User Manual

8
True-rms Measurements
For ac measurements most DMMs average the ac input signal and display the
result as an estimated rms value. This average-responding method is accurate for
sinusoidal waveforms, but can be very inaccurate for distorted waveforms. To
ensure the most accurate measurements, always use a true-rms DMM when
measuring ac voltage or ac current on circuits for the following kinds of
applications:
• Power Supplies - diodes
• Controllers
• Power Limiting - SCR or Triac
• Starting - motors
• Florescent Lighting - ballasts
• Speed Control - motors
• Pulsed Signals
• Any non-sinusoidal ac waveform
The 38XR-A displays the dc, ac, or ac+dc components of a voltage or current
measurment. The dc component is the shift level or baseline. The ac component is
the area under the waveshape. The ac+dc component is the combined effect of the
ac and dc components.
MIN MAX AVG Measurements
Note: The MIN MAX AVG feature does not work the dBm nor the dutycycle function.
The MIN MAX AVG feature reads and updates the display to show the maximum,
minimum, or average value measured after you press the MIN MAX AVG button.
Pressing the MIN MAX AVG button for less than 1 second will put the meter into a
mode of displaying the maximum, minimum, average, or actual readings. Each time
the button is pressed, the meter will cycle to the next display mode as shown in the
table below. Press the MIN MAX AVG button for more than 2 seconds to disable
this feature.
Button Display Value Displayed
< 1 second
REC MAX
Maximum value after feature activated
REC MIN
Minimum value after feature activated
< 1 second
REC AVG
Average value after feature is activated
< 1 second
REC
Actual reading, min max being recorded.
> 2 seconds
Exit MIN MAX AVG
Normal measurement, actual reading
Peak Hold Measurements
Note: The peak hold feature calibrates itself to meet the specifications.
The function of the peak hold feature is to record and store the positive and
negative peak values that occur while measuring ac current or ac voltage. To enable
the peak hold feature press the PEAK ± button for more than 2 seconds. The
display will show CAL to indicate the calibration cycle is in process. After the CAL
indication clears, press the PEAK ± button again to display the maximum (P+)
value for the ac voltage or ac current being measured. The display will toggle
between the P+ and P- readings each time the PEAK ± button is pressed. Press
the PEAK ± button for more than 1 second to exit the PEAK function.