Hello
I am a newbe to this and have a basic question about PNP transistors . I want to set up a board to drive some soleniods in a gearbox , the solenoids are earthed inside the box so I have to supply 12v positive to them to operate , so I can't use the NPN as listed in the quide .
I looked at the circuit for a PNP transistor and selected a tip137 , alas it does not work , hence my question about emiter-base voltage .
When I selected the PNP it said Base voltage 5v , great , what the arduino puts out etc . But reading the data sheet it states Emmiter-base voltage 5v .
As the Emmiter is 12v and the base is 5v , I guess this leaves me a Vebo of 7v at best and the transistor is never turning off .
Is that correct ?
Using PNP (or P-type FETs) is tricky. They are often needed as so called "high side drivers" - exactly as in your case - because otherwise you would have to provide a voltage ABOVE the Vcc to make them switch. This is not an issue when you use collector resistors, but this is nothing you want to do in high current switching..
When using a PNP you just have to provide 5 volts BELOW Vcc (where the emmitter is connected in this case.).
In your situation - I think - ground is not necessarily connected to Arduino ground!?
In this case I should propose to just use a standard NPN transistor, connect Arduino ground to the emitter, and the base through a 1k or something to the output pin.
When you do have to use the PNP this needs connecting Vcc of the Arduino to Vcc of the other device, and the base as said, you than switch it "active low".
In both cases make absolutely sure that there is no ground connection!!
Also, lookout that the Arduino is not powered by the same 12 volts supply as your device. This is exactly a "ground connection", but you might not recognize it immediately ....
That is the next best solution; you can of course use any open collector driver to , e.g. another transistor.
In fact a resistor connection +12V --- 47k ---x---- 470k ---- ground should also work with a MOSFET. The Arduino line and the gate is connected at x, and must be toggled between INPUT (high impedance) and LOW. This however might switch a little bit slower than the other solutions.