Theoretically: Constant voltage is the voltage output does not change regardless of the load. The constant current is that the current output of the power supply does not change regardless of the load. To achieve constant voltage, the internal resistance of the power supply is required to be infinitely small. No matter how the external current changes, the loop current will change, and the voltage drop will not be caused by the internal resistance of the power supply, thus ensuring a constant power supply of the power supply, and achieving constant current. It is necessary to go to the power supply with an infinite internal resistance. No matter how the external load changes, the loop resistance changes, and the relatively infinite power supply internal resistance does not cause any change in the impedance on the loop, and the coincidence obtains a constant current power supply.

However, from the power supply point of view, the realization of constant voltage and constant current circuit is different, the internal resistance of the ideal constant voltage circuit is zero, and the internal resistance of the ideal constant current circuit is infinite. That is to say, the actual so-called constant voltage or constant current circuit is limited to a limited load within a certain resistance range.

The real difference between constant voltage and constant current is the application. The signal sent by the constant voltage source can be connected in parallel with a constant load. For example, when connected to a voltmeter, an oscilloscope, an AD converter, etc., the signal sent by the constant current source cannot be used in this way; the signal sent by the constant current source is suitable for series connection. Load, but when the long-distance transmission of the slowly varying signal is that the voltage drop on the line does not affect the amount of current obtained on the load, it is obvious that the constant voltage source cannot do this. It can be seen that the essential difference between constant voltage and constant current is reflected in their application.

For the problem of constant voltage and constant current in the circuit at the same time, for the ideal resistance, constant voltage is realized to achieve constant current, but this has no practical application significance. The discussion of constant voltage and constant current only makes sense in the changing load. For a variable load, it is obvious that it is impossible to achieve both at the same time.