Design Fundamentals of DC Electronic Loads

In the series circuit of DC electronic load, the current at each point is the same, and the circuit needs to work with constant current. As long as the current flowing through one component is controlled in the series circuit, the constant current output we control can be achieved.

 A simple constant current circuit, usually used in applications with low power and low requirements. In other applications, this circuit is powerless, such as: when the input voltage is 1V and the input current is 30A,

This requirement cannot guarantee the work at all, and it is not very convenient for the circuit to adjust the output current.

 One of the most commonly used constant current circuits, such a circuit is easier to obtain stable and accurate current values, R3 is a sampling resistor, and VREF is a given signal.

The working principle of the circuit is, given a signal VREF: When the voltage on R3 is less than VREF, that is, the -IN of OP07 is less than +IN, the output of OP07 is increased, so that the MOS is increased and the current of R3 is increased;

When the voltage on R3 is greater than VREF, -IN is greater than +IN, and OP07 reduces the output, which also reduces the current on R3, so that the circuit is finally maintained at a constant given value, which also realizes constant current operation;

 When the given VREF is 10mV and R3 is 0.01 ohm, the constant current of the circuit is 1A, the constant current value can be changed by changing the VREF, the VREF can be adjusted by the potentiometer or the DAC chip can be used to control the input by the MCU,

The output current can be adjusted manually by using a potentiometer. If the DAC input is used, a digitally controlled constant current electronic load can be realized. Fixed layout

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Circuit simulation verification:

Constant voltage circuit

A simple constant voltage circuit, just use a Zener diode.

The input voltage is limited to 10V, and the constant voltage circuit is very useful when used to test the charger. We can slowly adjust the voltage to test the various responses of the charger.

The voltage on the MOS tube is divided by R3 and R2 and sent to the operational amplifier IN+ for comparison with the given value. As shown in the figure, when the potentiometer is at 10%, IN- is 1V, then the voltage on the MOS tube should be 2V.

Constant resistance circuit

For the constant resistance function, in some numerically controlled electronic loads, no special circuit is designed, but the current is calculated by the input voltage detected by the MCU on the basis of the constant current circuit, so as to achieve the purpose of the constant resistance function.

For example, when the constant resistance is 10 ohms, and the MCU detects that the input voltage is 20V, it will control the output current to be 2A.

However, this method has a slow response and is only suitable for occasions where the input changes slowly and the requirements are not high. Professional constant resistance electronic loads are realized by hardware.

Constant power circuit

Constant power function Most electronic loads are implemented by constant current circuit. The principle is that the MCU calculates the output current according to the set power value after sampling the input voltage.

Post time: Oct-19-2022
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