LM317T problem with V out

[skmdmasud]

Datasheet of my LM317t : https://www.electro-tech-online.com/custompdfs/2012/10/LM317.pdf

I have a power supply 30VDC, 5A. I want to use LM317 to get an output of 13.75V by using the following resistor

Vout to Adj - 1KΩ

Adj to ground - 10KΩ

After connecting the resistors i get 30V output. There is no change in V. What am i doing wrong?

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When i finished writing the above i felt like double checking my circuit, then i found out that i was using wrong resistor. After fixing the resistor it works great its V is very accurate and regulated.

Thank you for reading. Read resistor marking carefully. Some time i wonder why didnt they just printed the value in number.

Regards

 

[ericgibbs]

hi,
On the LM317 the R1 [Vout to Adj ] should be 240R, try a value of 2400R for R2 the [Adj to ground]

Use the formula Vout = 1.25( 1 + [R2/R1]) for an approx value of Vout

 

[Grossel]

Hi.

My guess is that you have a defect 10K resistor, or that it isn't properly connected. Check that first.

[edit]
According to tatasheet, the Adjustable Pin Current can vary from 46 to 100 μA. Using the formula in the datasheet, that gives an output range between 14.21 and 14.75 volts. That assumes the resistor have exact values (wich they don't have) so the possible min and max output will have a little greater window than that.

That said, you should get a little more voltage than you first calculated.

 

[MikeMl]

Couple of suggestions. The LM317 requires a minimum load of 30mA . Using 1K and 10K as the voltage adjust resistors doesn't meet the minimum load requirement.
If that doesn't work, you have a wiring error.

 

[audioguru]

No. Look at the datasheet.
The more expensive LM117 can use a 240 ohm resistor from its output to its ADJ pin. The cheaper LM317 needs 120 ohms or less to prevent its output voltage from rising without a load.

The LM117 needs a load of at least 5mA (1.25V/240 ohms= 5.2mA).
The LM317 needs a load of at least 10mA (1.25V/120 ohms= 10.4mA), not 30mA.

 

[MikeMl]

AudioGuru,

Actually, we are both wrong. Here is the snip taken directly from the OP's posted data sheet:

There is no requirement that R1 be 120 or 240 Ohms; only that if it is higher, like the op's 1K, then he shouldn't be surprised if the voltage rises if the load is removed from the output of the LM317. In some battery powered applications where battery life is important, and when the minimum load spec is otherwise met, it is not necessary to have such a low value for R1.

Show me on the data sheet where it says to use a 120 Ohm resistor for R1. I just read todays version of the data sheet, and couldn't find it.

 

[audioguru]

Fairchild did not invent the LM117 and LM317, National Semi did so I look at THEIR datasheet.
Almost every circuit on National Semi's datasheet shows 240 ohms for the more expensive LM117. Its minimum load current is 5mA and the minimum load current for the cheaper LM317 is 10mA, not 12mA as said by Fairchild.
Yes they say at an input to output of 40V but you might as well do the same for any voltage.

I am talking about NO LOAD. Then the 240 ohm or 120 ohm resistor is the only load.

 

[MikeMl]

...

I am talking about NO LOAD. Then the 240 ohm or 120 ohm resistor is the only load.

And how is that a useful configuration? If the regulator has no load, then why bother.

 

[panic mode]

the point is that voltage regulator should regulate ...voltage.
no load condition is common when this is part of bench power supply for example.
you don't want power supply output to change during connecting or disconnecting load.
in order to provide voltage regulation, any regulator will need stable voltage reference.
datasheet in 1'st post shows that the voltage reference is within spec when output current is at least 10mA.
common way to ensure that output current is never less than those 10mA is to place resistor between output and Vadj terminal.
since Vref is some 1.2V it can be easily seen that that resistor need to be 1.2V/10mA=120 Ohm (when no other load is present).
this current need to go to gnd through voltage setting resistor such as potentiometer of suitable value.
if there is a dedicated permanent load that always draws at least 10mA (and it is never disconnected) then values of R1, R2 can be increased.
if the actual load is not permanent, value of 120 Ohm is needed to make this regulator work correctly.
to get some 13V output, R2 would need to be about 1.2k. exact value will depend on unit and need to be trimmed.

 

[skmdmasud]

Thanks Guys..
I later connected 14.5k and 1k and got 20V regulated with and without load. I do my calculation using a software in my phone its very handy.

Regards.

 

[audioguru]

Why do you need software in your phone to do simple arithmatic?
Why do some people need an internet "Wizard program" to do simple arithmatic?
Don't school teachers teach simple arithmatic anymore?

 

[skmdmasud]

Why do you need software in your phone to do simple arithmatic?
Why do some people need an internet "Wizard program" to do simple arithmatic?
Don't school teachers teach simple arithmatic anymore?

lol schools dont teach the way they use to.... all this techno. things are making use/me lazy.... when I was a kid we use to play football and other outdoor sports nowadays kids love to play 3D games and xbox... not to mention 24hr cartoon channels.