Product Details Supplier Info More products

Powervar explains the need for low impedance in power transformers.

A low-impedance circuit has the ability to provide current (amps) in large amounts relative to the power needs of the load that is plugged into it.

A desktop computer is an example.

Modern computer power supplies draw power from an AC circuit in short-duration gulps that occur about 120 times per second.

Each of these current gulps or peaks is several times more than the average current used by the computer.

Engineers use the term crest factor to describe the ratio between the average current and the peak current consumption.

With a typical crest factor of 3, a computer with an average amperage rating of only 2A will require 6A of peak current from the power line.

An AC branch circuit rated at 20A would be a good low-impedance source for this computer, as it is rated at several times the peak demand of the computer.

But the question arises of what happens if it is necessary or desirable to install a power conditioner with an isolation transformer on this computer, and whether one should be installed that is rated at the full 20A rating of the AC branch circuit.

This would not be advisable, as it would be quite expensive; it would be preferable to install a power conditioner rated at about 3A.

But doing so would be a problem if the transformer was unable to provide for the crest factor current demand of 6A.

This is where transformer impedance becomes important.

Some transformer technologies are not low-impedance and cannot provide for the peak amperage demands of modern computer power supplies.

If a low-impedance load like a computer power supply is connected to a high-impedance source or transformer, a variety of negative consequences may occur.

Isolation transformers that have a low-impedance design are capable of supplying the peak amperage demands of high crest factor power supplies.

For more information on impedance and its importance as a power quality issue, read Powervar white paper 210: ‘What’s This About Impedance?’.

View full profile