LM317 Variable power supply

[MP1]

I have built quite a few of these. There is a LM317 calculator in the "Articles" section of this site. On the page, there is a link to a tutorial with some added information. In addition, if you could post your schematic including the information written on the back of your wall wart, I am sure we can figure it out.

One thing to check for now: which wire is + and which is - from the wall wart. They are not always Center +.

MP

 

[mikratt]

Thanks LM, I got it to work.

I talked to a friend who knows a little about electronics. I got the schematics from him, it's the same one as on the calculator page and told him the problem with the lack of current.

I showed him the my posts and your answers and the link to the tutorial and he noticed that in the tutorial it use a 1k ohm resistor (R1) and not a 240 ohm resistor as in the schematic I was using. So we tried different values with the LM317 Calculator and figured out we needed more resistance. He suggested trying the 1K resistor.

When I got home I didn't have that size resistor. I changed the 5K potentiometer to a 25K and used the 5K pot as the R1 and adjusted it as I needed to get it to work and it did.

Thanks again
Mikratt

 

[MP1]

Yes, the feedback resistor determines the current. A little about this can also be found in the LM317 data sheet.
I am glad to hear that all is well.

MP

 

[Enac]

Actually, my data sheet lists 3.5 mA as minimum typical load current for the LM317.
<snip>MP

MP, the National datasheet says the typical minimum current is 3.5ma, but the maximum minimum current (sounds like an oxymoron) is 10ma. Whose datasheet are you using?

 

[MP1]

I actually have two data sheets on this device. One from BayLinear and the other from National. The National Data Sheet is dated May 2003. Both data sheets list the same specs. 3.5 mA as Typical minimum load current "Min". The BayLinear Data Sheet lists the Minimum load current" Max" as 5 mA., whereas the National gives this a 5 mA spec for the LM117 and 10 mA for the LM317.
I doubt that there is any difference in the BayLinear and National chips and National is probably just playing it safe. Oddly enough, the Minimum load current "MIN" is what really makes a difference, so it is pretty much a mute point. If it is a real problem for you, just go to baylinear's website and buy their LM317.
http://www.baysemi.com/LM317%20Data%20Sheet%20REV%2009142002.pdf
They also send samples, so if you would like to compare theirs with the National, you can do so without risk of cost.


MP

 

[ante1]

Hi MP.

Isn

 

[MP1]

They are test standards of the parameters used by the chip manufacturer. Your application might very well be different. If your application falls within the generic specs, then sure. If you are pushing the edge, then you have to look closer. You can still achieve the manufacturer's specs if you use proper designing techniques.

For example, when a data sheet states that the device will work between -40C and +150C, they usually list a min max and typ to show their results. What they might not tell you is that 50% dropped out at +142C, and thus they can still compete with their competitors and advertise the +150C top end tolerance for temperature. But if you are manufacturing this device to be sent to the equator, you will want to provide more testing before you package it. This would be considered an extreme for any company. If you are using the device in your lab, you are pretty safe with the min tolerances given in the data sheet.

So, in answer to your question, yes, you can rely on the specs. If your application is extreme, you might have to add something to the circuit to continue to achieve this.

MP

 

[jandro]

???

Hi MP,

What would I have to do to control the voltage output of the LM317 VPS using a 4-20mA signal in place of the potentiometer?

I am trying to build a variable power supply I can "talk to" with a 4-20mA loop controller.

Thanks for your help.

 

[MP1]

Hi Jandro,
I have never seen anyone use this application. Perhaps the simplest way to do it would be to have the current change the intensity of light which activates a light sensitive resistor where the light sensitive resistor takes the place of the pot. You could make your own custom package if needed.
I am sure there are many other ways to do it as well.

MP

 

[MP1]

Jandro,
Did you find a solution?

MP

 

[jeff1]

Hello, i was wondering if it was possible to use a 450 ma 25 volt transformer on this lm317 power supply

 

[MP1]

Hi Jeff,
You should have no problems with this transformer.

MP

 

[jeff1]

I tried it with this and i have a two amp fuse on one side of the input and it is blowing???

 

[MP1]

Have you double checked the pin out of this regulator? It is different than the others. The pin out does not match the schematic symbol.

MP

 

[jeff1]

I bought the lm317t from radio shack. It shows on the package that 2 is the out or the middle pin. As the picture goes I think with the pins pointing down looking at the thing with the heat sink part laying on the table pin 3 is on the left and then pin 2 is center and pin one is right. Here is the schematic of my power supply, only I have a 25 v 450 ma transformer. I had it working for a minute and put a meter on the output and the fuse blew. I tried again after checking it and it blew as soon as i turned it on.

View attachment 35530

 

[MP1]

Jeff, with the heat sink tab on the table and pins down, your pin assignment is left to right. This should correct the problem.

MP

 

[jeff1]

Thanks for the help. The package for the lm317 is not clear as to which side is down. I got it working and am using it for a 4-H Fair as a demonstrations. Thanks for all the help.

View attachment 35544

 

[MP1]

Jeff, glad to see you got it working. You are right, there is really nothing on this device which indicates the pins are numbered from which end. You will want to memorize this since it is standard on all devices with this package type. This will surely come in handy on another project in the future. Nice looking project!

MP

 

[Emiliano86]

Pinky...i am planing to build this project, a regulated PSU with the LM317, i was searching on the net and i found a very similar regualtor the same voltage butinstead of 1.5 A, it can manage 3 A.......but bell I have some questions......for the LM317 PSU:

1 - if Iwant a 30VDC at maximun output, with 1.5A (very demandin), which is the transformer I need, because I have a 12 - 12 V 75VA?, or I have to use a 30 V one?...

2 - I am very intereting on the idea that you told at the beginig of the forum, that we can use a 12+12 transformer and a circuit to conect the 24V when i need more than 12.....but I can't find a circuit.... or something symilar, to have an idea.....Have you got a circuit or soemthing like that, or a croquis...???

3 - another question I have is about the ripple.....how can I improve the filter section in the ciruit to make better the ripple rejection?.....is a good idea use a choke input filter?......please help me...

4 - may be the LM317 could be replaced by the lm350K.......

Thank you very much!!!....and sorry for my bad english.......

 

[MP1]

Some notes on ripple:
If you have power supply ripple getting past the regulator, you should increase the "input" side cap, not the output side.

The output side of a reg should already be a very low impedance, and adding caps to it beyond that needed to suppress HF noise should not be necessary, and can actually drive a reg into oscillation under some conditions.

On the input side, you must simply ensure that at the max load current, the bottoms of the AC ripple are above the min dropout voltage of the regulator. More cap = less ripple, until you get above the dropout voltage.
On this regulator, you can also use a bypass cap on the adjustment ternminal and get excellent ripple rejection. Try this if you find you still have a problem after using a good sized bypass cap on the input pin.

In regards to the voltage needed, I think you need a minimum of 3 volts more "in" than what you are trying to get "out" of it. You can check the data sheet to verify. This refers to the voltage after rectification.

MP