brodin said:
Thanks a lot Goodwin for the scematics. I am not good in analog electronics, so if someone could tell me what kind of diodes to use i would be glad. As for the transistor, will it work with a BC547?
It's a PNP transistor, a BC557 should be OK, any silicon diodes will do, IN4148 are a standard small signal silicon diode which I often use, they would be fine.
If you wanted to use a BC547 you could invert the entire circuit, and have the transistor in the negative leg of the LED's.
If someone feel for it, please give me a short description of how a circuit like this works.
It's very simple, the two silicon diodes drop around 1.4V across them, and this will be fairly constant. So this fixes the base voltage at 1.4V less than the positive rail. The transistor drops 0.7V between base and emitter, so the emitter is 0.7V less than the positive rail - again this voltage will be fairly constant.
This means the emitter resistor (which is a constant value) has a constant voltage across it, which means (by simple ohms law) it MUST have a constant current through it. Because of the way a transistor works, most of this current will be provided from the collector current of the transistor (with a very small amount from the base current), so the current through the collector will be constant as well.
As the only place the collector can provide this current from is through the load, the load current will remain constant as well.
It's not a 'great' circuit, but it's simple and cheap - and will work a LOT better than a resistor.
To alter the current simply change the value of the emitter resistor, lower is more current, higher is less current.