Design of DC regulated power supply and leakage protection system based on LM358

LM358 pin diagram and functions

LM 358 has a total of 8 pins, each pin has different individual functions. The following is the LM 358 pin diagram and functions.

Pin 1 and Pin 8 are the comparator outputs.

Pin 2 and Pin 6 are inverting inputs.

Pin 3 and Pin 5 are non-inverting inputs.

Pin 4 is the GND terminal.

Pin 8 is VCC+.

Operational Amplifier:

Operational amplifiers, or op amps for short, are essentially DC-coupled high-gain voltage amplification devices. They are ideal for signal conditioning, DC amplification, filtering, and can be used with external feedback components such as capacitors and resistors between their outputs. ) together with the input terminal.

An op amp performs different functions depending on its feedback configuration, whether resistive, capacitive, or both, and based on this, it can use a differential amplifier, integrator, or adder.

Non-inverting input:

The non-inverting input of the operational amplifier is marked with a "+" sign on the circuit diagram of LM358, and the non-inverting input is 3

pin. It is found that a positive voltage is applied to the non-inverting input, then it will not change, it will produce a positive swing at the output.

If you apply a changing waveform (eg a sine wave) to a non-inverting input (eg pin 3 in the LM358) then it will appear at the output the same way, it has not been inverted.

Inverting input:

The inverting input of the operational amplifier is marked with "-" in the circuit diagram of LM358, and the inverting input is pin No. 2.

When a positive voltage is applied to the inverting input, a negative voltage swing will occur. Therefore, applying a sine wave to the inverting input will result in inversion at the output.

LM358 working principle

A Proteus simulation of LM358 is designed here, which can give you a better understanding of the working principle of LM358IC. In this simulation, a small automatic LED switching circuit is designed based on the LDR value. The image is shown below:

You can see in the picture above that I connected the LDR to the input pin and the LED to the output pin of the LM358.

Now, when the LDR dims, the LED will remain off, but when the LDR lights up, the LED will also light up.

The variable resistor is used for sensitivity purposes, and in the image below, its ON state is shown:

You can see in the image above that the LED is now on because the LDR is on.

Design of DC regulated power supply and leakage protection system based on LM358

Using LM358, field effect transistors, and operational amplifiers, a low-voltage, wide-range linear DC regulated power supply is designed and produced. The power supply has reliable voltage stabilizing characteristics under almost no-load and large load conditions.

1. Design plan

This system mainly consists of a DC voltage stabilizing module, a leakage detection module, a shutdown protection module, etc. These modules are introduced below.

1.1 Stabilized power supply circuit

The regulated power supply circuit is shown in Figure 1. This circuit module uses LM358 as a stable amplifier and a P-channel MOS transistor. LM358 has two independent, high-gain, internal frequency compensation dual operational amplifiers, which can achieve input voltage When the voltage changes from 5.5 to 25V, the output terminal should be stabilized at 5±0.05V.

Figure 1 Stabilized power supply circuit

1.2 Leakage detection circuit

The leakage protection device requires that when the input voltage is 5V, the output voltage should not be less than 4.6V. The maximum voltage across the leakage detection resistor R1 is 0.4V. The normal operating current of the leakage protection circuit is 250mV. It is calculated that the resistance of R1 should not be greater than 1.6Ω. Therefore we choose a 1Ω resistor as the detection resistor. In the circuit shown in Figure 2, a 1Ω resistor is connected, and the current is calculated by detecting the voltage change at both ends of the resistor. Since the voltage is too small, a subtraction circuit is used to amplify it, and the amplified signal is much larger than the input offset voltage. Then use a voltage comparator to determine the leakage current. This circuit is stable and reliable, and after testing, it has achieved high voltage detection accuracy.

Figure 2 Leakage detection device circuit

1.3 Analysis of shutdown protection

The shutdown protection is shown in Figure 3. R1 is the detection resistor. Adjust the value of resistor R1 appropriately. When the leakage current of the circuit is 30mA, the voltage difference between both ends of R1 is sent to the base of the triode through the two-stage amplification of LM358, causing the triode to conduct Pass, relay K1 is closed, the normally closed contact is disconnected, and the load power supply is disconnected. At the same time, the normally open contact is closed to form a self-locking. After the leakage fault is eliminated, press the normally closed reset button S1 and the circuit will return to the initial state.

Figure 3 Shutdown protection circuit

2. System analysis and testing

2.1 Stabilized power supply circuit test

Test plan for voltage regulation rate: The load R is fixed at 5Ω. When the input voltage changes from 5.5 to 25V, record the measured output voltage and observe whether the result changes within the range of 5±0.05V. Calculate the voltage regulation rate, Su=|Uo2- Uo1|/Uo2×100 (Uo1 is the output voltage when the input voltage is 5.5V, Uo2 is the output voltage when the input voltage is 25V).

2.2 Leakage protection circuit test

Remove the 5Ω load from the regulated power supply and switch to the leakage protection position. At this time, the working voltage is 5V. Connect a 20Ω load and a 330Ω adjustable resistor in series with the analog branch. Use a digital multimeter in parallel to monitor the analog leakage branch current to observe the reliability and sensitivity of the leakage protection circuit.

3.Test results and analysis

3.1 Test results

The test results of voltage regulation are shown in Table 1.

3.2 Result analysis

After testing, when the branch current reaches 29.6mA, the relay absorbs and self-locks, disconnecting the load. Reduce the branch current to 26mA, press the reset button, and the circuit returns to normal detection status. After repeated testing, the circuit is reliable and stable and meets the requirements for safe use. The absolute value of the operating current error=|30-29.6l/30×100%=1.3%.