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LM-317 Voltage Regulator Designer
by Martin E. Meserve

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Introduction

Regulator Design

Precision Resistors

Variable Resistors

Introduction

This web page is intended to aid the user in developing a voltage regulator that can be used to obtain tighter load variations and reduce ripple voltages. The power supply, described in the Un-Regulated Power Supply Design web page, is intended to be a front end to this regulator. However, any unregulated power supply can be used to feed this regulator as long as it meets the current and voltage requirements of the regulator.

The heart of the regulator circuit is the LM317 3-terminal adjustable regulator. The LM317 is a floating regulator and therefore sees only the Input-to-Output differential voltages. This allows the LM317 to be used in power supplies of several hundred volts, as long as the maximum Input-to-Output differential of 40 Volts is not exceeded.

There is a wide variety of configurations for the LM317, but this web page will only deal with a couple of them. If you want to look deeper into the LM317 you can view the LM317 Specification available from National Semiconductors Inc..

The simplest configuration is shown in Figure 1. It's very simple and consists of the regulator and two resistors. The un-regulated power supply voltage is fed into the terminals on the left and the regulated voltage is taken from the terminals on the right.

The key to determining the resistor values, for a specific regulator voltage, is the fact that the voltage between the ADJ terminal and the OUT terminal will always be 1.25 Volts.


Figure 1 - LM317 Regulator Basic Configuration

Based on this, the voltage across R3 will be 1.25 Volts. R3 is normally chosen to be between 100 and 470 Ohms. Then, knowing that the voltage across R4 must be the Output voltage, minus the voltage across R3, we can use the set of equations on the right to determine the value of R4 for any output voltage.

Pretty simple, huh? Well, a usable circuit will be a little more complicated than that. As shown in Figure 2, most manufacturers recommend By-Passing the Input and Output leads with 1 uF Tantalum Capacitors soldered directly to the regulator leads, as close as possible to the points where the leads project through the heat sink.

If one of your requirements is to be able to adjust the output voltage over a range of voltages, R4 could be relpaced with a variable resistor, as shown in Figure 3. This is not a very good method to control the output voltage as control near the high end of the range will be rough. And, unless you actually do want your output voltage to go down to 1.25 Volts, it would be better to use a fixed/variable combination to vary the output over a small range of voltages.

The section at the end of this web page, on determining the value of a Variable Resistor, contains a more indepth explanation.


Figure 2 - LM317 Regulator with 1uF By-Pass Capacitors


Figure 3 - LM317 Regulator with Variable Output Voltage

And, last but not least, is your current requirements. If your regulator needs to manage more than 1.5 Amps, the circuits above will not be sufficient. To extend the current carrying capacity of the regulator circuit, a power transistor will need to be added.

Figure 4 shows the addition of a resistor, R3, and PNP Power transistor, that would be required to shunt some of the current around the LM317 regulator. This web page does not cover this kind of arrangement, but I may add a follow-on to these pages later.


Figure 4 - LM317 Regulator with Power Transistor

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Introduction

Regulator Design

Precision Resistors

Variable Resistors

LM317 Regulator Design Data

On the right, enter the minimum regulator Input Voltage. For the LM317 regulator this voltage must be greater than, or equal to, 3.7 Volts.

Then, enter your required Output Voltage. This voltage can be set as low a 2.5 volts as long as, (1) the voltage is 1.25 volts lower then the Input Voltage and (2) the voltage differential, between the input and output, does not exceed 40 volts. You will be notified if either of these conditions is not met.

If all is well with the Input/Output Voltages, select a value for R3 and the tolerance of the resistor R4, that will be used in the circuit. The selectable values for R3 are 5% tolerance resistor values, however, R4 can be a 5% or 10% tolerance resistor. The range of resistors available in 5% tolerance is greater than 10% and will give you a output voltage that is closer to the output voltage you require.

You can pick any resistor and tolerance you want, for initial calculations, and the change it later at any time. All calculations are re-done any time any input data changes.

Input Voltage

Output Voltage

x

Value of R3

Tolerance of R4

A x resistor for R3 produces a current through R3 of x. Assuming the Output Voltage to be x, and the voltage across R3 to be 1.25 Volts, the voltage across R4 should be x with a current of x. The calculated value for R4 would then be x.

