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# LED current limiting using FET

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#### david90

Can I use a fet and biased it to limit current going thru LEDs instead of using resistor?

Yes it can.

### david90

Points: 2
thanks for the link! helped +1.

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.

Thanks.

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....)

see: https://en.wikipedia.org/wiki/Current_source for examples.

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 ..

For comparison, here is similar (adjustable) MOSFET circuit:

Regards,
IanP

IanP said:
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 ..

For comparison, here is similar (adjustable) MOSFET circuit:

Regards,
IanP

Do you have a circuit that limit current but works with a pwm signal for dimming?

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) ..

Regards,
IanP

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) ..

Regards,
IanP

can any body tell me how to design a foldback current limter?

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