LM317 Current Regulator Output Low With 7x1W LEDs, 24V Supply, 4.3Ω Resistor

Hi - I'm running 7 x 1W leds in series, which should be happy with 300mA,  7x3.2=22.4V

The simple setup is a 24v supply, and an LM317 with a resistor between  ADJ and OUT.  According to the data sheet, the resistor value should be 1.25/0.3A = 4.2 ohms. I used 4.3, 1%, which should give 290mA through the leds.

Unfortunately, it's not 290 mA, it's about 170.
I cranked the input voltage up from 24v to 26v (the highest the variable psu would go), then I got 250mA, still way off.

In case it was something to do with the psu, I used a 24v switch mode mains adaptor instead of the vpsu, with more or less the same result: 180mA out.

Just thought: I didn't bother using any capacitors.  Is that likely to make a difference?

Thank-you,
Graham
London UK

GrahamTry it with 5 or 6 LEDs and see whether it limits at 300mA. My theory is that whilst you have got the LM317 set up as a current limiter, it is far from an ideal (in the theoretical sense) part. There is a voltage drop and so the sum of voltages from supply to 0V reaches its limit before the current gets to its limit. Figure 3 of the ST datasheet suggests that the volt drop across the LM317 at 300mA will be about 3V.In other words you have the 22.4V of the LEDS plus the 3V of the LM317 =~25.4V which is right at the limit of your supply voltage. Also as the LM317 warms up, the volt drop across it is going to increase, compounding the effect.This seems to link nicely to Elizabeth Simon's series of blogs on how to read data sheetsHow to Read Data Sheets: Linear RegulatorsandHow to Read Data Sheets: Linear Regulators (More Power)

Looking at the TI datasheet I found here
http://www.ti.com/lit/ds/symlink/lm317.pdfI see on page 14 (section 8.3.7) that a similar circuit is discussed with the following requirement noted
Vi > Vo + 4.25V

So the problem is that you need more voltage (in your case about 27V)

The reason that you need so much voltage is that the LM317 needs 3V between input and output and the ADJ pin is another 1.25V below that.
Oh and by the way, your LM317 will need to dissipate about 1W so it will get a bit warm...

You might want to do a search for low dropout adjustable regulators or perhaps give up on the 7th LED if you only have a 24V supply.

Ok, good, thanks to both.I did wonder about the voltage drop across it, but ON Semiconductor's data sheet here:
https://www.google.co.uk/url?q=https://www.on...&usg=AOvVaw0XkHmE3prQz40PzmSCYTW2...doesn't seem to mention it. I was going by the "Input-Output differential",  given as -3 to 40V, which I told myself should surely include 0! There is no page 14.  I'll use the one you linked to from now on  instead.

Standing ready with a fire extinguisher ,  I'll reduce the led count to 6 and see what happens.
Thanks again, Graham.

Fig 10 of ON semis data sheet shows what you need to know, cheers,Richard

Oh, ok, I'll look again.Anyway, it all works fine now with 6 leds.