Simple Transistor Help

[Iawia]

Hi. I thought I knew how to use transistors, but apparently not. I am trying to just get the transistor to behave properly to be used in an H-Bridge. I am trying to check proper operation with LEDs.

I cannot get my TIP31C transistors to work properly. I have stared at the literature and internet diagrams, but I am still a bit confused.

Here is my basic understanding:

1) I have hooked up my load to the emitter (LED + 1k resistor).
2) The collector (middle pin), I hook to the power supply (in this case for the LED 5v, or for my 12v motor, 12v)
2) The signal to turn the NPN transistor on or off with 0v or 5v is sent to the base.

This setup is not working. What am I doing wrong? (Sorry bout the crude diagram)

Thx.

 

[Roff]

The emitter voltage should be about 4.3v (Vbe≈0.7V). What is your LED forward voltage? Current should be Iled≈(4.3V-Vfwd)/1k.

 

[MikeMl]

It should work for the led at 5V input to both base and collector. As pointed out by Ron, it is an emitter follower, so the you lose a Vbe drop between base and emitter, so it may not turn on a white Led (Red/Green likely ok)

If the collector is at at 12V, and the base at 5V, it can only pull the emitter to ~4.2V, so it will never work with a motor.

 

[Iawia]

The voltage across the LED to ground (w the resistor also), is 4.2v. The problem I am having is that the LED stays on when only the base is high (5v) and the collector has no voltage on it (separate power supply is off). Why is the LED on if there is not collector voltage???

Thanks.
-Tom

 

[Roff]

The voltage across the LED to ground (w the resistor also), is 4.2v. The problem I am having is that the LED stays on when only the base is high (5v) and the collector has no voltage on it (separate power supply is off). Why is the LED on if there is not collector voltage???

Thanks.
-Tom

The LED is getting power though the base-emitter junction, which, in the absence of any connection to the collector, is just a diode. If you measure base current, it will drop by a factor of beta when you reconnect the collector to +5V or +12V.
The base (or the emitter, in some applications) can be used as the controlling port. The collector is generally not used for control.

 

[Iawia]

The LED is getting power though the base-emitter junction, which, in the absence of any connection to the collector, is just a diode. If you measure base current, it will drop by a factor of beta when you reconnect the collector to +5V or +12V.
The base (or the emitter, in some applications) can be used as the controlling port. The collector is generally not used for control.

Hi Roff,

I will chk the beta drop, the thing is the led stays lit if there is collector voltage on or off. Is this the correct behavior? I am puzzled because it seems to be hooked up right, and you are saying (I think) that it is operating properly, it is just behaving not according to the truth table.

 

[Roff]

Hi Roff,

I will chk the beta drop, the thing is the led stays lit if there is collector voltage on or off. Is this the correct behavior? I am puzzled because it seems to be hooked up right, and you are saying (I think) that it is operating properly, it is just behaving not according to the truth table.

Show us the truth table for an emitter follower.

And yes, the circuit is working as it should.

 

[Roff]

Maybe this will help.

 

[MikeMl]

You haven't yet acknowledged that an emitter follower type of driver can only drive its emitter to base voltage - 0.7V even if its collector is connected to 12V.

 

[Iawia]

Ok thanks for the diagram Roff. I will study this closely.
Much appreciated to everyone's comments!

 

[Iawia]

Apparrently, I was using fairly old resistors, so the connections were weak because there was some oxidation on some of the ends. Something easily overlooked. I got them out of an old box at school, they seem super old, at least 30 years. I thought nothing of it until now. Another great lesson learned! The tip31s work great now. Thanks.