# Using Linear Voltage Regulator In Parallel With Two Different Voltage Outputs

#### Harsh_21

Joined Sep 23, 2021
9
Hello Everyone,
I am using a two LM317 in parallel with the same source and one LM317 should give 12v and other is 5v as a output voltage . But i am not getting the perfect output as i want. The LM317 which is for the 5v output is getting perfect output . But the the other LM317 is not getting the 12v as a output.
I am driving a controller with 5v . And 12v is further going to the my circuit .

#### click_here

Joined Sep 22, 2020
444
Are you able to draw a quick circuit diagram?

#### vu2nan

Joined Sep 11, 2014
188
The LM317 requires an input-to-output differential voltage greater than 3 V, for proper functioning.

That would mean an input voltage greater than 15 V for 12 V output.

Nandu.

Last edited:

#### Harsh_21

Joined Sep 23, 2021
9
The LM317 requires an input-to-output differential voltage greater than 3 V for proper functioning.

That would mean an input voltage greater than 15 V for 12 V output.

Nandu.
I am Giving The Input Voltage of 20V...

#### Harsh_21

Joined Sep 23, 2021
9

#### vu2nan

Joined Sep 11, 2014
188
Please recheck for the correctness of R1 and R2 values.

Nandu.

#### Harsh_21

Joined Sep 23, 2021
9
Please recheck for the correctness of R1 and R2 values.

Nandu.
Calclulated through online calclulator for LM317

#### vu2nan

Joined Sep 11, 2014
188
The formula is V = 1.25 ( 1 + R2 / R1).

That would be 12.7 V with R1 = 240 Ω and R2 = 2.2 kΩ.

Nandu.

Last edited:

#### Harsh_21

Joined Sep 23, 2021
9
240 Ω and 2.2 kΩ would give 12.7 V.

Nandu.
But I want the 12v So that's giving me when tried single.

I want to know what if i give the 13v input to the LM317 for output of 12v . What should it give 12v or something lesser then 12v.

#### ericgibbs

Joined Jan 29, 2010
13,864
hi,
As vu2nan posted:
The LM317 requires an input-to-output differential voltage greater than 3 V, for proper functioning.

So Vin must be a minimum of 15Vdc for a 12Voutput

The 500R resistors should be 240R as specified in the datasheet for the LM317.

E

• vu2nan

#### Harsh_21

Joined Sep 23, 2021
9
Actully the problem with the thing is that my DC voltage source is ranging from 25 to 12v And i want these two output of 12v and 5v continuosly . Can help me out with this??

#### BobTPH

Joined Jun 5, 2013
3,814
The 25V is too high unless you are using very low current, and the 12V is too low.

At 1A, the 5V regulator would dissipate 20W. And at 12V, the 12V regulator would not regulate.

You would be better off with two DC to DC converters. A buck for 5V, and a buck boost for 12V.

Bob

• #### Harsh_21

Joined Sep 23, 2021
9
You could use a Buck/Boost switch mode reg for the 12V out, then feed the 5V reg from that.
A buck/boost covnverter will give an output voltage below and above the input voltage.
https://www.ebay.com.au/itm/142535580645?hash=item212fc85be5:g:CeMAAOSwLTNZ3e~1&frcectupt=true
The problem with the buck/boost converter is that when i am feeding the 12v then it will drop to some other voltage.
For example : we set the buck/boost converter to convert the 25v to 12v then it is running perfeectly.
but at the time of 12v input instead of 25v then it will drop below 12v to 9 - 10 v ..

I have mentined in the post#11 that my input DC source ranging from 25v to 12v.

#### BobTPH

Joined Jun 5, 2013
3,814
The problem with the buck/boost converter is that when i am feeding the 12v then it will drop to some other voltage.
No, a buck boost converter can output a constant voltage with an input less or greater than the output.

Bob

Joined Feb 20, 2016
3,924
The problem with the buck/boost converter is that when i am feeding the 12v then it will drop to some other voltage.
For example : we set the buck/boost converter to convert the 25v to 12v then it is running perfeectly.
but at the time of 12v input instead of 25v then it will drop below 12v to 9 - 10 v ..

I have mentined in the post#11 that my input DC source ranging from 25v to 12v.
A Buck converter will need more than the o/p voltage to work, maybe 14V min i/p for 12V out.
A Boost converter needs less than the o/p voltage to work.

That is why you use a Buck/Boost as it can run from less than the o/p voltage, as well as greater than the o/p voltage.
A Buck/Boost reg will give 12V out on less than 12V in, and 12V out when greater than 12V in.

The one from Ebay is listed as.......

Module Properties: non-isolated step-up & down module (BOOST & Buck)
Input voltage: 5-32V
Output voltage: (1) continuously adjustable (1.3-35V)
Output Current: 1.5A (MAX)
Input Current: 4A (MAX)
Output power: natural cooling 20W, plus heat sink 25W (MAX) real power

#### Harsh_21

Joined Sep 23, 2021
9
A Buck converter will need more than the o/p voltage to work, maybe 14V min i/p for 12V out.
A Boost converter needs less than the o/p voltage to work.

That is why you use a Buck/Boost as it can run from less than the o/p voltage, as well as greater than the o/p voltage.
A Buck/Boost reg will give 12V out on less than 12V in, and 12V out when greater than 12V in.
Can you send the link for the circuit which consist of buck boost converter??

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