LM317 wrong output voltage

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?

Thank you all in advance.

Alessandro Mulloni

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

See

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.

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Peter Bennett, VE7CEI  
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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.

--
Terry Pinnell
Hobbyist, West Sussex, UK

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

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

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 | | | |---------| | | | | | | | | | | '---------' | | | | | | | | | | | | ADJ OUT IN

created by Andy´s ASCII-Circuit v1.24.140803 Beta

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

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 adj. pin is connected at the load side.

--
ciao Ban
Bordighera, Italy

Peter Bennett wrote: >

Ban wrote: >

Thanks for all your replies.

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 output and the adj pins.

Thanks everyone. Alessandro