Using a LM3117 as a current regulator to control the current applied to a 12 V yellow LED and to a 5mW green diode laser, I found a difference in the behavior of the LM317 when controlling both LED and laser.

I tested the circuit with three different resistors.

Here the values and the relevant current-limit, given by 1.25/R1.

R1 = 22 ohm I lim = 56.82 mA

R1 = 47 ohm I lim = 26.60 mA

R1 = 100 ohm I lim = 12.50 mA

Here you can see the results:

R1 = 22 ohm

R1 = 47 ohm

R1 = 100 ohm

I would like someone to explain to me why, in the case of the laser diodes, after reaching the target value I = 1,25/R1 the current tends to decrease and then to grow again, unlike the LED, for which the current is constant as expected. This effect becomes more evident as resistance grows.

Thanks in advance.

Paolo

I tested the circuit with three different resistors.

Here the values and the relevant current-limit, given by 1.25/R1.

R1 = 22 ohm I lim = 56.82 mA

R1 = 47 ohm I lim = 26.60 mA

R1 = 100 ohm I lim = 12.50 mA

Here you can see the results:

R1 = 22 ohm

R1 = 47 ohm

R1 = 100 ohm

I would like someone to explain to me why, in the case of the laser diodes, after reaching the target value I = 1,25/R1 the current tends to decrease and then to grow again, unlike the LED, for which the current is constant as expected. This effect becomes more evident as resistance grows.

Thanks in advance.

Paolo

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