# constant current source circuit 1550e

I am trying to design current source of 20mA for LED 1550e(thorlabs).The circuit designed gives accurate result in proteus simulation that LED is giving a voltage drop of around 1.2V and drawing 20mA current. But in actual the current passing through LED is around 70-80mA. So what is the error i am making. Schematic of circuit is as folows

simulate this circuit – Schematic created using CircuitLab Results of my simulation are shown in the picture

• What package is the transistor in? May 17, 2016 at 9:25
• current is about 70-80 micro ampere not mA May 19, 2016 at 10:37

If either D1 or D2 are not functioning i.e. there is an open circuit in the diodes, the base current (due to the 10k ohm) will be about 1mA and, given that the transistor will be ~close to saturation, it's hFE could be about 70 and hence you get ~70 mA through the LED.

Check the voltage on the base with respect to ground - if it appears to be about 2V then an open circuit (or reversed) diode is the problem.

The forward voltage drop of a 1N4003 is 1.1V You have used two in series so the voltage at the base of the transistor is 2.2V. The current through the transistor is calculated by the voltage at the emitter divided by the value of the emitter resistor. Ice=Ve/Re. The voltage at the emitter is 2.2V-0.7V(Vbe)=1.5V 1.5V/20ohms = 0.075A!

Try a 750ohm resistor. R=V/I (1.5/20mA=750ohms)

A diode current of 70 to 80 mA implies about 1.4V across the resistor. That could happen if you have shorted out the base emitter junction of the transistor.

If that is true, then the base and emitter will be pinned at two diode drops above ground (due to the two diodes in the base circuit), about 1.4V at room temperature which would yield about 70mA in the resistor (and therefore in the LED).

This actually is a useful troubleshooting tip; if you see errors in circuits with multiples of about 0.6V to 0.7V, there is probably a diode involved.

• If base and emitter were shorted the transistor wouldn't work. May 17, 2016 at 10:15
• Well everybody sorry a correction current is about 70-80 micro ampere May 19, 2016 at 10:33