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PQ Tip: Computer Monitors and Jitters: It's Location, Location

Sep 1, 1998 12:00 PM, By John A. DeDad, Editor-in-Chief

Your monitor's location dictates its susceptibility to magnetic fields. Close proximity to certain electrical equipment could spell problems and wiggles.

Have you ever tried working with a jittery computer monitor? If you have, you can attest to the irritation, eye strain, even nausea.

But don't automatically blame the monitor. Magnetic fields may be causing the problem. These fields may be interfering with the magnetics of the monitor's picture tube. In fact, this is the classic symptom of magnetic field interference.

Basically, a magnetic field must be greater than 8 milligauss to cause monitor jittering. This depends on the monitor's size. The larger the monitor (measured in diagonal inches on the screen), the greater the susceptibility to magnetic fields. So, where do these fields come from?

Any time electric current flows through an appliance or wire, it creates a magnetic field. You measure this field in units of milligauss. According to a Power Quality Network Application paper (No. 4), these appliances cause magnetic fields:

• Laser printers

• Fans

• Refrigerators

• Electric stoves

• Microwave ovens

• Air conditioners

• Electric heaters

Building transformers, circuit breaker panelboards, power switchboards, some UPS devices, and high concentrations of magnetic ballasts also cause magnetic fields. Even overhead or underground power cables carrying large currents can produce these fields. You shouldn't worry about internal building wiring, if it's properly installed. That's because the fields respectively generated by neutral and phase conductors cancel each other out. Why? Because these conductors are usually routed together.

But, you can get significant magnetic fields when currents flow "unopposed" in building wiring and conduit. Wiring errors cause these currents, which are sometimes called stray currents. These errors include the following NEC violations:

• Improper grounding,

• Bonded or reversed neutral and ground wires in panelboards and receptacles, and

• Individual circuit wires not routed with other wires.

So, how do you get rid of the jitters? Here are some viable solutions.

Verify grounding. Make sure the equipment is properly grounded per NEC requirements and the connections are tight. This is a cardinal rule in troubleshooting any power quality problem.

Use elimination method. With the monitor on, turn off any lights and appliances one at a time. See if the jitters go away with any one switching. If they do, you've located the magnetic field source. Then it's just relocate the monitor or the source (if possible).

Verify proximity to field generating equipment. If the monitor is near a transformer, copy machine, circuit breaker panelboard, or UPS, it's "case closed." But, don't forget to look behind shared walls. This equipment may be directly opposite the monitor but separated by a party wall. If you can't move these field-generating devices, try relocating the monitor to see if the jitters cease. Sometimes, just turning a monitor does the job.

Look at fluorescent ballasts. If you've narrowed down the field sources to your magnetic ballasts, you may have to replace them with electronic ballasts. Or, you can simply keep the overhead fluorescent light fixtures off during computer use. Simple desktop task lighting will suffice.

Change scan rate of monitor. You can reset this rate, which is also called refresh rate, to a different frequency by using a program called SETVSCAN (for IBM-compatible computers). You can use it for any DOS-shell programs. The closer you can get the scan rate to 60 Hz, the less the monitor will jitter when affected by a 60 Hz magnetic field.

However, the minimum refresh rate acceptable for ergonomic performance is 72 Hz. Any setting below 72 Hz (80 Hz for many people) creates flicker that may have a directly negative effect on productivity.

Most Windows programs run at 59.9 Hz; as such, magnetic fields won't affect them. If you're using Windows to call up DOS-shell programs that are affected by these fields, execute the SETVSCAN program before you open Windows.

Reconfiguring your monitor's video card will also reset the scan rate, but you may have to contact the monitor manufacturer as to how to do this.

Install magnetic shields. This should be the last resort because they're expensive and won't quite "fit in" with your office decor.

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© 2012 Penton Business Media, Inc.

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