The concept of constant current power supply

When the grid voltage and other influences change with a certain range, it can provide a stable output current power supply.

What is constant current? What is constant current power supply?

Constant current can also be called steady current, which is similar in meaning and generally does not need to be distinguished. Compared with the concept of constant voltage, the concept of constant current is more difficult to understand, because constant voltage sources are more common in daily life. Storage batteries and dry batteries are DC constant voltage power supplies, while 220V AC can be considered a kind of AC Constant voltage power supplies, because their output voltage is basically unchanged, does not vary greatly with the changes of the output current .

First, give an example: a constant current value adjusted to 1A and a maximum output voltage of up to 100V. When you turn on the power switch of this constant current source, you will see what value the power supply’s voltmeter and current meter. What? It can be seen with certainty that the output voltage is 100V and the output current is 0A. Someone once asked, are you not a 100V 1A constant current source? Why is the output not 100V 1A? Here we still need to use Ohm’s law to explain. Theoretically, it can be calculated like this: the output voltage of the power supply U=IR, where U is the output voltage, I is the output current, and R is the load resistance.

The following is divided into 5 situations to explain:

If the power supply is no-load, R can be represented by infinity, U=I* ∞, because the power supply can output 1A current, if the power supply current is 1A, then U=1A* ∞ = ∞, and the power supply voltage can only output 100V at most Undoubtedly, the power supply can only output its maximum voltage of 100V. Since the power supply cannot output an infinite voltage, the current can only be a very small value, that is, the current output is 0A, that is, I=U/R=100V/ ∞ =0A.

If the load resistance R=200 ohms, then because the power supply can only output 100V, the current can only be 0.5A, that is, I=U/R=100V/200R=0.5A

If the load resistance R=100 ohms, because the power supply can output 100V, the current can reach 1A, that is, I=U/R=100V/100R=1A, and the output current just reaches the constant current value of the power supply.

If the load resistance continues to decrease, change it to 50 ohms. According to the formula I=U/R=100V/50R=2A. But the key here is that our power supply is a power supply with a constant current value of 1A, so the output current at this time It can only be forced to be restricted to 1A instead of 2A, so the output voltage can only be forced to drop to 50V instead of 100V. Here we still have to comply with Ohm’s law, that is, U=IR=1A*50R=50V

If the load resistance becomes 0 ohm (that is short circuit), then since the output current can only be 1A, the output voltage can only be 0V, that is, U=I*R=1A*0R=0V

From the above 5 examples, it can be seen that if the load resistance is too large, the output current of the power supply cannot reach the constant current value, then the output voltage of the constant current source will automatically rise to the maximum output voltage of the power supply, only when the load resistance is small to a certain value The output current of the power supply reaches a constant current value, and the power supply is truly in a constant current working state. With the gradual decrease of the load resistance value, the output voltage also drops regularly to keep the output current constant. This is the concept of constant current.

In general, whether it is a constant voltage power supply or a constant current power supply, they are essentially the same. Their output is voltage and current. Of the two quantities, the power supply can only control one of them, or stabilize the voltage, Either stabilize the current, the other quantity must be determined by the load resistance, and the load resistance is determined by the user, so one of the two output quantities of the power supply must be determined by the user. Only in accordance with the logic, in accordance with Ohm’s law, can it be used by the user, it does not matter whether the output voltage and the output current can be given at the same time.

Post time: Aug-26-2021