Problems with transistor on test circuit (beginner)

[w-mo]

Hello!
I am fairly new to electronics and have a problem understanding what's wrong here. I always get the 3V3 from the base on the NPN (2N2222A, in the picture is a 2 missing) emitter but not the 5V from the collector.

I have tried pretty much every possibility I could think of, including using a different (or no) resistor, flipping the NPN around and exchanging it, but always i get the voltage (or slightly less of it )from the base. I could even let the collector without anything on it - same result. Do I have a faulty batch of NPN or do I have something very basic about the NPN not understood?
Thanks for your time!

 

[davenn]

Hello!
I am fairly new to electronics and have a problem understanding what's wrong here. I always get the 3V3 from the base on the NPN (2N2222A, in the picture is a 2 missing) emitter but not the 5V from the collector.

hi there
welcome to EP

Well you have the base permanently tied to 3.3V, so you will always get about 3.3V, minus a small voltage drop across the 10 Ω resistor

The collect is tied to 5V, so it should always see 5V there as well

so is the motor turning on or not ?
is the ground connection on the motor the same ground connection as for the 5V and 3V ?

where are the 3V and 5V rails coming from ?


Dave

 

[Minder]

Where did you get the circuit from, BTW?
M.

 

[AnalogKid]

What is it you want this circuit to do?

ak

 

[w-mo]

Thanks for the answers!

the circuit is self made. I just wanted to switch the motor on with the smaller 3.3V current. The power comes from a usb powered supply for a breadboard that has a 3.3V and 5V output.

The idea is that a switch the (vent) motor on and of with an output signal from a raspberry pi. The motor should run on the 5V supply. It can run on 5V but stays off with 3.3V. with the upper setup it stays off as it only gets the 3.3V from the base. If I plug the base into the 5V supply without the resistor it runs.

Edit: Yes the ground is the one from the 5V and 3V.

 

[davenn]

OK

The transistor only requires 0.7V to turn it on. maybe you have damaged it when you applied 5V directly to the base
as there would have been no current limiting resistor

try a new transistor, apply 5V to the base via a 1k resistor and let us know the results

at this stage, without seeing a pic of your construction, we are assuming you have correctly identified the
correct base, emitter and collector pins of the transistor



Dave

 

[w-mo]

Okay, ill give you an update tomorrow. If it's not working ill give you a foto.

Good night

 

[cjdelphi]

You designed it wrong, motor says 5v, and i presume the 3v is to turn on the motor?

If so, this is far more efficient

 

[davenn]

You designed it wrong, motor says 5v, and i presume the 3v is to turn on the motor?

and he is driving it with 5V via the transistor !!

why do you say it's wrong ?
what is your point of the 80 Ohm resistor in the 5V line ?

 

[cjdelphi]

The resistor/80ohm resistor is just the load...

He's driving the motor with 3v (schem says 5v)

 

[(*steve*)]

The problem is the load being in the emitter rather than the collector (assuming the motor requires 5V).

However a diode across the motor would be advised me (and the absence may have caused damage to the transistor).

 

[cjdelphi]

Takes a lot to stop a 2n2222

But for a motor use a ceramic cap?
For relay, a diode in reverse across it?
But for a cfl, capacitor and resistor / snubber?

All for 1 thing inductive voltage spikes, any more ways of snubbing/shunting!

Just thinking out loud, but a TVS diode could be used ? For any of these applications? (MOV for high voltage?)

 

[davenn]

He's driving the motor with 3v (schem says 5v)

no, he's using 3V to turn on the 5V through the transistor
look again

and your 80 Ohm resistor isn't needed ... I see no point in it


But I prefer Steve's advice and I also prefer to always put the load ( the motor) in the collector cct

 

[cjdelphi]

"no, he's using 3V to turn on the 5V through the transistor
look again
-----------------
No, you look again, yes the transistor will switch on, however the motor only gets 3v not 5v like the op wants!




and your 80 Ohm resistor isn't needed ... I see no point in it
-----------

There is no point ... it's simply a dummy load as there's no motor in the simulator i was using


But I prefer Steve's advice and I also prefer to always put the load ( the motor) in the collector cct
-------

He's refering to the circuit i posted!


Maybe it's time to brush up on your transistor skills!

 

[davenn]

No, you look again, yes the transistor will switch on, however the motor only gets 3v not 5v like the op wants!

so you are saying there will be a 3V drop across the collector - emitter jnct ??

that's a crappy transistor mate

 

[cjdelphi]

No, that's how npn's work, the voltage on the base -.6v = out

2.8-3v to the motor! (The transistor is acting as a pass transistor)

 

[cjdelphi]

The op clearly states he wants to run it with 5v not 3v...

 

[cjdelphi]

The idea is that a switch the (vent) motor on and of with an output signal from a raspberry pi. The motor should run on the 5V supply

 

[AnalogKid]

"no, he's using 3V to turn on the 5V through the transistor
look again
-----------------
No, you look again, yes the transistor will switch on, however the motor only gets 3v not 5v like the op wants!

Maybe it's time to brush up on your transistor skills!

Glass houses...

No, the transistor will not switch on. The transistor is configured as an emitter follower. It will not saturate, so there is no switching and no "on". However, the OP does not want an emitter follower, he does want a switch, and his circuit is not correct for that. There is nothing in either OP post that indicates that he wants 3 V across the motor. He does say that he wants to control the motor with 3.3 V, which is not the same thing.

If the OP wants to stay with the same logic polarity out of the uC, his options are either to add a PNP transistor (and keep one end of the motor grounded), or use the 2222 as a common-emitter saturated switch with one end of the motor tied to 5 V.

ak

ps. There is no "collector-emitter junction" in a BJT.

 

[cjdelphi]

In this instance, for all intents and purposes it's switched on 3v on the base, it might not be fully saturated, but it's more "on" than "off"

Ps i made a circuit to do what he wants, hence all the fuss!