Simple Adjustable Power Supply Circuit using IC LM338

Many electronic devices need a steady flow of clean DC power to work well.

This post teaches how to build a circuit that turns messy DC power into a smooth adjustable DC power using a common chip called the LM338.

This circuit can take an unsteady DC voltage anywhere from 5 to 34V and turn it into a clean voltage from 1.25 to 30V.

The knob on the circuit adjusts the output voltage to what the device needs.

Circuit Working:

Parts List:

Vin Pin 3 of the LM338 IC is connected to an unregulated DC input power supply with a voltage range of 5V to 34V.

An internal voltage reference and an error amplifier are advantages of the LM338.

The error amplifier makes a comparison between the output voltage and the voltage reference which is 1.25V.

Pin 1 Adjust of the LM338 is connected to ground through the variable resistor RV1.

By adjusting RV1 the user can set the reference voltage influencing the output voltage.

Resistors R1 and R2 form a voltage divider network.

The ratio of R2 to R1 along with the reference voltage calculates the output voltage according to the formula mentioned below.

The output screw terminal of the LM338 is connected to Pin 2 Vout.

This connection generates the controlled output voltage.

The user can regulate the output voltage within the specified range of 1.25V to 30V by modifying the values of R1 and RV1.

The maximum output current obtained from IC LM338 is 5 Amp

Capacitor C1 provides a soft start to the output load.

Diodes D1 and D2 are used for reverse voltage protection.

It prevents damage to the circuit in case of a reverse voltage at the input.

A controlled steady output voltage with little ripple is ensured by the resistor, capacitor and built-in IC LM338.

The LM338 works well in applications requiring dynamic power since it maintains a steady output voltage despite various loads.

A maximum constant current of 5 Amp is generated by the IC LM338.

Formulas:

Below mentioned formula are used to calculate the output voltage Vout of a circuit which involves an adjustable voltage regulator IC LM338.

V_out = 1.25V (1 + R2 / R1 ) + I_adj * R2

where,

• The voltage calculated as the output is referred to as Vout.
• Value regulators like the LM317 often use a constant reference value of 1.25V.
• R1 and R2 represent two external resistors connected in series to form a voltage dividing circuit by adjusting their values one can modify the output voltage.
• This resistor which is located between the output capacitors positive terminal R1 and the 1.25V reference voltage is connected to it.
• The voltage regulators adjust pin and ground pin are connected to an R2 resistor.
• The adjustment current or Iadj pin is a very small current that flows through the voltage regulators often its value is small when compared to the current that flows through the resistors.

Steps to calculate Vout:

Calculate the values:

To complete the calculation one need the values of R1, R2 and most likely a Iadj pin.

When choosing these resistor values the expected output voltage and the voltage regulators datasheet requirements are usually taken into mind.

Calculate what the voltage divider term is:

Calculate the term (1 + R2 / R1) first.

This is an example of the voltage gain that the voltage divider resistors provide.

There is a great advantage if R2 is higher than R1.

Using the formula:

The estimated voltage divider term (1 + R2 / R1) is included during the calculation along with the known reference voltage of 1.25V.

If required the voltage drop that results from Iadj is (Iadj ≅ R2).

V_out = 1.25V (1 + R2 / R1 ) + I_adj * R2

Remember:

Considering that Vout appears on both sides of the equation it is possible that this formula cannot always be solved for Vout directly.

In practical applications Vout can frequently be calculated using repeated steps or circuit modeling tools.

For voltage regulators such as the LM317 the Iadj term is often small and may be ignored in the majority of calculations

The correctness of the component values resistors and if necessary the use of factors like Iadj are used to measure the calculated Vouts accuracy.

How to Build:

To build following steps are required to follow while designing Simple Adjustable Power Supply Circuit using IC LM338:

• Connect pin 3 vin of the LM338 directly to the positive terminal of the input power supply.
• Connect pin 2 vout to the output screw terminal.
• Connect pin 1 Adjust to ground through the variable resistor RV1.
• Connect R1 and R2 according to the desired output voltage using the formula mentioned earlier.
• Adjust RV1 to achieve the desired output voltage.
• This provides flexibility in setting the regulated voltage.
• Connect capacitor C1 between resistor R1 and GND and the ADJ pin.
• Confirm correct polarity and follow the datasheets recommended values.
• For reverse protection make use of diodes D1 and D2.
• In order to avoid damage from reverse voltage connect them correctly.
• Connect the input power source between 1.5V to 35V to ensure the regulator runs as planned.

Testing:

• After turning on the circuit check the output voltage with a multimeter.
• As one makes the modifications to RV1 see how the output voltage changes.
• A controlled output should be within the desired range.
• Keep an eye out for ripples and confirm that the output is constant.
• Capacitor C1 provides a soft start voltage for the output.
• Check diodes D1 and D2 provide effective reverse voltage protection.

Conclusion:

By following these construction details one can build Simple Adjustable Power Supply Circuit using IC LM338 by providing a regulated DC output and by modifying a specific voltage requirements and current upto 5 Amps.

When working with electronic components always be sure to take safety precautions and see the IC LM338 datasheet for further information.

References:

LM338 5-Amp Adjustable Regulators datasheet