Re: lm317 as current regulator

"James Thompson" <Jamesthompson2002@xxxxxxxxxxx> wrote:
"ian field" <dai.ode@xxxxxxxxxxxx> wrote in message
news:pgDng.1748$ST2.629@xxxxxxxxxxxxxxxxxxxxxxx

"William P.N. Smith" <news2006c@xxxxxxxxxxxxxxx> wrote in message
news:avss92h58bpg4e0vr4n08ig7dd0fcclfus@xxxxxxxxxx
"James Thompson" <Jamesthompson2002@xxxxxxxxxxx> wrote:
When using the lm317t voltage regulator with a 1.2 ohm 5 watt resistor
for 1
amp limiting, my question is: Is the input limit on the ic still limited
to
about 40 volts?

No, it's the differential across it that's limited to 40V.

But if you're drawing 1A with a drop of 40V you'd better check if the chip
can handle 40W!

If the specs say it can handle 1.5 amp with a differential of 37 volts,
would that not mean it can handle 55.5 watts. ??

No. :) You might get 1.5A, OR 37V safely handled, butthere are things like heat
dissipation to consider. Even then, I doubt the very best preparations will
keep the junction temperature low enough if you try for both at once.

My circuit will have a differential of 20 volts at 1 amp, which means it
will disapate 20 watt to heat. I have a fairly good sized heatsink for it
so all should be well. My transformer can source around 4 amp, so it also
should stay fairly cool. I am designing it and pcb in a rectangle shaped
caseing with a cooling fan on one end to dirct airflow over it all. Other
than the led's, I have a second winding with 17 volts to run a lm7812 for
running the lcd electronics.

Please read my other posts, I've tried to address this 20 volt drop. It's a
great waste that could be easy to avoid. I've probably made a gaff or two but
nothing uncorrectable. The main weakness of my case might be in assuming
excellent consistency in the voltage drops of your LED's, but if you're wanting
960, I guess you'll be buying 1000 from the same batch, so it's a reasonable
assumption that they're close enough to allow a low series resistance for each
chain of LED's.

If I were making this panel, I'd use a different transformer, or a cheap
effcient power converter if I could get one, to make 140 volts or more. If I
used an old valve transformer which could put out 280 - 300 VAC there need only
be 6 chains of 80 LED's, easy to wire in your array. Each would have a single
LM317T configged as current regulator. This way you need 6 of them, but you
don't need to waste power in series resistors, and most of the current will be
passing usefully through LED's, and each regulator will be running at around
170 mA, so would not need a heatsink at all, and probably no forced air
cooling, either. :) A further benefit is that you can guarantee equal
brightness from each chain without having large series resistors to even things
out if the LEDs' voltage drops vary too much. Yet another benefit of valve
(vacuum tube) PSU transformers is they have low volt windings as well.
.