HT instruments HT401 - Manual (Page 5)

HT401
EN - 4
2. GENERAL DESCRIPTION
The instrument HT401 carries out the following measurements:
 DC and AC TRMS voltage
 DC and AC TRMS current
 Resistance and Continuity test
 Frequency of voltage and current
 Capacitance
 Diode
test
 Temperature with type K probe
 DC/AC voltage measurement with low impedance (AutoV)
Each of these functions can be selected using the 10-position rotary switch, including an
OFF position. The instrument is also equipped with function keys (see § 4.2) a bargraph
and backlight. The selected quantity appears on the LCD display with the indication of the
measuring unit and of the enabled functions.
The instrument is also equipped with an Auto Power OFF function which automatically
switches off the instrument approx. 20 minutes after the last time a function key was
pressed or the rotary switch was turned. To switch on the instrument again, turn the rotary
switch.
2.1. MEASURING
AVERAGE VALUES AND TRMS VALUES
Measuring instruments of alternating quantities are divided into two big families:
 AVERAGE-VALUE meters: instruments measuring the value of the sole wave at
fundamental frequency (50 or 60 Hz).
 TRMS (True Root Mean Square) VALUE meters: instruments measuring the TRMS
value of the quantity being tested.
In the presence of a perfectly sinusoidal wave, the two families of instruments provide
identical results. In the presence of distorted waves, instead, the rdg shall differ. Average-
value meters provide the RMS value of the sole fundamental wave; TRSM meters,
instead, provide the RMS value of the whole wave, including harmonics (within the
instrument’s bandwidth). Therefore, by measuring the same quantity with instruments from
both families, the values obtained are identical only if the wave is perfectly sinusoidal. In
case it is distorted, TRMS meters shall provide higher values than the values read by
average-value meters.
2.2. DEFINITION OF TRUE ROOT MEAN SQUARE VALUE AND CREST FACTOR
The root mean square value of current is defined as follows: “In a time equal to a period,
an alternating current with a root mean square value with an intensity of 1A, circulating on
a resistor, dissipates the same energy that, during the same time, would have been
dissipated by a direct current with an intensity of 1A”. This definition results in the numeric
expression:
G=

T
t
t
dt t g
T
0
0
) (
1
2 The root mean square value is indicated with the acronym RMS.
The Crest Factor is defined as the relationship between the Peak Value of a signal and its
RMS value: CF (G)=
RMS
p
G
G
This value changes with the signal waveform, for a purely
sinusoidal wave it is 2 =1.41. In case of distortion, the Crest Factor takes higher values
as wave distortion increases.