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Old 18th September 2010, 09:45 PM   #1
cbj591 is offline cbj591  United States
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Default LM317 calculator for filament circuit

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
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Old 18th September 2010, 09:49 PM   #2
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by cbj591 View Post
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!
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Old 18th September 2010, 09:54 PM   #3
cbj591 is offline cbj591  United States
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Sorry - They all seem to be the same.

Here you go.

LM317 / LM338 / LM350 Voltage and Current Regulator Calculators

LM317 Calculator

Thanks
Brian
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Old 18th September 2010, 09:56 PM   #4
jony is offline jony  Poland
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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
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Old 18th September 2010, 09:57 PM   #5
M Gregg is offline M Gregg  United Kingdom
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It is the actual voltage out of the regulator!
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Old 18th September 2010, 10:03 PM   #6
cbj591 is offline cbj591  United States
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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
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Old 18th September 2010, 10:06 PM   #7
M Gregg is offline M Gregg  United Kingdom
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The regulator has an internal ref voltage derived from the input voltage
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Old 18th September 2010, 10:07 PM   #8
cbj591 is offline cbj591  United States
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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
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Old 18th September 2010, 10:08 PM   #9
cbj591 is offline cbj591  United States
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ok, ill look at the specs and figure that out

thanks
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Old 18th September 2010, 10:16 PM   #10
M Gregg is offline M Gregg  United Kingdom
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Look at page 15.

http://www.national.com/ds/LM/LM117.pdf
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