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transistor as a switch

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asa!!

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hi.please help.i have a circuit consisted of a white led and a resistor of 0,12KOhm.the voltage offered to the circuit is around 15v.i want to adjust to that circuit a transistor to work as a switch.i have bought a 2N3904 transistor.the current i want to have to the led is 125mA.i assumed that the beta value of the transistor is around 100.so the base current should be 1,25mA.

Without being sure , i beleive that the base needs a voltage of around 0,7V to work as a switch meaning to go from the cut of area to the saturation area.i have adjusted a resistor to the base of a value of 5,6KOhmso i figured that i need a voltage of 7v=0,7v=7.7v at the base in order for the transistor to work as a switch.

problem is it does not work.what is wrong?please help
 

You would be better off using the BJT as a current source. Put a resistor between the emitter and ground. Put the LED between the + supply and the collector. Then the current will be your voltage at the base (minus 0.7 V) divided by the resistor at the emitter.
 

Hi
the white led can not handle 125 ma current and it will burn rapidly reduce the current to about 15 ma if the led diameter 5mm and if it is 3mm reduce the current to about 3-5 ma that will be safe.I have aquestion does that transistor can hanle 125 ma current?. Also you didn t tell what happens when you switch the circuit on so I can be more useful to you .
regards
 

By switching this LED, what are you trying to achieve: just simple ON/OFF switch or changing the LED's light intensity?
In each case the LED's current will be different: for static ON/OFF situation it will be around 20mA, for dimming it can be even >100mA ..
Better post your circuit schematic, then it will be easier to address your problem ..
Regards,
IanP
 

how can we make a "switch on-off " LEDs, with a transistor or darlington conencted transistors?

(assume VCC=12V and there are 7 LEDs with a 33ohm connected serial , on the Collector branch. )
 

usE the BJT as a current source. Put a resistor between the emitter and ground. Put the LED between the + supply and the collector. Then the current will be your voltage at the base (minus 0.7 V) divided by the resistor at the emitter.
 

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