A classic power supply circuit, analyzed very thoroughly!

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A classic power supply circuit, analyzed very thoroughly! [Copy link]

The schematic diagram of the circuit is shown below.

Simplified circuit of switch circuit

Before the circuit is powered on, the switch "TEST" is turned off and the microcontroller is not powered by VCC. At this time, the base of T1 is grounded through R9 (100k) and is in the off state. Test, which is connected to the base resistor R7 of T3, is in the off state, so T3 is also in the off state.

The power supply +9V is isolated by T3 and is not loaded on the voltage regulator chip IC2. The output VCC of IC2 remains at a low level.

Circuit closed state

Press the button "TEST" to start the circuit, the base of T3 is grounded through R7, Test, and T2, so that T3 is turned on. At this time, +9V is added to the IC2 voltage regulator chip through T3. IC2 output VCC is added to the microcontroller.

After the microcontroller works, it outputs a high voltage through IO2, and turns on T1 through R8. At this time, even if Test is released, the base of T3 can be grounded through R7, LED1, and T1, realizing the self-locking opening of the power supply.

Press TEST to start the circuit

After the circuit is started, the MCU provides the base voltage of T1 to keep T3 on.

Afterwards, the microcontroller software can make the IO2 port return to a low level, turning off T1 and then T3.

The switch state of T2 can be read according to the IO1 port to determine whether the user presses the function key. After determining that the user presses Test, IO2 can be set to a low level after the user releases Test.

It is also possible to automatically delay power-off according to software functions, thereby reducing power consumption.

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