HT61 - HT62
EN - 4
2. GENERAL DESCRIPTION
The instrument carries out the following measurements:
DC
Voltage
AC
TRMS
voltage
DC/AC voltage with low impedance (LoZ)
DC
Current
AC
TRMS
Current
Resistance and Continuity test
Diode
test
Capacitance
(HT62)
Current and voltage frequency
Duty
Cycle
Temperature with K probe (HT62)
Each of these functions can be selected by means of the appropriate switch. The
instrument is also equipped with function keys (see § 4.2), an analogue bargraph and
backlight. The instrument is also equipped with an Auto Power OFF function (which can be
disabled), which automatically switches off the instrument 15 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 ANDTRMS 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.
With a perfectly sinusoidal wave, the two families of instruments provide identical results.
With distorted waves, instead, the readings shall differ. Average-value meters provide the
RMS value of the sole fundamental wave; TRMS meters, instead, provide the RMS value
of the whole wave, including harmonics (within the instruments 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 of 1A intensity, circulating on a
resistor, dissipates the same energy that, during the same time, would be 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.
