# LM317 wrong output voltage

Discussion in 'Electronic Basics' started by Alessandro Mulloni, Sep 13, 2004.

1. ### Alessandro MulloniGuest

Hi everyone,

I must state first that I'm a total beginner in electronics.

I have a problem with a simple Ni-Cd battery recharger circuit. The
diagram is the sequent

---------------------- 1N4004 diode
+ o----|----------|input (LM317) output|----|----------->|----o +
| | adj | > U
input | ---------------------- > 47ohm U
= 0.1uF | | battery
| |---------------- U
- o----|--------------------|---------------------------------o -

and the input is between 4V and 12V DC.

What I would like to get is 1.25V between the adj and output pins (as
should be from the LM317 specs)

What I get is a varying voltage depending on the input one (that is,
3.3V if the input is 4V, 4.8V if the input is 5.5V, and so on..)

Do you all know why this is possible? Shouldn't the LM317 in the above
circuit guarantee 1.25V _always_ between the adj and the output pins?

Alessandro Mulloni

2. ### Byron A JeffGuest

-Hi everyone,
-
-I must state first that I'm a total beginner in electronics.

Well congratulations.

-
-I have a problem with a simple Ni-Cd battery recharger circuit. The
-diagram is the sequent
-
- ---------------------- 1N4004 diode
- + o----|----------|input (LM317) output|----|----------->|----o +
- | | adj | > U
-input | ---------------------- > 47ohm U
- = 0.1uF | | battery
- | |---------------- U
- - o----|--------------------|---------------------------------o -
-
-and the input is between 4V and 12V DC.

Looks like a standard constant current LM317 charger circuit.

-
-What I would like to get is 1.25V between the adj and output pins (as
-should be from the LM317 specs)

Not exactly. NiCads want constant current and are not too concerened about
the voltage requirements. Hence the single 47 ohm resistor which will for
a constant 1.25V/47 Ohm -> 26 mA of current across the battery.

-
-What I get is a varying voltage depending on the input one (that is,
-3.3V if the input is 4V, 4.8V if the input is 5.5V, and so on..)

That's exactly how it's supposed to work. Measure the current. You'll find that
the current stays the same even as the voltage fluctuates.
-

-Do you all know why this is possible? Shouldn't the LM317 in the above
-circuit guarantee 1.25V _always_ between the adj and the output pins?
-

Nope. The current is fixed, not the voltage.

BAJ

3. ### Terry PinnellGuest

There are some aspects of your schematic that I'm unclear about. Is
the lower end of your 47R connected direct to ground, as apparently
drawn? Or to the wiper of a pot, as would be the case for variable
control? If the latter, what is its value? And, although not strictly
relevant to your question, what voltage battery are you charging?

Anyway, begging answers to those, here are a couple of simulations
that may help.

http://www.terrypin.dial.pipex.com/Images/317Source.gif

4. ### Alessandro MulloniGuest

Nice diagrams, thanks the lower pin is connected directly to the
ground, like in your first diagram.

My circuit is indeed _exactly_ the one drawn in your first diagram,
except from the voltage I get.

The battery is a 1.2V AA or AAA. With the 47R I would like to get some
27mA load so I can recharge even the 250mAh batteries, maybe I'll change
the resistor when I'll realize I only have batteries with more mA so to
recharge them in a shorter time. Without a constant voltage between the
output and the adj pins I cannot precisely do this calculation (that's
simply I = V / R).

Alessandro

5. ### Alessandro MulloniGuest

Yes, I need 1/10 of the mAh of the battery to recharge it, but with a
non-constant voltage between the output and adj pins how can I calculate
properly the value of the resistor (currently set to 47ohm)?
Ok, but what I really do not understand is that by specifications the
LM317 should guarantee 1.25 of voltage between those pins. Is there
maybe some wrong connection in my circuit?

Alessandro

6. ### Peter BennettGuest

In that case, I suspect that you have the 317 connected incorrectly.

See http://vancouver-webpages.com/peter/lm317.gif for the correct
pinout for a TO-220 package. That drawing shows a fixed-voltage
regulator. For the fixed-current regulator that you want, the Adjust
pin should not be grounded, and the output is taken from the junction
of the adjust pin and the resistor.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter

7. ### CFoley1064Guest

Subject: LM317 wrong output voltage
Hi, Alex. Are you sure you've got the pinout on the IC right? It's different
than the LM78XX. This shows the TO-220 IC as viewed from the front of the
package (view in fixed font or M\$ Notepad):

LM317 Front View
.---------.
| o |
| |
|---------|
| |
| |
| |
| |
| |
'---------'
| | |
| | |
| | |
| | |

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

One of the famous newbie problems with the LM317 -- check your wiring and
pinout. I did it myself once many years back.

Good luck
Chris

8. ### BanGuest

http://www.national.com/ds.cgi/LM/LM117.pdf page 17 precision current
limiter,
look your schema is wrong, there is no connection to gnd, just inserted into
the 12V line. Also the rsistor has to be in series with the load and the

9. ### Alessandro MulloniGuest

The problem was not in the pinout, but in the circuit itself. I actually
got it right at the first time (that is the Ban's circuit), but it
wasn't working, so I changed it to the one posted to this newsgroup.

When I went back to the original circuit, I noticed that something was
wrong.. I didn't have 12V in input, but -12V I know it's a stupid
mistake but I didn't even had a tester the first time I made the circuit.

Anyway, the problem is gone, the input + and - are connected in the
right way into the circuit, and I have some nice 1.25V between the