Dual-way power switch sequence control circuit

In some electronic products or in their production experiments, dual power supply is required. For example, a control board with a single-chip microcomputer, with an LCD or other peripheral devices. The control board uses +5V power supply, and the peripheral devices need another positive or negative regulated power supply in addition to the +5V (VCC) power supply, such as a -10V power supply (VEE) to provide a driving display contrast adjustment voltage for the LCD. In these applications, the switching sequence of the power supply is very important. When turning on the power, it is necessary to ensure that the +5V power supply is connected first. Then connect VEF. When turning off the power, VEE must be disconnected first, and then VEE. Otherwise, when the +5V logic power supply of the circuit does not exist or is not fully established, the driving power supply VEE is fed to the logic circuit, exceeding the withstand voltage range of some components, causing component damage. Therefore, we made the following circuit, connected to the power supply, and controlled the switching sequence of the power supply, and achieved good results.

How it works

In the figure, Vcc IN is connected to +5V power input, VEEIN is connected to -10V power input, S1 is the power switch, and VcC 0UT and VEE OUT are power outputs.

When the power is turned on, S1 is closed, and C1 and C2 are charged. Since the value of C1 is much smaller than that of C2, V1 is turned on first after a period of time, driving the relay K1 to operate, closing the contacts below the diagram, connecting K1's 1-2 and 4-6, so that the Vcc input is connected to the VCC output, and the logic power is powered on. At this time, the V bandit input is connected to the 4th pin of K2, preparing for the next step of VEE connection. After a period of delay (the delay time depends on the values ​​of R1, R2, C1, and C2, generally tens to hundreds of milliseconds), V2 is turned on, driving the relay K2 to operate, closing the contacts below the diagram, connecting K2's 1-3 and 4-6, so that the VEE input is connected to the VEE output, the driving power is powered on, and at the same time, a Vcc path is connected. Diode D1 is used to isolate the paths controlled by the two switches so that they do not affect each other. The light-emitting diode D2 indicates that the power is turned on.

When the power is off, S1 is disconnected, C1 and C2 are discharged, V1 is cut off first, and the control relay K1 is disconnected, cutting off the path of VEE. At this time, K2 is still connected, and the path of VcC is maintained by the 1-3 contacts of K2. Therefore, the driving power supply VEE is disconnected before the logic power supply VcC. After C2 is discharged, V2 is cut off, K2 is disconnected, and both power supplies are cut off. R5 is a discharge resistor, which allows the VEE load charge to be discharged as quickly as possible through it, reducing the maintenance of the VEE voltage on the load, and further ensuring the reliability of the power-off sequence.

In the attached figure, the relay uses the common Panasonic DC relay on the market, the driving voltage is 5V, and the other components are also common components. During production, the input and output ends of the voltage are soldered on the circuit board using terminal blocks to facilitate power leads. The no-load power consumption of the entire circuit is 36 mA when Vcc=+3V, and 50 mA when VCC=+5V, which has little effect on the power supply.

Conclusion

Connect the load with dual power supply directly to the voltage stabilizer. Sometimes these voltage stabilizers do not have independent switches for each power supply, or in order to reduce the operation links or careless operation in production or experiment, the main switch is used to switch the power supply directly. It is easy to cause circuit damage caused by frequent switching of the power supply. After this set of switching circuit transfer, the power supply is controlled by one switch, which is convenient to operate, safe and reliable, and has achieved obvious results, greatly reducing damage and improving work efficiency.