problem with LM317(Current limitter)

[yaghoubi]

hello
there are 6 power LEDs in series and 24vdc power supply.I used the attached circuit but the current is not constant(when the LEDs get warm , the current increases)
[when I increase power supply in 20VDC the circuit sinks 370ma and the LEDs sink 350ma[after one minute the leds sink 380ma.lm317 can not limit the current] .when I increase the power supply to 24VDC the leds sink 800ma (after one minute the leds sink more current.lm317 can not limit the current)].



I read this link but it didn't help me https://www.edaboard.com/threads/201189/

 

[milind.a.kulkarni]

One of the think that I will like to share here is LED is not a passive load .....it is an active load .....I mean to say LED is a semi-conductor device and one can not consider this as direct resistive load .....so In my view what is happeing is one LED drivers some current it's get dissipated in term of I2R and as a result of it the temperatuer is getting increased.... In case of active devices made using semi-conductors have generally negative temperature coeffeicent of resistance....forcing the resistance of LED of drop ....and hence there is an incerese in current..... Now related of question why LM317 is not able to control the current will be the load(LEDS) that is it driving at two point of time is different....

Good Luck

 

[betwixt]

There are a number of possible reasons.

1. To stabilize the current, the input voltage must be at least the LM317 overhead voltage + Vref + combined Vf of all the LEDs and must of course be able to maintain that at the load current.
2. The LM317 is not a precision reference device, it will slightly change it's reference and regulation current as it warms up.
3. The current is decided by the voltage drop across the resistor, is it's value staying the same as it warms up?
4. Depending on wiring lengths, a single 0.1uF capacitor at the input may not be sufficient to ensure stability. Try a capacitor of 10uF or more in parallel with it.

Brian.

 

[FvM]

To start with a strange point in your post

when I increase power supply in 20VDC the circuit sinks 370ma and the LEDs sink 350ma

Do you say, the input current to the circuit is different from the LED current? This can be only the case, if your real circuit is different from the shown datasheet schematic. If so, all furthers guesses are void. If it's just an erroneous measurement, how can we know that the other measurements are correct?

 

[yaghoubi]

the input current to the circuit is different from the LED current?

I use a power source for circuit(and I can see the current which the circuit sinks). then I use a multimeter to measure the LEDs current.
maybe the multimeter doesn't have enough precise and accuracy(next time I will measure both with one device and say result).

3. The current is decided by the voltage drop across the resistor, is it's value staying the same as it warms up?

I didn't measure the Vref of LM317(and voltage across the resistor) but I will do it and say the result next time.

 

[yaghoubi]

hello again
I found the problem(I had connected LEDs to out put pin of LM317 and it is causes the resistor not be on the rout)

but as I had said about the current difference between LED and circuit; see attached file(It is because of poor multimeter)


but now there is another problem:
LM317 gets hot in 40sec. actually there are 6 LEDs (and Vf is 2.9v until 3.3v) with the 24VDC power supply.
I measured the Vf of LEDs and it was 2.9v.

Is there any problem in circuit that causes LM317 gets hot soon?

 

[FoxyRick]

If your LED's each have a Vf of 2.9V, then the whole string of them has 17.4V.

So, the LM317 has the remaining 24-17.4 = 6.6V

With 0.37A flowing through the LM317, with a voltage of 6.6V, the LM317 is dissipating 2.44 watts. It is normal for it to get hot with that dissipation - it will need a small heatsink.

 

[betwixt]

Not necessarily a poor voltmeter, suppose the real value was 0.3699999 Amps, the error would be tiny.
It's just a limitation of the measurement resolution, you will always see the last digit reading +/- 1 of the true amount, more digits would meant the last one was less significant.

Brian.

 

[yaghoubi]

hello again
I have another question about LM317.
I want to use 18 pieces LM317 on one heat-sink in current regulator mode (actually outputs will be shorted ).
Is there any problem with that?if yes why?

 

[milind.a.kulkarni]

In my view using One heat sink is Bad idea bcz thermal profile distribution may be different at different places on the heat sink....However if you make proper distance and design then your idea will work....But I strongly suggest you to go for individual heat-sink .....

Good Luck