Re: lm317 as current regulator

James Thompson 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?

You probably need a resistor for each string, to equalize currents.
And there's another reason you need extra resistance in the circuit;
your regulator will get too hot.
Even assuming all the led strings have equal current and voltage, look
at the circuit --
you drop 40 volts across the led's (12 X 3.3) and 1.2 volts across the
resistor,
leaving 18.8 nominal across the terminals of the lm317t. At one amp
that's 18.8 watts, way to much power
unless you have one hell of a heatsink. So use a resistor in each led
string.
It will equalize currents among the strings and get rid of extra heat.
You want to drop about 15 volts.
Exactly how much depends on how stiff your power supply is. But for now
we'll say 15 volts.
Now since you want 25 mA in each string, you will get a 15 volt drop
with
about 600 ohms (or nearby standard value) resistor in each string. Use
a half watt or one watt resistor.
If your power supply sags with an amp load, you could start with a 560
ohm resistor and see how that works.
If your supply doesn't droop you can go for more than 600 ohms. The
next standard value is 630.
The higher you go with the equalizing resistors, the cooler your
regulator chip will run, so try different values until
you find the right one.

.

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