Simple transistor / LED circuit problem

[batvink]

Hi,

I am testing a simple circuit that I will ultimately drive from a Raspberry Pi. At the moment I am testing on a breadboard. But I cannot get my transistor to switch off, and so my LED is continuously lit. Here is my circuit, more details after the image...


There is 0.6 V between ground and the transistor base. The emitter and base are definitely tied to the same ground, albeit via 2 resistors for the base.
There is 9 volts across the LED
There is 5 volts across the colleco and emitter of the transistor.

The only possible issue I can see is that I am using an old 12 Volt adapter for the power; as you can see it is actually providing 14 volts. However, the emitter and base are tied to the same adapter ground point, so this should eliminate any issues there.

If I disconnect the base from ground, the LED stays lit, and at 0.6V across the base/emitter.

I have replaced the transistor - same result.

The ultimate aim is to feed the base from the 12 volt line, by tying it between the 2 resistors and giving a 6V signal.

Any help gratefully received!

Regards,
Steve.

 

[Bluejets]

without any bias on the base as your drawing shows, then one has to assume the transistor is kaput.
Either that or you have a pnp transistor.

 

[batvink]

Thanks for the quick reply Bluejets.
Now I have the confidence that somebody who knows what they are doing says the circuit looks correct (I got an electronics qualification 25 years ago!), I re-checked the circuit.
Specifically you said to check the transistor to see if it is Kaput or a PNP transistor. Having checked the specification sheet again, I can see I had the collector and emitter back to front

Thanks for your input!

Regards,
Steve.

 

[(*steve*)]

With the collector and emitter connected backwards in this circuit you will have significantly damaged the transistor.

If you have spares, throw this one out and use another one. If not, get a new one and replace this one.

Oh, without a series resistance for your LED, you've probably damaged that too. Replace it and connect the appropriate series resistor to save the life of your LED and transistor.

At least the resistors will be ok

 

[hevans1944]

ALWAYS use a current-limiting resistor in series with ANY LED. See EP's resource on driving LEDs.

 

[batvink]

Thanks for the replies and the additional advice, much appreciated.
Regards,
Steve.