# LM317 - 3 terminal voltage regulators question (Help needed)

#### Spow

Joined Apr 13, 2009
2
Hey,
I have a problem with the question i attached with this thread. I know there is an equation for getting the output voltage, it goes like this:
I would really appreciate if someone showed me the way and helped me understand it.

Peter

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#### bertus

Joined Apr 5, 2008
22,094
Hello,

Take a look at the application hint:

Is the value of Vadj also given?
This can vary between 1.2 and 1.3 Volts.

Greetings,
Bertus

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#### italo

Joined Nov 20, 2005
205
basicaly is the ref v=1.25 ~v that is fixed if you add a 100Ω from out to adj then 1.25v/100Ω=12.5ma now if you want 12v the is this current across a resistor +1.25v will get the output volts lets try it for 12v 12-1.25/12.5ma=rΩ or 860Ω. so 100 and 860 to gnd get you ~ 12v . change the 100 to something else is the same procedure good luck.

#### Engr1001

Joined Apr 15, 2009
3
10/4.7 =
2.1276596
ans+1 =
3.1276596
ANS*1.25 =
3.9095745
50e-6*10e3 =
0.5
ans+3.909 =
4.409

Seems like you claculate 4.4v and this is not one of the choices in the figure you provided?
Show you work amd state the answer you have arrived at.

#### Audioguru

Joined Dec 20, 2007
11,248
The school-book question is wrong.
R1 is supposed to be 120 ohms for an LM317 with such a high input voltage. With 4.7k for R1 then the output voltage will rise to about 30V without a load and will not be regulated.

#### italo

Joined Nov 20, 2005
205
audioguru the resistor it is not suposed to be nothing like 120Ω or 100Ω this resistor set up a bias adjustment of typ 50ua or 100ua max and that also change with temp of the chip -50° = 40ua +150 = 59ua this is the bias current variation as temp changes read your specs. to the circuit as shown the output is 3.9096v kind of impritical but this is school work.

#### Wendy

Joined Mar 24, 2008
23,052
A LM317 absolutely requires a minimum of 10ma to work, it is what powers the internal circuitry. For this reason a 120Ω is usually considered a minimum resistance (1.25V/120Ω = 10.4ma). The 10ma requirement is in the spec sheet.

You start with this as a given value and calculate from there.

#### Vishg14

Joined Apr 16, 2009
4
V(ref) for LM317 is 1.25volt :
According to the formula the answer is 4.4volt.

#### SgtWookie

Joined Jul 17, 2007
22,227
4.41v would be the "roughly correct" answer, based upon a Vref of 1.25v and the other parameters given, which is nominal for an LM317. Since that answer is not in the list, the closest being 4.25v with no tolerance specified, the question is not correct.

However, regulation is not guaranteed unless >=10mA current load is placed upon the output of the regulator. Since the previously mentioned 120 Ohm resistance is not R1's value (which would guarantee a minimum 10mA load for any Vref within the specifications), the output voltage may be significantly different than calculated, and vary widely over temperature.

#### Wendy

Joined Mar 24, 2008
23,052
Adding to what Wook said, if you can guarantee a 10ma load, you can lighten up on the feedback resistors. This is a common restriction on power supplies, which is why most of my designs have a LED circuit hanging off the end.