Re: lm317 as current regulator
- From: Lostgallifreyan <no-one@xxxxxxxxxxx>
- Date: Sun, 25 Jun 2006 20:56:20 GMT
"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?
What I am doing is, I have a supply voltage of 60 volt dc and I want to
limit the current draw to 1 amp for an led board I made. the leds will be a
series link of 12 with 40 of these in parallel. By limiting the current
available to this panel, it should put the voltage of each led at 3.33 volt.
White leds here.
Would the lm317t work in current reg mode since it will only see 20 volt
across it.
Or should I first pre-regulate the 60 volt down to say about 42 volt?
How much does your 60V power supply vary its voltage?
Your main need would be to avoid wasting power, if you're considering dropping
20 out of 60 volts, that's one third of your power gone. 20 watts lost at one
amp, when it's easy to limit that to around 3 to 5 watts.
As at least one earlier poster said, you'll want a single current limiter
resistor for each series string of LED's. If you try one LED at the maximum
current you want it to take, measure the voltage. Better to do this empirically
if you want to cut it close to save power.
LED voltage drops will vary, but not much, so each string's resistor should not
need to drop more than maybe a fifth of what any single LED will drop. Your
single LM317 current regulator (and it's control resistor) need only bear the
difference between minimum and maximum expected supply variation, plus up to
one LED's worth of voltage drop, plus its own total voltage drop at 1 amp. (A
low dropout type might save another watt at 1 amp).
Setting the optimum number of LED's is easy, start with one string of a few
more than you need, and cut back one at a time till the string lights up with
the lowest expected supply voltage applied. LED voltage drops are so well
behaved that if your PSU is stable to within less than two LED drops, you can
probably do it right with simple caclulations and not worry about overcurrent
or waste.
.
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