problem with lm317t - Page 1
Forum section: electronics / beginners

author: saipavan
author_rank: Contributor
timestamp: August 09, 2013, 08:51:41 am
content: 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?

author: Alana
author_rank: Frequent Contributor
timestamp: August 09, 2013, 09:59:12 am
content: Post your schematics with elements value.

author: saipavan
author_rank: Contributor
timestamp: August 09, 2013, 10:20:18 am
content: 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 ceramicR2 10k ohms single turn potR1 240 ohmsc2 1micro farad electrolytic 50 volts..

author: Andy Watson
author_rank: Super Contributor
timestamp: August 09, 2013, 10:39:42 am
content: 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.

author: ptricks
author_rank: Frequent Contributor
timestamp: August 09, 2013, 10:56:43 am
content: Quote from: Andy Watson on August 09, 2013, 10:39:42 amThat and use at least 2 diodes for the AC-DC.

author: saipavan
author_rank: Contributor
timestamp: August 09, 2013, 11:44:13 am
content: i used a 5000 micro farad capacitor onthe output.i used two diodes for the conversion ac to dc.

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 09, 2013, 11:57:39 am
content: 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?

author: Andy Watson
author_rank: Super Contributor
timestamp: August 09, 2013, 12:06:38 pm
content: 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.

author: lewis
author_rank: Frequent Contributor
timestamp: August 09, 2013, 12:14:30 pm
content: Definitely got the pins round the right way?

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: August 09, 2013, 01:31:38 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2013, 01:36:20 pm
content: Quote from: saipavan on August 09, 2013, 11:44:13 amAre you talking about C1 or C2?C1 needs to be 5000uF.

author: saipavan
author_rank: Contributor
timestamp: August 09, 2013, 02:52:35 pm
content: 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.

author: mariush
author_rank: Super Contributor
timestamp: August 09, 2013, 03:16:05 pm
content: How did you connect the 10k potyou have 3 leads for that 0 - wiper - 10kIf 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...

author: smashedProton
author_rank: Frequent Contributor
timestamp: August 09, 2013, 03:33:21 pm
content: " 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.

author: free_electron
author_rank: Super Contributor
timestamp: August 09, 2013, 04:08:56 pm
content: Quote from: smashedProton on August 09, 2013, 03:33:21 pmAs 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 killerand don;t start about flicker and popcorn noise ...

author: Andy Watson
author_rank: Super Contributor
timestamp: August 09, 2013, 05:18:29 pm
content: Quote from: smashedProton on August 09, 2013, 03:33:21 pmYeah, apologies for breaking into jargon.

author: ptricks
author_rank: Frequent Contributor
timestamp: August 09, 2013, 07:24:11 pm
content: Quote from: saipavan on August 09, 2013, 02:52:35 pmReplace 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.

author: lewis
author_rank: Frequent Contributor
timestamp: August 10, 2013, 12:29:34 am
content: 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.

author: c4757p
author_rank: Super Contributor
timestamp: August 10, 2013, 12:52:48 am
content: I like "furry".What the hell is a kettle of ballgames?

author: lewis
author_rank: Frequent Contributor
timestamp: August 10, 2013, 04:24:54 pm
content: A cross between 'a different kettle of fish' and 'a whole different ballgame'.

author: saipavan
author_rank: Contributor
timestamp: August 11, 2013, 09:58:09 am
content: 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)

author: vikasvjois
author_rank: Newbie
timestamp: August 11, 2013, 10:38:47 am
content: Quote from: saipavan on August 11, 2013, 09:58:09 amUnlikely 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