# LM317 question

Discussion in 'The Projects Forum' started by adriano08, Sep 11, 2010.

Apr 25, 2010
33
1
Hi guys!

I need a 5.5v and 3.3v supply to power a circuit (which got two supply). So I decided to use
LM317 variable voltage regulator. My idea was a 9v battery and step down to 5.5v and a 3.3v. I looked up the datasheet and patched up the circuit as followed:

http://www.national.com/mpf/LM/LM317.html#Overview

So I used two of LM317 to get a 5.5v and 3.3v output. I managed to test it out via a multi-meter after adjusting the variable potentiometer. It went correctly. A 5.5v and 3.3v output each.

The weird thing was when I used the output to power my circuit, the output voltage dropped. From the multi-meter it reads 1.9v(supposed to be 5.5v) and 2.7v(for a 3.3v output). The voltages was simply too low to get my circuit working.

I really don't understand why.

Wasn't it supposed to be 5.5v and 3.3v powering my circuit? When connected up to the power, why it became 1.9v and 2.7v???

Uhhh. Anyone??

2. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,224
Have you measured the battery voltage then you have this voltage drop? Also how much current do your circuit draw

3. ### bertus Administrator

Apr 5, 2008
17,137
3,007
Hello,

Do you realize that 9 volt block batteries have a pretty large internal resistance?
The voltage will drop pretty quick when the load is to high.
Measure the input voltage when there is no load and when there is a load.
You will see quite some difference.

Bertus

4. ### Markd77 Senior Member

Sep 7, 2009
2,803
595
Have a look at the dropout voltage graph on the datasheet. Worst case the 317 needs 2.5V more than the output, so 8V for the 5.5V output.

Apr 25, 2010
33
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Hey hello!

I see.. so how do I overcome this? I mean I tried using 9v power supply to step down. When it is okay, then I will switch to a 9v battery. Unfortunately not.

Because I need to use a 9v supply... any idea?

Apr 25, 2010
33
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It's a 9v battery I supposed to use. Now I am using power supply for testing..

Hmm.. not sure about the current.. maybe around 0.1A to 0.25A?

7. ### bertus Administrator

Apr 5, 2008
17,137
3,007

Apr 25, 2010
33
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I looked through the datasheet.. what do you mean by 2.5v more than the output, 8v for the 5.5v output? I don't understand the theory in it..

Is it when the temperature goes higher with more current (load) exist, the difference of the Dropout voltage will be huge?

9. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,224
Do you have a multi or volt meter you can use to measure the battery voltage? If yes can you give us the voltage then connected and not connected

Apr 25, 2010
33
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Hey!

Thanks for your info. I have thought of your idea as well but this is the worse case scenario. My main aim is to use a single 9v supply.. and since LM317 seems to have some hope for me to step down to 5.5v and 3.3v, I assume it can just do the tricks.

Hmmm..... how?

11. ### Markd77 Senior Member

Sep 7, 2009
2,803
595
This graph is from the National datasheet - other manufacturers will be similar. It just shows the amount that the supply voltage has to be higher than the desired output voltage at different currents and temperatures.

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12. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
202
9V batteries were never meant to supply any sort of current, I think they were first developed to power transistor radios. The transistors available at the time weren't exactly state of the art and the output stage typically consisted of two transistors in a simple push/pull circuit driving a center tapped transformer.

Alkalines improved matters but they were still just jamming 6 cells similar to a AAAA (yes, 4 A's) into the standard case size. The regular ones were (and probably still do) sandwich 6 oddball sized rectangular cells in series.

You can get a pretty good initial current from a modern 9V alkaline battery but only for a short period of time, the internal cell resistance builds up quite quickly.

Common linear regulators are going to eat up a decent amount of your capacity due to their internal losses. Adjustable switching regulators would be a better option.

Also, as mentioned above, if you're not limited as to this being the actual 9V package you're going to be better off using multiple cells of a larger size. Even common AA batteries are extremely good now depending on the brand, lots of competition due to word of mouth as to how many pictures someone can get or how long their game will run on Duracell vs Energizer vs Ray-O-Vac etc. so there's an incentive to build them the best they can.

If you are limited to the 9V package size I believe they have lithium 9V batteries out now and those can last anywhere from 2 - 10 times as long.

You sure about the 5.5V? 5.1V was a far more common voltage.

Apr 25, 2010
33
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Now I don't use battery... for testing I used power supply. I set it to 9v. But the result is the same..

When connected the output to power my circuit (its a small circuit btw with a relay, a logic gate, a voltage comparator, resistors)...

the output goes to the power rail of tha circuit:

Result:

5.5v becomes 1.9v, and 3.3v to 2.7v.. as measured from the multi-meter.

14. ### bertus Administrator

Apr 5, 2008
17,137
3,007
Hello,

What happens to the input voltage?
Is that stable without and with the load?

Do you use the recomended capacitors at in and output?

Bertus

15. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,224
As I see it. It could be that your LM317 is not connected properly. Also it may not be anything wrong with your voltage regulator setup. But the unit you are connecting to your voltage regulator

Apr 25, 2010
33
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Hey! Thanks for that great info man.

I will be using 9v Energizer block battery.

Yes, too bad my circuit need that 5.5v to work. Anything less than that won't work.

Apr 25, 2010
33
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It is stable without the load. With load, the voltage drops.

Yes I followed the everything on the setup datasheet.

Apr 25, 2010
33
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I think I'd agree with you.

When I supplied 5.5v and 3.3v directly from a power supply, my circuit works properly.

But when I used the vol reg circuit which I adjusted to 5.5v and 3.3v and connected it to my circuit power source, the voltage drops.

When disconnected, I checked the voltages again, it reads 5.5v and 3.3v from the output.

Maybe....

19. ### bertus Administrator

Apr 5, 2008
17,137
3,007
Hello,

What happens to INPUT voltage?
You only mention the OUTPUT voltages.

Can you post a schematic of what you have upto now?

Bertus

20. ### Wendy Moderator

Mar 24, 2008
21,012
2,745
Both regulators should go to the 9V power supply, and not be interconnected.

Even 6X AA batteries will beat a 9V battery and run at the same voltage.

Do you have the filter caps on the inputs and outputs? You need a minimum of 0.1µF on both outputs and the common input, and probably a low capacitance electrolytic too (say 33µF). You might think of a larger electrolytic for the common inputs (say 220µF).