# How to adjust the LM317T voltage regulator?

Discussion in 'Electronic Basics' started by [email protected], Mar 7, 2006.

1. ### Guest

I got the LM317T from Radio Shack and I am trying to wire up an
external battery pack to run an mp4 player which has an internal 3.7v
battery.

Can someone explain to me (in plain english, I'm new with this stuff)
what I need and how to adjust this thing?
I want to run this mp4 player on a rechargable 9.6v battery that I
have.

2. ### FigaroGuest

|I got the LM317T from Radio Shack and I am trying to wire up an
| external battery pack to run an mp4 player which has an internal 3.7v
| battery.
|
| Can someone explain to me (in plain english, I'm new with this stuff)
| what I need and how to adjust this thing?
| I want to run this mp4 player on a rechargable 9.6v battery that I
| have.
|
| Thanks in advance.
|

Try the 1st circuit on

http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm

You'll need 192 ohms for R2

3. ### Guest

Ok well I'm a total "noob" at all this but I think that diagram makes
sense. I'll see what I can make of it. Thanks.

4. ### Byron A JeffGuest

Be aware that you'll burn up much of your power in the LM317. So don't
expect long battery life with this method.
In plain English: The output pin regulates to a voltage that is 1.25V
higher than the voltage on the ADJ pin. The two resistors form a voltage
divider to set the ADJ pin voltage. You can use the formula on this page

http://www.circuit-innovations.co.uk/LM317.html

To compute the values of the resistor.
It won't last long. You really need a switching regulator to get any
real usable battery life.

BAJ

5. ### Guest

What more can you tell me about a switching regulator? What would it
take to use one of those instead of the LM317T?

6. ### Bill BowdenGuest

Switching regulators make a lot of noise. You might get better sound
with the LM317. The battery life will be a about 1/2 as much compared
to a switching regulator since the input is about twice the output.

Basically, a switching regulator charges a capacitor very quickly to
the desired output voltage and then waits for the capacitor voltage to
fall a small amount and then blast it again with a pulse of current to
bring the voltage back up. It sort of switches on and off as necessary
to keep the output fairly constant. But it won't be as smooth as the
linear LM317 and so needs more filtering on the output to get rid of
the noise and small variations in output voltage.

-Bill

7. ### ehsjrGuest

Here's a "picture".

+-----------------------> Gnd To MP3 (-)
|
------ | ---------
+---|470ohm|---+---|-BATTERY+|---+
| ------ --------- |
| |
| ------ |
| | o | |
| |______| |
| /______/| |
| | || |
| |LM317 |/ |
| ------ |
| | | | |
+----------+ | +---------------+
| |
| |
| ------ |
+---|240ohm|--+------------------------> 3.7V To MP3 +
------

With the 240 and 470 resistors shown, you'll be within
..003 volts of target (3.7) per the computation. You do
not need any capacitors if you keep the LM317 relatively
close to the battery.

Ed

8. ### Guest

Thanks a lot. So far that is working perfectly. I will post a link to
my project once I have the site up, for anyone who is interested.