If a x, x% Tolerance, resistor is used for R4, the output voltage will be x. If a x, x% Tolerance, resistor is used for R4, the output voltage will be x. These are the closest resistors values, above and below the calculated value, for the tolerance you specified.

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Introduction

Regulator Design

Precision Resistors

Variable Resistors

Create a Precision Resistor from
Two Standard Resistors for R4

If the standard resistors, calculated for R4 in the previous sections, doesn't bring the output voltage close enough to the value you want, you can put two resistors in parallel to do the job.

This section calculates the values of two standard value resistors Ra and Rb which, when connected in parallel, will result in a net resistance R that will be within very close tolerances to the required value of R4. This web page only does a calculation for the resistor R4, that is determined within this web page. For a more versatile web page and a more detailed explanation on how this calculation works, go to the section on Precision Resistors.

Resistor Tolerances for
Ra & Rb

For precision resistors less than 10 Ohms, refer to the HamCalc program Copper Wire Programs for data on copper wire resistors.


For a target resistance of x, with a voltage of x across point X to Y, and using x% tolerance resistors, a value of x for Ra and x for Rb have been selected. Ra will conduct x of current and dissipate x of power. Rb will conduct x of current and dissipate x of power. Select appropriate power ratings for these resistors.

The actual resistance between points X and Y, using the above resistors, is x which will produce an output voltage of x. This is within x% of the required output voltage.

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Introduction

Regulator Design

Precision Resistors

Variable Resistors

Replacing R4 with a Variable Resistor

A variable resistor in place of R4 will allow you to make the regulator output variable over a range or enable you to fine tune the regulator output. You have two choices here. You can replace R4 directly with a variable resistor, or, replace R4 with a fixed resistor and small variable resistor, in series.

Replacing R4 directly, even though it minimizes parts, is not really a very good selection. As can be seen from the drawing, as the potentometer is adjusted, part of R4 is placed in series with R3, continually changing the relationship of the resistances above and below the tap point that goes to the ADJ pin of the LM317.


Figure 5 - LM317 Regulator with Variable Output Voltage

Having the tap point vary over a wide range will cause the output to vary widely and will, more than likely, cause the regulator to go into the Dropout range, where it will no longer be effective.

To illustrate my point, lets assume that you need a regulator that will have 20 Volts on the input and 15 Volts on the output. Further, selecting a 220 Ohm resistor for R3 requires a 2420 Ohm resistor for R4. If you choose a 2,000 Ohm variable resistor for R4, you won't be able to bring the output to the full 15 Volts. Choosing the next highest variable resistor avialable, 5,000 Ohms, the chart on the right could be made. The chart assumes a linear variable resistor and shows the percentage of rotation versus the output voltage.

In the chart, note that almost 50% of the output voltage swing is confined to the upper 10% of the variable resistor. Unless your dealing with a 10 turn variable resistor, this would be a little difficult to adjust in that range. Also, at or near the 100% setting, the differential voltage is too small for the regulator and it drops out of regulation.

Rotation Percentage v.s Output Voltage

VR %

V Out

Notes

VR %

V Out

Notes

100 %

17.5 V

Dropout

45 %

2.03 V

 

95 %

13.88 V

 

40 %

1.88 V

 

90 %

9.06 V

 

35 %

1.75 V

 

85 %

6.73 V

 

30 %

1.64 V

 

80 %

5.35 V

 

25 %

1.55 V

 

75 %

4.44 V

 

20 %

1.46 V

 

70 %

3.79 V

 

15 %

1.38 V

 

65 %

3.31 V

 

10 %

1.31 V

 

60 %

2.94 V

 

5 %

1.25 V

 

55 %

2.64 V

 

0 %

1.19 V

RP = Min

50 %

2.4 V

 

 

 

 

Replace R4 With a Fixed Value Resistor and a Small Variable Resistor

A better solution to the control problem is to replace R4 with a small fixed value resistor in series with a variable resistor, RP. As you can see in Figure 6, the wiper of the variable resistor is connected so that it shorts out part of the resistance as it is rotated.

If you replace R4 with a x potentiometer, and a standard value resistor of x in series, you would be able to vary the output voltage from x to x.

Use the button on the right to view the Variable Resistor Setting v.s. Output Voltage.


Figure 6 - LM317 Regulator with a Small Variable Resistor and a Fixed Resistor