Author Topic: problem with lm317t  (Read 7689 times)

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Offline saipavanTopic starter

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problem with lm317t
« on: August 09, 2013, 08:51:41 am »
while making a voltage regulator i followed the circuit given in the datasheet of lm317t (st electronics) for voltage regulation.

but when i set up the circuit the output voltage is not uniform.It fluctuates around 10 to 100 milli volts.
iam powering it with a 12-0-12 volt transformer.

can anyone say how to reduce the fluctuations?
« Last Edit: August 09, 2013, 09:28:13 am by saipavan »
 

Offline Alana

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Re: problem with lm317t
« Reply #1 on: August 09, 2013, 09:59:12 am »
Post your schematics with elements value.
 

Offline saipavanTopic starter

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Re: problem with lm317t
« Reply #2 on: August 09, 2013, 10:20:18 am »
i have a diode and a capacitor to the ac supply of transformer and i got a dc supply of about 17 volts.

In the schematic ,
C1  0.1 micro farad ceramic
R2 10k ohms single turn pot
R1 240 ohms
c2  1micro farad electrolytic 50 volts..
 

Online Andy Watson

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Re: problem with lm317t
« Reply #3 on: August 09, 2013, 10:39:42 am »
The supply from the transformer is very lumpy, you need a big capacitor (100µ - 10,000µF) in addition to C1 to smooth-out the lumps.
 

Offline ptricks

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Re: problem with lm317t
« Reply #4 on: August 09, 2013, 10:56:43 am »
The supply from the transformer is very lumpy, you need a big capacitor (100µ - 10,000µF) in addition to C1 to smooth-out the lumps.

That and use at least 2 diodes for the AC-DC.
 

Offline saipavanTopic starter

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Re: problem with lm317t
« Reply #5 on: August 09, 2013, 11:44:13 am »
i used a 5000 micro farad capacitor onthe output.

i used two diodes for the conversion ac to dc.
« Last Edit: August 09, 2013, 11:45:45 am by saipavan »
 

Offline TerminalJack505

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Re: problem with lm317t
« Reply #6 on: August 09, 2013, 11:57:39 am »
What's the frequency of the fluctuations?  How are you measuring them?  Are you using a scope or multimeter?

If they're high frequency then are you using the low inductance ground attachment of the probe?
 

Online Andy Watson

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Re: problem with lm317t
« Reply #7 on: August 09, 2013, 12:06:38 pm »
What load are you putting on the output? The datasheet suggests that proper regulation requires a minimum load current of about 1.25mA (basically it has to dump the current needed to operate the internals of the regulator - it does this through the output terminal.) In your circuit, the only load is the 10k feedback R1 - this is barely adequate to sink working current of the regulator. Try it with a load of at least 10mA.

Edit: Sorry, ignore that. I'd managed to interchange R1 and R2. Although, you might still have problems if R2 is too high in value - consider what happens to the LM317's operating current.
« Last Edit: August 09, 2013, 12:12:18 pm by Andy Watson »
 

Offline lewis

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Re: problem with lm317t
« Reply #8 on: August 09, 2013, 12:14:30 pm »
Definitely got the pins round the right way?
I will not be pushed, filed, stamped, indexed, briefed, debriefed or numbered.
 

Offline Jon Chandler

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Re: problem with lm317t
« Reply #9 on: August 09, 2013, 01:31:38 pm »
R1 should be 120 ohms on an LM317 to meet the minimum load requirements.  For some reason, all the data sheets show sample circuits based on the LM117 which has half the minimum load requirement.
 

Offline Zero999

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Re: problem with lm317t
« Reply #10 on: August 09, 2013, 01:36:20 pm »
i used a 5000 micro farad capacitor onthe output.

i used two diodes for the conversion ac to dc.
Are you talking about C1 or C2?

C1 needs to be 5000uF.
 

Offline saipavanTopic starter

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Re: problem with lm317t
« Reply #11 on: August 09, 2013, 02:52:35 pm »
I am measuring the voltage with a normal multimeter.
The capacitance C1 is 5000 micro farad.
 
I have tested the same circuit with a 1k resistor on the output but got the same results.sometimes voltage changes by 10 millivolts or about 0.1 volt ,its unsteady.

