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The current going thru the LED depends of the characteristic of the fet? For my circuit I would like to limit the current to 500mA because I have about 20 LEDs.
For 20 leds you can better use a current source with more power like with a transistor. (ok you need a few extra components but less then 20 resistors....)
If your voltage is enough try to place a much leds in series and place the strings in parallel with each other.
I have made this kind of circuit with 2x 6leds (blue) at 28 Volts and an transistor current source and that works very nice and reliable ( >200 circuits working over 4 years )
Regardless of the configuration of the LED array, if you need 500mA use the LM317 in it's 'constant current' configuration .. see picture below ..
Resistor's value for 500mA is 2.5Ω (1W) ..
Max. input voltage is <40Vdc ..
It is just easier to rely on repeatable characteristic of this voltage regulator than on an individual properties of FETs ..
Regardless of the configuration of the LED array, if you need 500mA use the LM317 in it's 'constant current' configuration .. see picture below ..
Resistor's value for 500mA is 2.5Ω (1W) ..
Max. input voltage is <40Vdc ..
It is just easier to rely on repeatable characteristic of this voltage regulator than on an individual properties of FETs ..
It can be done with PICAXE-08 microcontroller (PIC12F683 with BASIC, 8-pin DIL) + one external transistor ..
Here is 4-20mA circuit:
For 500mA, all what have to be done is to replace current sensing resistor with, say, 0.1Ω, use more powerfull BJT, BD135, for example, and slightly modify software (shouldn't take more than 5mins) ..
It can be done with PICAXE-08 microcontroller (PIC12F683 with BASIC, 8-pin DIL) + one external transistor ..
Here is 4-20mA circuit:
For 500mA, all what have to be done is to replace current sensing resistor with, say, 0.1Ω, use more powerfull BJT, BD135, for example, and slightly modify software (shouldn't take more than 5mins) ..
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