What's the difference between these transistors in my circuit?

I built this OR gate with 2N2222A transistors. I did the calculations based on the datasheet but there's still a CE current leak which dimly lights the LED when no buttons are pressed. When I replace the transistors with BC547, the leak seems to go away. Functionally these transistors are supposed to be swappable so why would they be acting differently?



Vin = 9V

R1 Resistance

LED VF = 2V
LED IF = .02A
2N222A VCE(sat) = .3V

R1 = (9V - 2V - .3V) / .02A = 335Ω


Transistor Base Current

IC = .02A
β(MIN) = 100
Overdrive factor = 10

IB = .02A / 100 x 10 = .002A


R2 and R3 Resistance

VBE(sat) = .6

R = (9V - .6V) / .002A = 4200Ω

You should never leave the base (or gate) of a transistor "floating", as circuit leakage (internal to the transistor or external) can cause erratic behaviour. They could even be picking up and amplifying AC "hum" via the switch wires, in your circuit.

Add a resistor between base and emitter of each transistor; 10K or something like that, it's not critical except in really high speed applications.

Interesting, that worked. So basically you want allow the base another way to discharge so it doesn't leak EM interference or something into the transistor?

BobbyHood said:

So basically you want allow the base another way to discharge so it doesn't leak EM interference or something into the transistor?

Click to expand...

You also want to have a path for the collector-base leakage current, which is otherwise amplified by the transistor current-gain to a much high value collector current.

Although the leakage current of a couple of transistors will probably not give enough light to make LEDs visible, it can be worth noting that there may still be a voltage across the LED when turned off. A high-value resistor, 10 k or 100 k, will stop that.

Also, if you have a circuit that relies on the LED voltage dropping when turned off, always add a resistor in parallel. The voltage across an unconnected LED depends on the ambient light level. All LEDs are solar cells to some extent.

Maybe your 2N2222A transistors were made with a bit of rice inside them.
I have bought thousands of transistors but none from ebay.

audioguru said:

Maybe your 2N2222A transistors were made with a bit of rice inside them.

Click to expand...

No, but it have to be a red star symbol inside tha package somewhere