I have also checked the datasheet and wired the pins appropriately.
« Last Edit: August 09, 2013, 02:54:38 pm by saipavan »
 

Offline mariush

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Re: problem with lm317t
« Reply #12 on: August 09, 2013, 03:16:05 pm »
How did you connect the 10k pot

you have 3 leads for that  0 -  wiper - 10k 

If you connected between wiper and 10k , then you may have a high resistance between adjust and ground and get a bad voltage on the adjust then...
 

Offline smashedProton

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Re: problem with lm317t
« Reply #13 on: August 09, 2013, 03:33:21 pm »
" The supply from the transformer is very lumpy, you need a big capacitor (100µ - 10,000µF) in addition to C1 to smooth-out the lumps."


Lumpy!  I'm going to add that to ny technical vocabulary.
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Offline free_electron

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Re: problem with lm317t
« Reply #14 on: August 09, 2013, 04:08:56 pm »
" The supply from the transformer is very lumpy, you need a big capacitor (100µ - 10,000µF) in addition to C1 to smooth-out the lumps."


Lumpy!  I'm going to add that to ny technical vocabulary.

As long as you don't have 'GRASS' or 'floppy the banjo clown' spikes , it's ok ... lumpy is the least of your worries... floppy is a real killer
and don;t start about flicker and popcorn noise ...
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Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Online Andy Watson

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Re: problem with lm317t
« Reply #15 on: August 09, 2013, 05:18:29 pm »
Lumpy!  I'm going to add that to ny technical vocabulary.
Yeah, apologies for breaking into jargon. ;)
 

Offline ptricks

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Re: problem with lm317t
« Reply #16 on: August 09, 2013, 07:24:11 pm »

I have tested the same circuit with a 1k resistor on the output but got the same results.sometimes voltage changes by 10 millivolts or about 0.1 volt ,its unsteady.


Replace the pot with a resistor and see if the voltage still changes. I have seen adjustable power supplies that are correct on paper  but fluctuate because of a bad quality pot.

Try to use a 10 turn version if you can, those single turn ones are generally crap for stuff like this.

If you need even more stability, put a 10uf capacitor parallel with R2 and also add a diode in parallel with R1, cathode on R1 to lm317T adj pin.
« Last Edit: August 09, 2013, 07:31:48 pm by ptricks »
 

Offline lewis

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Re: problem with lm317t
« Reply #17 on: August 10, 2013, 12:29:34 am »
I always use 'furry' if there's HF riding over a sine wave.

Or 'Fucked' if something can't be repaired.

Also, I invented the phrase 'Different kettle of ballgames' and am trying to popularise it. Feel free to use this in general conversation.
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Offline c4757p

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Re: problem with lm317t
« Reply #18 on: August 10, 2013, 12:52:48 am »
I like "furry".

What the hell is a kettle of ballgames? :P
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline lewis

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Re: problem with lm317t
« Reply #19 on: August 10, 2013, 04:24:54 pm »
A cross between 'a different kettle of fish' and 'a whole different ballgame'.
I will not be pushed, filed, stamped, indexed, briefed, debriefed or numbered.
 

Offline saipavanTopic starter

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Re: problem with lm317t
« Reply #20 on: August 11, 2013, 09:58:09 am »
before putting on the breadboard i soldered wires on to the lm317 pins with a 25 watt soldering iron.(tried the lm7805 with the same transformer and everything worked fine).

I want to know if lm317 device is heat sensitive (that is with a 25 watt iron)???
 

Offline vikasvjois

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Re: problem with lm317t
« Reply #21 on: August 11, 2013, 10:38:47 am »
before putting on the breadboard i soldered wires on to the lm317 pins with a 25 watt soldering iron.(tried the lm7805 with the same transformer and everything worked fine).

I want to know if lm317 device is heat sensitive (that is with a 25 watt iron)???

Unlikely if its a TO-220 package because it has lot of thermal capacity. How long did you keep the iron on each pin during soldering? 2-3 seconds seems to be the limit to stay out of risk.
Post a picture of your circuit and setup so that its easy to find if anything is obviously wrong in such a simple circuit.

Regards,
Vikas
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