LM317 wrong output voltage
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Posted by Alessandro Mulloni on September 13, 2004, 6:24 pm
Please log in for more thread options 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 | |---------------- 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 | ||||||||||||||||||||||||||||||||||
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Posted by Byron A Jeff on September 13, 2004, 3:50 pm
Please log in for more thread options - -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 +
- = 0.1uF | | battery
- | | adj | > U -input | ---------------------- > 47ohm U - | |---------------- 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 | ||||||||||||||||||||||||||||||||||
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Posted by Alessandro Mulloni on September 14, 2004, 2:34 am
Please log in for more thread options Byron A Jeff wrote:
> -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. > 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)? > -
> -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. 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 | ||||||||||||||||||||||||||||||||||
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Posted by Terry Pinnell on September 13, 2004, 10:59 pm
Please log in for more thread options
>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. 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 -- Terry Pinnell Hobbyist, West Sussex, UK | ||||||||||||||||||||||||||||||||||
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Posted by Alessandro Mulloni on September 14, 2004, 2:30 am
Please log in for more thread options Terry Pinnell wrote:
>
> >>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. >
> > 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 > 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 | ||||||||||||||||||||||||||||||||||
| | adj | > U
input | ---------------------- > 47ohm U