Many electronic devices need a steady flow of clean DC power to work well.
This post shows you how to build a circuit that turns messy DC power into 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 lets you adjust the output voltage to what your device needs.
Circuit Description
Parts List:
| Type | Component | Quantity | Details |
|---|---|---|---|
| Resistors | 120Ω | 1 | 1/4 W CFR |
| Potentiometer | 4.7k | 1 | |
| Capacitors | Electrolytic 10µF 25V | 1 | |
| Semiconductors | IC LM338 | 1 | |
| Diode 1N4007 | 2 |
Here is a detailed explanation of how the circuit works:
The unregulated DC input power supply ranging from 5V to 34V, is connected to pin 3 vin of the LM338 IC.
The LM338 has an internal voltage reference and an error amplifier.
The voltage reference is 1.25V and the error amplifier compares this reference voltage with the output voltage.
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 determines the output voltage according to the formula mentioned below.
Pin 2 vout of the LM338 is connected to the output screw terminal.
The regulated output voltage is obtained at this terminal.
By varying the values of R1 and RV1 the user can control the output voltage within the specified range 1.25V to 30V.
The maximum output current that can be obtained from IC LM338 is 5 Amp
Capacitor C1 provides a soft start to the output load.
Diodes D1 and D2 are employed for reverse voltage protection.
They prevent damage to the circuit in case of a reverse voltage at the input.
The combination of resistors and capacitor, along with the LM338 inherent stability ensures a regulated and stable output voltage with minimal ripple.
The LM338 maintains a stable output voltage even with varying loads making it suitable for applications with dynamic power requirements.
The LM338 generates a maximum constant current of 5 Amp.
Formulas:
Below mentioned formula can be used to calculate the output voltage Vout of a circuit, typically involving an adjustable voltage regulator IC LM338.
Vout = 1.25V (1 + R2 / R1 ) + Iadj * R2
where,
- Vout: This stands for the voltage you wish to compute as the output.
- 1.25V: value regulators such as the LM317 frequently use this constant reference value.
- R1 and R2: These stand for two external resistors that are wired together in series to create a circuit that divides voltage, you may change the output voltage by changing their values.
- R1: The positive terminal of the output capacitor is linked to this resistor, which is situated between it and the 1.25V reference voltage.
R2: This resistor is wired between the voltage regulators adjust pin and ground.
Iadj: This stands for the adjustment current, which is a very little current that passes via the voltage regulators adjust pin, its value is frequently insignificant in comparison to the current flowing through the resistors.
Steps to calculate Vout:
Determine the values:
To complete the computation, you will need the values of R1, R2 and (preferably) Iadj.
The voltage regulators datasheet requirements and the intended output voltage are often taken into consideration while selecting these resistor values.
Determine what the voltage divider term is:
Determine the term (1 + R2 / R1) first.
This is an illustration of the voltage gain that the voltage divider resistors provide.
There will be a greater advantage if R2 is higher than R1.
Examine Iadj (optional):
You may determine the voltage drop across R2 as a result of Iadj by multiplying Iadj by R2 (Iadj*R2) if its value is significant, which is typically not the case.
Utilize the formula:
Enter the estimated voltage divider term (1 + R2/R1), the known reference voltage (1.25V), plus if desired, the voltage drop caused by Iadj (Iadj ≅ R2) into the calculation.
Vout = 1.25V (1 + R2 / R1 ) + Iadj * 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.
Iterative techniques or circuit modeling tools are frequently employed in real world applications to ascertain Vout.
For voltage regulators such as the LM317, the Iadj term is often negligible and may be disregarded in the majority of computations.
The precision of the component values (resistors) and if relevant the taking into account of variables such as Iadj determine how accurate the computed Vout is.
Circuit Construction:
- 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.
- Ensure proper polarity and follow recommended values from the datasheet.
- Use diodes D1 and D2 for reverse protection.
- Connect them appropriately to prevent reverse voltage damage.
- Connect the input power supply within the specified range 1.5 V to 35V to ensure proper functioning of the regulator.
Testing:
- Power up the circuit and measure the output voltage using a multimeter.
- Adjust RV1 and observe changes in the output voltage.
- Ensure the regulated output falls within the desired range.
- Monitor the output for stability and observe for any ripples.
- Capacitor C1 provides a soft start voltage for the output.
- Verify that the diodes D1 and D2 provide effective reverse voltage protection.
Conclusion:
By following these construction details, you can build a regulated adjustable voltage power supply circuit using the LM338 IC providing a regulated DC output tailored to your specific voltage requirements and current upto 5 Amps.
Always refer to the LM338 datasheet for additional information and ensure safety precautions while working with electronic components.