Working conditions of three states of circuit

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In the study of electronic circuits, to learn how to analyze circuits, we must start by understanding the three states of circuits. The three states of circuits are: open circuit (load), short circuit, and open circuit (no load). The power supply voltages in the three states are U=E-IR, U=0, and U=E. The following content introduces the specific conditions of these three states respectively.

1. Access status

The path is when the switch in the circuit is closed and current flows through the load. In this state, the relationship between the voltage at the power supply end and the load current can be determined by the external characteristics of the power supply. According to the size of the load, it is divided into three conditions: full load, light load, and overload. The working state of the load under rated power is called rated working state or full load; the working state below rated power is called light load; the working state above rated power is called overload. Since overload can easily burn out electrical appliances, overload is generally not allowed.

2. Short circuit state

If the external circuit is connected by a conductor with a resistance value close to zero, the power supply is in a short-circuit state. In this state, the current in the circuit (short-circuit current) I≈E/R. We know that the internal resistance of the power supply is generally very small, so the short-circuit current may reach a very large value, which will cause the power supply to burn out and must be strictly prevented.

The most common way to prevent short circuits is to install a fuse in the circuit. The fuse in the fuse is made of a low-melting-point lead-tin alloy and silver wire. When the current increases to a certain value, the fuse is blown first, thus cutting off the circuit.

The terminal voltage of the power supply in the short-circuit state is:

U=E-IR≈EE/R*R=0

It can be seen that the main characteristics of the short-circuit state are: the short-circuit current is very large and the voltage at the power supply end is zero.

It should be noted here that the internal resistance of a power supply is usually basically constant and has a very small value, so the terminal voltage of the power supply can be approximately considered to be equal to the power supply electromotive force. In the future, if the internal resistance of a power supply is not specifically stated in the bid, it means that the internal resistance is very small and can be ignored.

3. Open circuit state

An open circuit is when the power supply is disconnected somewhere, no current flows through the circuit, and the power supply does not deliver power to the load. For the power supply, this state is called no-load. The main characteristics of the open circuit state are: the current in the circuit is zero. The voltage at the power supply end is equal to the electromotive force.

These three states can be seen everywhere in our lives. For example, when the light switch is turned on, the light is on, which is a state of passage. If the light is turned on and the refrigerator, air conditioner, rice cooker, TV, computer, speaker, and electric frying pan are turned on at the same time, the load is relatively large and overload may occur later. When overload occurs, the wires are prone to smoke and fire. When the switch is turned on, the light goes out, which is an open circuit state. When the outer skin of two wires (live wire and neutral wire) is damaged by mice, causing the two wires to touch each other, a short circuit will occur. If there is an overcurrent switch, the overcurrent switch will work immediately. If there is no overcurrent switch, smoke and fire will occur immediately.