Npn transistor question...

[Racer53]

I'm using an NPN 2n2222a transistor in my timer project, as a switch, and I need to know how many volts I need to get it to switch...

Any help is appreciated...

 

[crutschow]

The base emitter junction of an NPN transistor has a turn-on voltage of about 0.7V. But a bipolar transistor is a current operated device so you normally put a resistor in series with the base to limit the base current to the desired value. The typical base current to fully turn on (saturate) a bipolar transistor is Ib = 1/10 of the collector current. So you select the base resistor value to achieve that base current as determined by the control voltage or Rb =(V-0.7V)/Ib

 

[Racer53]

My problem isn't having too much voltage to activate the switch it's having too little...

There is 6volts coming into the collector, and I can use up to 1.5volts at the base/emmiter junction...I have tried this but it doesn't work...

Make sure I have this right...I have 6v. coming into the collector, and 1.5v. at the base emmiter junction, the circuit then continues into an LED and then the negative port of my battery...I am trying to use the small voltage to switch the large voltage, am I going about this the right way???

 

[Reloadron]

If you look back at Carl's post, see where he mentions the collector current? The collector current needs to be known so a base resistor can be chosen. The 6 volts doesn't say much as to collector current. Understand that you are using the transistor as a switch so you want to drive it into saturation. That is a given. If you have 6 volts applied to the transistor collector and the emitter is at ground and you turn on the transistor you have an effective short to ground. What is between the 6 volts and the collector? What is the load you are switching?

Take a look at this link to better understand the transistor circuit and base resistor selection.

Ron

 

[crutschow]

My problem isn't having too much voltage to activate the switch it's having too little...

There is 6volts coming into the collector, and I can use up to 1.5volts at the base/emmiter junction...I have tried this but it doesn't work...

Make sure I have this right...I have 6v. coming into the collector, and 1.5v. at the base emmiter junction, the circuit then continues into an LED and then the negative port of my battery...I am trying to use the small voltage to switch the large voltage, am I going about this the right way???

It sounds like you have the transistor configured as an emitter follower which won't work for what you want.

Post a diagram of your circuit.

 

[alec_t]

1.5V isn't too little. If you have applied 1.5V to the base of the transistor without using a series resistor to limit the current then the transistor is probably dead. Like-wise if you have applied 6V directly to the collector, rather than via a load resistor, and switched the transistor on. We need to see your circuit schematic.

 

[Grossel]

As mentioned, a schematic would help a lot to identify the error.

Until a schematig is given, the official explanation is that you've blown your pnp transistor with 1,5V.

 

[crutschow]

Attached is a circuit to turn on a 20mA LED using a 1.5V control signal.

 

[Racer53]

Sorry been busy all morning...That schematic is exactly what I've been looking for...I'll pick up another transistor and try again...

Thanks for all replies...