# LM317 calculator for filament circuit

#### cbj591

##### Member
Good afternoon all,

I was looking at a couple of LM317 calculators for R1 and R2.

Question. Is the VOut given in these calculators the voltage drop from the input voltage or the actual voltage output.

The Toroid tranny I have has a 17.8v tap so I was a little confused on what the calculator VOut giving.

Any help would be grateful

Cheers
Brian

#### M Gregg

##### Disabled Account
Good afternoon all,

I was looking at a couple of LM317 calculators for R1 and R2.

Question. Is the VOut given in these calculators the voltage drop from the input voltage or the actual voltage output.

The Toroid tranny I have has a 17.8v tap so I was a little confused on what the calculator VOut giving.

Any help would be grateful

Cheers
Brian

If you can give us a link to the calculator it will be easier to comment! #### jony

##### Member
Hmm, If I understand you correct the Vout is actual output voltage.
And for exapmle if you need Vo = 15V
Then R2 = ( Vout/1.25V - 1 ) * R1 = 12*R1 = 12 *120 = 1.5K
And input voltage must by at least 3V greater the Vout.
So Vin_min = 18V

#### M Gregg

##### Disabled Account
It is the actual voltage out of the regulator!

#### cbj591

##### Member
Then what input voltage is it basing the output on?

You can play with the R! and R2 values and get different output voltages. Example if my input was 17.8?

Not trying to be dim, just not getting the concept on how it knows the output without a given input.

thanks

#### M Gregg

##### Disabled Account
The regulator has an internal ref voltage derived from the input voltage

#### cbj591

##### Member
Or does the input not matter as long as it is 1.5v above the output?

7.8v to 30v input would give the same output of 6.3?

Thanks

#### cbj591

##### Member
ok, ill look at the specs and figure that out

thanks

#### jony

##### Member
As long as Vin is at last 3V above Uout. Input voltage does not matter.
But (Vin - Vout) and load current determine the power that will be dissipate in IC.
Pdis = (Vin_max - Vout_min) * Iload_max.
And without the heatsink power dissipation in IC can not be greater then 1.2W