LED does not completely switch of by transistor

[neazoi]

Hello, in case A on the attachment the LED is completely switched off when 0v is applied on ACC. But in case B, when 0v is applied on ACC, the led is not completely switched off but it is very dim. I measured the voltage on the transistor base when off and this was 0.6v as expected. Then I measured the voltage across an off LED and it was about 2.6v. On 3v LEDs this 2.6v means a very dim LED (I have used high efficiency blue LEDs). Does the transistor still conducts some electrons even when 0.6v on it's base? I have not a clue. It is ok but I would like to solve why this is happening.

 

[keith1200rs]

Try adding a resistor from the transistor base to ground - maybe 10k. Transistors don't turn on/off abruptly at 0.6/0.7V.

Keith

 

[thylacine1975]

Hey neazoi - aye, a transistor certainly does conduct some collector current with 0.6V on its base.
The late Bob Pease has an informative (and light hearted) discussion worth a read here: **broken link removed**

[Your situation would probably show the Vbe temperature dependance quite clearly! Connect ACC=0V on circuit B, and heat the transistor by putting the soldering iron on top of its case. I'd suspect your LED would start to glow more brightly!]

Given your high-efficiency LED's sensitivity to the circuit's leakage currents, you might be able to fix things by connecting a pull-down resistor across the transistor's base-emitter connections. Try a value of 1-10K and see if that improves the circuit operation.
Cheers

 

[SUMIT_GAMBHIR]

thylacine ... good suggestion