Transistor switch circuit base issue

[flashzac]

Hi, I am only just starting out with electronics so please bear with me.

I have a simple transistor switch circuit that lights an LED. The transistor base is forward biasing from 9v + through a resistor allowing the +9v connected to the collector to pass to the emitter; passing through the resistor and LED lighting it.

All works fine BUT, when I remove the wire from the +9v to the base of the transistor the LED is dimly lit ??? There is no voltage going to the base at all. When I put a volt meter on the emitter it reads 1.4 volts ????? It's almost like the bread board is leaking voltage into the base of the transistor. If I remove the base wire of the transistor so only the emitter and collector are in the bread board then the LED goes off and 0 volts is read from the emitter and just under 9 at the collector. Which is what I want..

Any suggestions? Faulty breadboard? I have tried 2 and they both do the same Just had a thought and I haven't measured the voltage at the base to see if there is a voltage leak.

 

[BobK]

You need a pull down resistor from the base to ground to insure that the transistor is off when there is no signal at the base. Something like 10K to 100K should work.

Bob

 

[flashzac]

You need a pull down resistor from the base to ground to insure that the transistor is off when there is no signal at the base. Something like 10K to 100K should work.

Bob

ah thank you, that makes sense

 

[(*steve*)]

Any suggestions? Faulty breadboard? I have tried 2 and they both do the same Just had a thought and I haven't measured the voltage at the base to see if there is a voltage leak.

Bob has told you how to fix it. I will answer your questions.

(Bob's advice is useful, mine may be less so)

Do you have a faulty breadboard? Maybe (but probably not). It could be that there is some stray conductive path between the tracks, or it could be that the tracks are acting as a tiny antenna, picking up stray radio signals. These tiny voltages may be enough to turn the transistor very slightly on. (There are many other possible factors too).

Can you measure that voltage at the base? It would be great to be able to, however I suspect that you would find two things if you connected your multimeter between base and emitter. Firstly, it would read zero volts. Secondly, the LED would go off.

The impedance of the meter (essentially its resistance) whilst very high is also probably low enough to short out the signal that is turning the transistor on.

However, it would be a really interesting thing for you to try. Do you get 0 volts? Does the LED turn off? Even if you don't get quite zero volts, the voltage you read may be substantially lower than the voltage present when the meter is not connected.

If you're interested in the WHY of this, you'll probably find that it is something that you will gain more and more insight into as your understanding of electronics increases.

 

[flashzac]

Hi, thanks for that. It gets stranger, soon after posting I measured the voltage and the voltage was 0 and the light did go off. The very next day I energised the project again and noticed that the LED was now off. I checked the power voltage to see if it had drained and it' had slightly (I haven't added the pull down resistor yet which, thanks to Bob expanded my knowledge on what these actually do after some wiki and google sessions, Another knowlege gap filled

Thinking about it the only other difference is that I had the bread board on my leg when testing, this time it was on the table and also the test was conducted during the day and sunlight had warmed the boarded slightly as I gave up the night before (around 2am in the moring) trying to figure out what was going on and left the project on the coffee table. Maybe the voltage is now low enough for this not to happen or the atmospherics were different. I will test with a fresh battery and see what happens before adding the pull down resitor. Thanks for the help guys

 

[BobK]

Yes, it is good to know why floating inputs can be a problem, but the real lesson is "do not have floating inputs". You will get away with this often with BJTs, but try it with a MOSFET once and you will learn!

Bob