Expert: Dave Nyce - 7/26/2002

Question
Dave,

I've prototyped another part of the circuit I'm designing. The two transistors in question each have a gain of about 100, but I'm getting a very very small current of only 2.6mA flowing into the LED.  The LED is very dim.  I can't figure out what is happening.  Are the transistors working properly? Any help welcomed! I need a current flow of 80mA into the LED (4 LEDs will actually be used in the final circuit).

The circuit is at (both the same) :-

http://www.pendulumphotography.com/proto2.htm
http://geocities.com/usuff_omar/proto2.htm

Many thanks,

Usuff


Answer
With your circuit, you should get about 4.3 volts to the LED circuit. If it is a red LED, it has a voltage drop of about 1.7 volts. So, there is 4.3-1.7 = 2.6 volts across the 390 ohm resistor = 6.7 mA. To increase the current, decrease the resistance. If you will have 4 LEDs, they should each have their own resistor. With 4 LEDs and 4 resistors of 130 ohms, each red LED will have 20 mA, for a total of 80 mA. (Make sure that you are not using an LED that already has an internal resistor.)

Hope this helps!

Dave