LM317 calculator

- tsp
Last update 21 Oct 2022
Reading time 3 mins

The LM317 is one of the most common used parts in the history of hobby electronics. It’s a flexible linear adjustable voltage regulator that can convert an input voltage between $3 V$ and $40 V$ into an output voltage in the range of $1.25V$ to $37V$. Note that it’s a linear regulator so excessive potential is simply thermally converted. The larger the difference between input and output voltage is the larger your heat sink needs to be. There even had been 2A $3V$ to $12V$ laboratory power supplies that just consisted of some resistors that could be dialed using a rotary dial, a transformer down to around $24 V$, a bridge rectifier, a few capacitors and a LM317 - mounted of course on a really huge heat sink since in this configuration the power supply will most likely dump most of the power.

The following information is just a summary and mini calculator for my own personal convenience - it does not substitute reading the data sheet of course.

Voltage regulation: The basic schematic

The basic schematic

Usually the LM317 is used in conjunction with:

The resistor network $R_1$ and $R_2$ selects the output voltage, usually $R_1 = 240 \Omega$. The regulator works by keeping the voltage drop over $R_1$ at a constant $1.25V$ above the adjust input.

[ V_0 = V_{ref} * (1 + \frac{R_2}{R_1}) + I_{adj} * R_2 ]

Usually $I_{adj} \approx 50 \mu A$ and is thus usually neglected from most design calculations. With constant $R_1 = 240\Omega$ one can simply derive:

[ V_{Out} = V_{ref} * (1 + \frac{R_2}{240 \Omega}) + \underbrace{I_{adj} * R_2}_{\approx 0} \\ V_{Out} = V_{ref} * (1 + \frac{R_2}{240 \Omega}) \\ V_{Out} = V_{ref} + V_{ref} * \frac{R_2}{240 \Omega} \\ R_2 = \frac{(V_{Out} - 1.25 V) * 240 \Omega}{1.25 V} ]

Of course it doesn’t get much more complicated in case one does not neglect the $I_{adj}$, usually it’s just not required

[ R_2 = \frac{V_{out} - V_{ref}}{\frac{V_{ref}}{R_1} + I_{adj}} ]

Resistor calculator

In case JavaScript is enabled in your browser the following calculator is able to calculate one of the three parameters in the input/output section (just leave one field empty):

Input and output parameters
Output voltage: V
$R_1$ Ohm
$R_2$ Ohm
Discard adjustment current

Current limiting

One can also use the LM317 to provide current limiting. This is pretty simply and also exploits the function that the LM317 just tries to maintain $1.25 V$ voltage difference between it’s output pin and the adjust input. One can simply use an sense resistor - in the following schematic this has been the potentiometer $R_{V1}$:

LM317 current limiter

[ 1.25 V = R_{V1} * I_{out} \\ R_{V1} = \frac{1.25 V}{I_{out}} ]

The idea is really simple - when there is too much current the regulator outputs less voltage, when there is not enough current the regulator increases the output voltage as far as possible. One can use this as current regulator or current limiter. In case the input power supply cannot increase over a given voltage it just caps there - providing an effective active current limiter. Note that in worst case the LM317 has to dump the whole power into thermal energy.

For example to provide a $100 mA$ current limit one would require a $12.5 \Omega$ sense resistor. For a $1 mA$ current limit on the other hand it would be a $1.25 k\Omega$ resistor.

This article is tagged: Tutorial, Basics, DIY, Electronics

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Dipl.-Ing. Thomas Spielauer, Wien (webcomplains389t48957@tspi.at)

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