5V Power Supply

[AtomSoft]

I want to create a Regulated 5V Supply for upcoming projects. I decided to try out the "LM317A" here are my schematics.
**broken link removed**
The one on top is the default way i seen in the data sheet
The one on bottom i made after reading PAGE 1 PARAGRAPH 3
"The adjustment terminal can be bypassed to achieve very high
ripple rejection ratios which are difficult to achieve with standard
3-terminal regulators."


My Question is will either or both work? (don't have parts yet to physically test it) Just got paid today going shopping tomorrow

 

[eblc1388]

Short answer is no. Both will not work.

Reason for top circuit is wrong resistor values.

Reason for bottom circuit is the wrong interpretation of the term "bypass" in datasheet, which here means connecting an additional capacitor between the ADJ pin and 0V.

Edit: Of course one still have to use the two usual resistors as they are needed to setup the correct output voltage.

 

[AtomSoft]

How about it on top:

R1 resistor = 120 ohms
R2 resistor = 360 ohms

Im just have trouble figuring what would be my A/mA outcome . I know the V is 1.250 now i assume if i use these resisitors i will get about 5v

VOUT = 1.25 * ( 1 + R2/R1 ) = 1.25 * (4) = 5V
which can be rearranged to give:
R2 = R1 * ( (VOUT/1.25) -1 ) = 120 * ( 3 ) = 360 ohms for R2

Am i correct ?

 

[AtomSoft]

oops didnt notice the 750K i didnt notice the k but this other 120/360 would be ok also
**broken link removed**

 

[mytech]

Why do you need 317 for constant 5v supply instead 7805?..
Anyway 317 is used for variable supplies..
Just put a variable resistor around 2k/4.7k instead 360.. by varying you can obtain 5v..

 

[Hero999]

The LM317 has a higher output current than the LM7805 and can drop 40V which the LM7805 can't do.

The LM317 doesn't have to be used in variable supplies, it can be used in a fixed supply if you use the correct resistor values.

For 5V use any of the folowing combinations of R1 and R2.

R1 = 100hm:
R2 = 300hm:

R1 = 110hm:
R2 = 330hm:

R1 = 82hm:
R2 = 240hm:

R1 = 68hm:
R2 = 200hm:

R1 = 56hm:
R2 = 180hm:

The above are all standard values that will give you 5V with a reasonable accuracy. Be warned about making R1 too low as the quiescent current will become excessive.

 

[mytech]

Yes i agree.. but in the above circuits it shows 9v input, thats why i told 7805..
Anyways thank for your good infos.. probably all our infos would help atom..

 

[Hero999]

Is this running from a 9V battery by any chance?

It won't last for long as it will stop regulating when the voltage drops below between 7V and 7.5V (depending on the current draw and how good your LM317 is) which isn't very long in to the discharge.

If you're using a 9V battery, then you need a low dropout regulator like the LM2936-5.0, LM3940, LM2937 etc. to maximise the battery life.

 

[AtomSoft]

No I have a 9v Transformer from an old cordless phone. Felt like keeping it alive.

Thanks all for your thoughts.
Hero999 I will use the:
R1 = 100
R2 = 300

most likely.

 

[Hero999]

Be warned that the output from your 9V adaptor could be much higher than 9V when there's no load connected so make sure the input capacitor is rated to 16V minimum, 25V recommended.

 

[silvarblade]

what happened to your hello all post. it was interesting.???

 

[AtomSoft]

Im waiting to get my hand on some new tools because mines are a pain. When i get them and am sure i can accurately measure my projects right ill keep posting lol but i just went shopping and forgot to buy a new multimeter. Gonna go out again in like 30 min so.. ill get it then

 

[RDL2004]

For all but the LM117, something around 120 ohms is recommended for R1.

edit: also, C2 is what they meant by "bypass the adjust pin"

 

[blueroomelectronics]

As pointed out to me in a similar thread the LM78S05 is a nice 2A 5V regulator.