Transistor activated by finger on base

mitocondrio

So... My transistor does the switching job when I put my finger on its base. I know that NPN transistor are activated with positive charge, so... What happens between transistor, my finger and the earth? What happens to the charge? I don't know much about this, I would like an introduction.
Also, PNP transistors should be activated with a positive charge on the base... So how do them work? Please help me! Thanks :)
Thanks!

carlgrace

You have built up some charge in your body. When you touch the base you are capacitively coupling that charge into the transistor. That's why ESD can kill electronics.

Seems like wikipedia would be a good place for you to start.

Crazymechanic

Hi there, well it's not that easy to imagine the specific setup you have there , well people have different body capacity and resistance , well if that is a BJT then as you probably know the current flows also from the base so unless you can make sufficient current flow the transistor should close after the current is too little to maintain emitter-collector flow.

A MOSFET could either stay open or closed depending on the charge put on it's gate but that is a different case because in a MOSFET the gate is isolated so acts as a capacitor but in a bipolar the base is part of the circuit, so where do you get the charge to put in that BJT to keep it open?
By the way it would be great if you could say the transistor part number/model.And tell more about the circuit you have.
Are you just measuring the transistor or is it in a circuit and then being measured or what?

CWatters

Could be your body picking up the mains and acting like a AC signal injector.

arydberg

see here
http://talkingelectronics.com/pay/BEC-3/PNP-or-NPN.html