problem using lm317

[saipavan]

I used lm317 chip along with smoothing caps to regulate voltage from a transformer rated 12 volts at 500 milli amps.

after rectification and smoothing the transformer gave around 17 volts.when i used a 470 ohm resistor between adjust and output pin it gave regulated voltage from 1.28 volts to 15 volts.but when the resistor was changed with the resistor that was prescribed in the data sheet that is 220 ohm the output voltage was not increasing beyond 3 to 4 volts.
I would like to know the reason for this behaviour and also its solution.
(all the voltage readings were taken without any external load).

thanks, in advance.....

[Stonent]

I used lm317 chip along with smoothing caps to regulate voltage from a transformer rated 12 volts at 500 milli amps.

after rectification and smoothing the transformer gave around 17 volts.when i used a 470 ohm resistor between adjust and output pin it gave regulated voltage from 1.28 volts to 15 volts.but when the resistor was changed with the resistor that was prescribed in the data sheet that is 220 ohm the output voltage was not increasing beyond 3 to 4 volts.
I would like to know the reason for this behaviour and also its solution.
(all the voltage readings were taken without any external load).

thanks, in advance.....

I would say there might be something wrong with your circuit. But without seeing it we can only guess.

Many times people will say they followed the datasheet's example but missed something.

[mariush]

The output voltage is determined by the ratio between a resistor and the default 240 ohm resistor recommended (it can vary a bit but around this value) :



Iadj x R2 can usually be ignored, as it doesn't produce much change in the output voltage.

So you Vout is 1.25 ( 1 + R2 / R1) where R2 is you potentiometer.  At 0 ohm, you'll have 1.25 ( 1 + 0) = 1.25v , to get 15v you'd need  R2 = 2640 ohm  because  Vout  = 1.25 ( 1 + 2640/240) = 15v

Your potentiometer is probably too small - I would guess you use a 1 kOhm resistor ?

[echen1024]

The LM317 uses a resistive divider between OUT and ADJ to form a 1.25V_ref. Therefore, when you adjust the pot, you are changing the ratio by which it divides, and the output voltage changes.

[AG6QR]

In addition to the previous comments...

You have to read the data sheet carefully, and pay attention to the minimum load current to maintain regulation.  Its max value is 12mA on the Fairchild data sheet I found (http://www.fairchildsemi.com/ds/LM/LM317.pdf) .  A 240 ohm value for R1 will give 1.25V/240ohm = 5.2 mA load current, assuming nothing else is connected to the output.  That won't always be enough to keep the regulator regulating.  Using a value of 120, or even 100 ohms for R1 is a safer bet than 240 ohms, if it's important to maintain full regulation at an open circuit output.

Once you've got the value of R1 set to get the right minimum load current, then you choose R2 to give you the output voltage range you desire.