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LM317 output voltage problem

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erfan_fossile

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Hai,

I am presently using LM317 in my design and using it to generate 3.3V by using 3.7K for R1 and 4.7K for R2. The formula used is

Vout= 1.25 [1+R2/R1] + Iadj R2. Iadj is assumed to be 100uA.

I am getting 7.5V instead of 3.3V. Different datasheet and applications notes are saying different formulae. I tried connecting Adjust pin directly to ground and without load and I observed 8.5V instead of 1.25V. All this experiments were done without load. Please help.
 

Are you certain you have the pins connected in the correct order? Grounding the 'adjust' pin should always give 1.25V out, if it gives anything else the LM317 is faulty.

Brian.
 

I tried with 2 IC's and both are giving the same out.

---------- Post added at 17:05 ---------- Previous post was at 17:01 ----------

pin 1 adjust, pin2 output, pin3 input
 

do not always use a fixed resistor, use a variable resistor at pin Adjust to find voltage what you want .. after that, measure the impedance variable resistor and replace it with a fixed resistor

00906301.jpg
 

I am presently using LM317 in my design and using it to generate 3.3V by using 3.7K for R1 and 4.7K for R2

R1 (output-ADJ) has to be low in order to maintain the min output current, use a 220 or 270 ohm value.

Alex
 

I have only 2 parts in hand and I tried both and they are giving same results. It can happen like both have gone bad or I am checking it in wrong way. Let say if I have connected higher resistor value for R1 then it may be giving some issue. but when I am connecting the adjust pin to ground that time atleast it should give out 1.25V properly right...
 

Use a low value resistor (220/240/270 ohm)from ADJ to OUT and you will get correct output, your 3K7 value is too high.

Alex
 

Your resistance combination is not correct. Log here for long variety of resistance and Vout combination pre calculated and you can have your choice added and calculated values.
LM317 Voltage Calculator - Electric Circuit
 

Regardless of the resistor values, connect the adjust pin to ground should always give 1.25V. Is it possible the regulator is oscillating? You should have a capacitor of at least 1uF between the 'in' pin and ground and another between the 'out' pin and ground. The capcitors should be mounted as close as possible to the LM317 and certainly not more than about 5cm away.

Brian.
 

Regardless of the resistor values, connect the adjust pin to ground should always give 1.25V. Is it possible the regulator is oscillating? You should have a capacitor of at least 1uF between the 'in' pin and ground and another between the 'out' pin and ground. The capcitors should be mounted as close as possible to the LM317 and certainly not more than about 5cm away.

Brian.

The datasheet says that the min output current is 3.5mA to 10mA, don't you think that 1.25v/3k7=0.33mA would be a problem?
There is also a graph in page 7 ( www.national.com/ds/LM/LM117.pdf ) that shown the min current.

Alex
 

You can find a LM317 resistance calculator software...where u just have to input your o/p voltage value and it gives u the required value.
This is the link.
**broken link removed**

and please chk with capacitor in place at the o/p
 
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