Odd LM317T question

Discussion in 'Electronic Design' started by Richie, Oct 17, 2003.

1. RichieGuest

I am trying to use a LM317T to drive some 3.8 volt LEDs. I built a
circuit using a 1k resistor for the fixed and 10k pot for the
adjustable. Using my trusty DMM i set the voltage to 3.8 volts and
preceed to connect the LED. When I connected the LED the voltage drops
to 2.8V and no matter how high I turn the pot the voltage of the LED
stays at 2.8V. Does anyone know why it is doing this?
Thanks
Rich

2. Paul BurkeGuest

Sounds like the LED is actually 2.8V and you are current limiting. Is
there a resistor in series with the LED? Does the LED require more
current than the LM317 can deliver?

Paul Burke

3. Robert BaerGuest

LEDs should be driven by a *current* source.
You may notice one or more things as you "turn up the pot":
a) the LED gets brighter
and/or
b) there is more ripple at the LM317T output
and/or
c) the LM317T drops completely out of regulation

4. AndreGuest

I'm assuming you have the LM317T connected round the right way and in
constant current mode not constant voltage

Check the current through the LEDs, your resistor should have some
effect , the change may just not be large enough to have an effect on
the voltage across the LEDs.

5. Bill SlomanGuest

You were trying to drive some 3.8V LEDs from *what* supply voltage?

If it was 5V, you would be out of luck. The LM317 needs at least 1.5V
from input to output beofre it can work. This limits your output
voltge to less than 3.5V, not enough to drive your LEDs.

Your 1k fixed resistor is too high - the LM317 data sheet recommends
120R - and this may be part of the problem.

This - incidentally - happens to be the wrong way to drive LEDs. If
you read the LED data sheet carefully, you will find that the "3.8V"
is specified at a specific current (usually 20mA) as 3.8+/-0.4V or

You need to design your circut so that each LED gets its 20mA (or
whatever) anywhere within the guaranteed voltage drop range around
3.8V. If you are trying to drive several LEDs in parallel from a
regulated voltage, most of the current will go through just one LED
(the one with the lowest voltage drop) and the others probably won't
light up - not what you want.

You can configure an LM317 as a current regulator, but it won't work
with a 3.8V LED and a 5.0V power rail. A low-drop-out linear regulator
might work, but they can be picky about the output impedance they see,
and the only handy data book doesn't show any of them configured as a
regulated current sink, which is a bit worrying.

You can use a current mirror to do the job. 1.2V of headroom is
probably just enough. Horowitz and (Win) Hill's "The Art of
Electronics" (ISBN 0-521-37095-7) will tell you all you need to know

6. Tim ShoppaGuest

Two points:

1. Do not drive LED's with regulated voltages. If you have to regulate
them, just regulate the current. There's a circuit in the LM317 data sheet
if you don't mind all the losses of linear current regulation. There
are many switch-mode current-regulated LED drivers out there if you care

2. You need 2.5 or 3V of headroom over the output voltage when the LM317
is used as a voltage regulator, as well as some bypass capacitors. What
are you putting in to the LM317, 5V?

Tim.

7. Fred BloggsGuest

Yes- the LED clamps the ADJ terminal of the 317 to the LED voltage
corresponding to Vref/1K=1.25mA ( less 280uA through the 10K pot at max
setting). Remove the 10K pot altogether and replace the current setting
resistor as shown below. If you want 10mA through the LED then
R=1.25/0.01=125 ohm- use a 120 ohm- etc. The LM317 IN terminal must be
at a minimum potential of 3.8+1.25+1.5=6.6V in this circuit.

Please view in a fixed-width font such as Courier.

LM317
+------------+
Vin >--+-----| IN OUT |------+
| | | |
| | | \
| +------------+ \
| | /
=== | |
0.1U +-------------+
| |
| | LED |
| + --- | 1.25V
| Vled \ / ~~ | I= -----
| - --- | R
| | \|/
+-----------+
|

8. happyhobitGuest

The LM317 has a 'minimum current to maintain regulation' requirement.
(Typical 3.5 ma, max 10 ma) With a 1K resister you only have 1.25 ma. That's
TOO LOW.

Also see recommendations above re. current limit being better that volt
Regulation for lighting LED's

Jay

9. Byron A JeffGuest

-I am trying to use a LM317T to drive some 3.8 volt LEDs.

Um. Why?

- I built a
-circuit using a 1k resistor for the fixed and 10k pot for the
-adjustable. Using my trusty DMM i set the voltage to 3.8 volts and
-preceed to connect the LED. When I connected the LED the voltage drops
-to 2.8V and no matter how high I turn the pot the voltage of the LED
-stays at 2.8V. Does anyone know why it is doing this?

Yes. Because you're mistaken about the role of voltage in driving an LED.

the critical attribute for an led is current.

Here's a nice thread from the PICLIST that talks about the role of current
and voltage in driving an LED:

http://tinyurl.com/rans

What's your source voltage? Is it regulated? If it is then you don't need
the LM317T to drive the LED, just a simple resistor whose value can be
computed from the info given in the thread above. The voltage plays its
role in fixing the value of the resistor in order to generate the correct
current for the LED.

Hope this helps,

BAJ