Test equipment and measurement of efficiency and light load performance

AN4345

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DocID025078 Rev 1

Figure 38. Switch in position 1-setting for standby measurements

In case of high UUT input current (i.e. for measurements in heavy load conditions), the

voltage drop can be relevant compared to the UUT real input voltage. If this is the case, the

switch in Figure 37 can be changed to position 2 (see simplified scheme of Figure 39) where

the UUT input voltage is measured directly at the UUT input terminal and the input current

does not affect the measured input voltage.

Figure 39. Switch in position 2-setting for efficiency measurements

On the other hand, the position of Figure 39 may introduce a relevant error during light load

measurements, when the UUT input current is low and the leakage current inside the

voltmeter itself (which is not an ideal instrument and doesn't have infinite input resistance) is

not negligible. This is the reason why it is better to use the setting of Figure 38 for light load

measurements and Figure 39 for heavy load measurements.

AM13844V1

V

+

-

A

~

AC

SOURCE

UUT

U.U.T.

AC

INPUT

Voltmeter

Ammeter

Wattmeter

AM13845V1

V

+

-

~

AC

SOURCE

UUT

U.U.T.

AC

INPUT

Voltmeter

Ammeter

A

Wattmeter