Kentucky Change Location

Tech Tip 03

Standby Power Supplies Can Lock Up Your PC


  • Computer System Lock Ups.
  • Standby Uninterruptible Power Supply (UPS) "Beeps".


  • UPS Has Highly Distorted Output Voltage.


  • Replace with an On-Line Uninterruptible Power Supply (UPS) with Sine Wave Output Voltage.
  • Adjust sensitivity setting for UPS.

A standby Uninterruptible Power Supply (UPS) is designed to provide battery backup for the normal 120V AC supply in the event there is an interruption of electrical service. This feature is designed to prevent loss of computer data and protect the load from power line disturbances.

A distributing company reported occasional computer lockups when one of their UPS units "beeped". A computer operator noted that the UPS usually "beeped" when she turned on one specific computer.

The UPS user's manual claimed a "Pseudo Sine Wave" output. A snapshot of the actual output waveform is shown in Figure 1. Replacing the UPS with an on-line UPS featuring a sine wave output was recommended.

The line-to-neutral peak was 200 volts, roughly 18% above the nominal peak of 170 volts. The higher voltage and nonsinusoidal output voltage waveform may have contributed to the computer lock-ups.

 Transfer to Battery Backup

Figure 1: Transfer to Battery Backup

Employees of a fast food restaurant reported enduring computer system lockups a few times a day. These lockups would halt the electronic cash registers until the computer system could be restarted. The output waveform of their UPS is shown in Figure 2.

The sensitivity setting of the transfer mechanism caused the UPS to switch to battery backup several times each day. The random computer lockups ceased following replacement of the standby UPS with an on-line, pure sine wave output UPS.

Transfer to Battery Backup

Figure 2: Transfer to Battery Backup

For some UPS systems it is possible to lower the voltage threshold setting that causes the unit to switch to the backup supply. This may be done by flipping mini-switches on the back of the unit or by software that is supplied with the unit.

The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.