.. The spec sheet indicates 150 ma with a gain of 50. That means the base current needs 150/50 =15/5 = 3 ma again round up to 5 ma. Then find a Resistor that can allow 5 ma at 4 V ......

R = Vr / I = 4 V / .005 = 800 ohms nearest real value is 750 ohms.

Finally what is the 2.2k Resistor Power = V x I =12 V x .005 A .06 watt


Thanks for the best answer.

Wanted to show you a simpler drive for your coil.




Some micros reset with all their outputs high, like the 8052, an NPN/PNP circuit can have the advantage that its off when the input is high.

ok, this looks like what I am working on. I just got the coils so I will be testing this out, Thanks.


I finally came across Yenka for th diagram part of the question.


2 years ago

IF you are smoking the 2N3906 PNP there are two possible causes.

A] The relay is using more current then the PNP is capable of supplying.

B] Or the reactive diode is wired backwards.

the resistors on the NPN collector are the wrong way round, and the 10k should be more like 2.2k


2 years ago

Blue led is backwards.

And a simpler voltage driver for you.


The math to size the LED resistor is

R = V / I = (9 - 3.3 - 0.3) / .02 = 5.4 / .02 = 270 ohm

I am new to drawing the diagrams, so in the real circuit it is facing the right way, and I need to be able to control the LED (which is a place holder for a magnetic coil, easier to check if it is working with the LED) uphill of the LED. I need to be controlling the ground and power of a grid, but I am using an Ardunio and need to pretty much step-up the 5v to control the PNP. I don't understand why this set-up isn't working.