Device for monitoring the ATX PSU standby voltage.
Please read this first.

Modern personal computers are usually equipped with ATX format PSUs. Besides of the main voltages such blocks also provide a standby voltage, which remains on even, when the system power is off. Most of the users believe that PSU generates no voltages when the computer is shut down. However, this is not the case and the standby voltage module remains active as long as the computer is connected to the power outlet. Since all fans of the system are off in this state, the standby voltage module works in difficult conditions. As a result its parameters deteriorate with time. Very often this leads to a higher standby voltage and the voltage powering the power supply PWM-controller. This, in turn, can result in combustion of the controller along with the motherboard.

Check here for some disaster photos.

In the cheap power supply blocks, which overwhelming majority of computers are equipped with, nothing is implemented to prevent this trouble. This is inacceptable and the proposed devices is designed to address this issue.

Its schematic is shown below.

PowerPC Control U Base

This device does not fix all computer power supply problems, but it is designed to deal with the most important one, namely control of the standby mode voltage. The device is based on a popular PIC12F675 microcontroller (MCU) from Microchip Technology Incorporated PIC12F675, which has a built-in analog-to-digital converter (ADC).

Monitoring of the standby voltage is performed in four levels (two top ones and two bottom ones). When the voltage goes out of the two middle levels the device starts to generate audio signals: upon exceedind the upper level the device signals "SOS", and when it drops below the lower level then Funeral March is played. The sounds stop as soon as the the voltage returns back to permitted values. If the voltage raises up to the fourth level or drop down below the first one then a continuous beep is added to the sounds. At the same time the MCU sets certain logic levels on its pins: high level on pin 3, high level after some delay on pin 6, and low level after some delay on pin 5. Those signals can be used to control the PSU protection module, if the PSU has one. Forcing the MCU to exit this mode can only be done by unplugging the power cord.

*The mentioned above delays in setting the logic levels are needed in order to ignore short drops of the output voltage, which are inavoidable in inexpensive switching PSUs during the load current bursts.

After one minute of turning the computer off, if the device detects any trouble with the standby voltage, it also starts to generate series of 9 short beeps reminding the user to unplug the computer from power. If in this mode a serious trouble is detected, the device works as described above.

For a proper operation of the device, it must be connected to a place in the PSU circuit that responds to the standby voltage changes. In some cases, this can be the PWM-controller power supply. To tune the device, measure the voltage at this point (measurement should be performed on a completely new PSU, or after maintenance targeted to a restorartion of the standby module functionality), and select R so that the voltage at MCU pin 7 is about 40/51 of the MCU supply voltage.

Flash presentation, explaining the device operation on a computer simulator.

Here is a version of the device with some external components added.

Doc 1.1 (28.06.2010).

PowerPC Control U Doc 1.1
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