BK Precision 5335B, User Manual

Page: 23 / 52

Measurement Setup
Central to setting the proper range of the instrument is the Crest Factor. Crest factor is the ratio of the peak value of
a waveform to the RMS value of the waveform. For example, a perfect sine wave crest factor RMS value is 0.707*Peak.
The crest factor is the inverse of the normalized RMS value,
0.707
−1
= 1.414
. In many cases, like the current waveform
of a AC-DC power supply, the crest factor is a larger value. For accurate measurement, the signal measured should stay
within the measurement range of the instrument. For example, a 100V RMS signal with a crest factor of 5 has peaks of
500V. If the range selected is smaller than 500V than inaccuracy will occur as the signal will be clipped.
3.1 Crest factor
The Crest Factor, the ratio of the peak to the RMS value, has two settings, 6 or 3. This setting applies to both current
and voltage simultaneously, and modifies the available ranges listed in Section 3.2 . Essentially, by knowing the RMS
signal level and the peak value, the setting of this parameter is determined. For an 117V RMS AC line signal with little
distortion and a purely real load (power factor = 1), the peak value is
1.414 ∗ 117𝑉 = 165𝑉
. 1.414 is the approximate
Crest Factor of a Sine wave. So, the appropriate setting will be 3 (CF3).
𝐶𝑟𝑒𝑠𝑡𝐹 𝑎𝑐𝑡𝑜𝑟(𝐶𝐹 ) =
𝑃 𝑒𝑎𝑘
𝑅𝑀 𝑆
As this meter also measures the DC level of the signal, this must also be accounted for when choosing the Crest Factor.
If the the signal will exceed the range of the unit (crest factor * range) then the measurement value will be clipped and
the value inaccurate. Conversely, by choosing a Crest Factor and Range combination that accommodates the peak value
of the signal, a larger crest factor than set may be measured. The trade-off is then the accuracy, and is listed in the
specifications, Chapter 9 .
3.1.1 Example - Range=150 V, CF = 6
The maximum input signal and crest factor are as follows:
𝑉
𝑟𝑚𝑠
= 100 𝑅𝑎𝑛𝑔𝑒 = 150𝑉 𝑉
𝑚𝑎𝑥𝑟𝑎𝑛𝑔𝑒
= 900𝑉
So, a signal with a peak that is 9 times that of the RMS value (Crest Factor 9) is measurable using these settings.
3.2 Set Measurement Range
The appropriate measurement range (voltage and current range) must be set for accurate measurement.
1. In the “Meter” interface Press the soft key corresponding to either “U-RANGE” or “A-RANGE”, and use the knob
or the arrow keys to select the voltage or current range desired. See Table 3.1
2. Press the “Enter” key to confirm the setting. Otherwise the instrument will automatically confirm and exit the setting
after 5 seconds of no activity.
Parameter CF 3 CF6
Voltage 15, 30, 60, 150, 300, 600V 7.5, 15, 30, 75, 150, 300V
Current 5, 10, 20, 50, 100, 200, 500mA, 1, 2, 5, 10, 20A 2.5 ,5 ,10 ,25 ,50 ,100 ,250mA, 0.5, 1, 2.5, 5, 10A
Table 3.1 Current and Voltage Ranges per Crest Factor setting
3.3 Fixed Voltage and Current range
Select the required range from a number of options. For the voltage range, when the crest factor is 3, the maximum
option is 600V and the minimum option is 15V. When the crest factor is 6, the maximum option is 300V and the minimum
option is 7.5V. See Table 3.1