unless i'm missing something here... When in current regulation mode, it requires a low value resistor to get any kind of reasonable current?...
eg - To get a regulated 961.588ma of current, the Resistor has to be 1.3 ohms, and at least 1.5watts to cope.
Yeah, I have a load of uses for 1 watt, - 1 - 10ohm resistors?!, So for each LM317, I might as well spend $10 on a 2watt potentiometer, just to get a steady current... I might as well just use 1.3ohm resistor
and omit the lm317 from the circuit all together!
And before you say, ah yes but doing it like this means you can feed in any fluctuations in Voltage and you'll always get a nice steady 30milliamps out... But you would also get a steady 30ma out, if you used the LM317 as a constant voltage source then using a resistor (CV/CC) .... you'd also end up with a steady current out.
(Zener Diode + Resistor anyone? = Almost CC/CV lol)
Thanks for the interesting musings. You got me thinking too...
cjdelphi:
unless i'm missing something here... When in current regulation mode, it requires a low value resistor to get any kind of reasonable current?...
eg - To get a regulated 961.588ma of current, the Resistor has to be 1.3 ohms, and at least 1.5watts to cope.
That's certainly how I read it, and that's supported by a bit of google-fu too (for example this page)
cjdelphi:
So for each LM317, I might as well spend $10 on a 2watt potentiometer, just to get a steady current... I might as well just use 1.3ohm resistor
and omit the lm317 from the circuit all together!
I guess you could, but that would be a current that fluctuates with input voltage (a current limiter rather than a true regulator) so you'd still be adding some kind of voltage regulation. Besides which by my maths the LM317T & a 5W 1 Ohm cement resistor is 42c all up (using as a reference pricing from my usual, Tayda for 5W 1ohm cement resistor and LM317T) so since LM317 comes in as a winner on cost, even if you use one for voltage and one for current, why would you choose the (comparatively expensive) 2W pot?
And since your pot will be doing the current limiting directly in series with the load, wouldn't it run out of spec at 5V at 400mA (as an example) whereas at that LM317 can produce constant current at far higher voltages. In the example linked above his LEDs had a 12V supply and consumed 320mA each - 3.84W which would let the smoke out of the 2W pot alternative - or am I missing something in your design?
cjdelphi:
(Zener Diode + Resistor anyone? = Almost CC/CV lol)
Can you get a constant current circuit from just those, or constant voltage only?
Really! - I presumed for some reason >1watt resistors were pricey... well in that case, I certainly shall go buy a couple
But, which would be better?
LM317t in current regulation + resistor? or LM317 in voltage regulation + resistor?
with exception to 1 less resistor? which is the better method to use? ... I'm only curious because as you pointed out, lm317's are pretty darn cheap these days
so when it's time to use one, I shall (for low power applications, otherwise i'll stick switching regulators)
Right now, bugger all.. I ordered 10 LM317's for less than 2 bucks, so I thought I'd read up on how to fully exploit them...
After all this, I think i'll be using these purely as voltage regulators; and handle a current a different way... (even a 555 with pwm! and a cap/resistor or Transistor, undecided yet i'll play when they arrive)
