Transistor Help Please

[kittydog42]

I am trying to make a circuit that takes a 5V pulse in and provides a matching 12V pulse out. It doesn't matter if the output is inverted because it is changing state 3 times per second at a minimum. Both of the pulses have a common ground, and 12V power is available.

I used an NTE128 (NPN transistor). I wired the 5V pulse through a 1000 ohm resistor to the base. I wired the common ground to the collector. I wired a 1000 ohm pull-up resistor from the emitter to the 12V power source. The problem is, I am measuring only 8.5V on my oscilloscope at the emitter. It does pulse up and down to 0.7V as I was expecting, but I thought that the output would be 12V, not 8.5V. Another thing that I can't understand is that the voltage seems to rise when I increase the pull-up resistor value, which is the opposite of what I expected.

Are the any ideas as to why I am only seeing 8.5V at the emitter, when the pull-up is tied to 12V? Could it be my choice of transistor? Any help would be appreciated.

 

[ericgibbs]

I am trying to make a circuit that takes a 5V pulse in and provides a matching 12V pulse out. It doesn't matter if the output is inverted because it is changing state 3 times per second at a minimum. Both of the pulses have a common ground, and 12V power is available.

I used an NTE128 (NPN transistor). I wired the 5V pulse through a 1000 ohm resistor to the base. I wired the common ground to the collector. I wired a 1000 ohm pull-up resistor from the emitter to the 12V power source. The problem is, I am measuring only 8.5V on my oscilloscope at the emitter. It does pulse up and down to 0.7V as I was expecting, but I thought that the output would be 12V, not 8.5V. Another thing that I can't understand is that the voltage seems to rise when I increase the pull-up resistor value, which is the opposite of what I expected.

Are the any ideas as to why I am only seeing 8.5V at the emitter, when the pull-up is tied to 12V? Could it be my choice of transistor? Any help would be appreciated.

hi,
If this is correct NTE128 (NPN transistor)., you have the transistor upside down.

 

[kittydog42]

As in, wire the 5V pulse to the emitter, and the pulled-up output to the base? I am not sure what you mean by upside down, but thank you for the reply.

 

[ericgibbs]

As in, wire the 5V pulse to the emitter, and the pulled-up output to the base? I am not sure what you mean by upside down, but thank you for the reply.

hi,
Its a NPN transistor that means the collector should be at a positive voltage with respect to to the emitter also the base should be positive wrt the emitter.

Connect the collector to +12V thru the 1K0 resistor, the emitter to 0V

Connect the incoming +5V signal to the base via a 470R resistor and measure the output between the 0V line and the collector.

The signal will be inverted but you say thats not a problem.

 

[Boncuk]

Are the any ideas as to why I am only seeing 8.5V at the emitter, when the pull-up is tied to 12V? Could it be my choice of transistor? Any help would be appreciated.

How do you see 8.5V at the emitter if it's tied to ground?

No matter what the NPN transistor does, an emitter should always have ground level.

Take a look at the screenshot. It shows clean 12V with arriving 5V pulses. (measured at the collector)

Boncuk

 

[Roff]

How do you see 8.5V at the emitter if it's tied to ground?

No matter what the NPN transistor does, an emitter should always have ground level.

Take a look at the screenshot. It shows clean 12V with arriving 5V pulses. (measured at the collector)

Boncuk

He has the collector and emitter swapped, as he said. The transistor is still operating as a low (reverse) beta NPN, and the Vbe breakdown of ≈7.8V, plus 0.7V Vbc, is giving him 8.5V on the output when the transistor is off, instead of the 12V he was expecting

 

[Boncuk]

He has the collector and emitter swapped, as he said. The transistor is still operating as a low (reverse) beta NPN, and the Vbe breakdown of ≈7.8V, plus 0.7V Vbc, is giving him 8.5V on the output when the transistor is off, instead of the 12V he was expecting

Aha! I should have known that.

 

[kittydog42]

Yeah, that was it. I had it 'upside-down'! Thanks for the help.