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minutes approximately or little bit more it' s restored to the nominal value. On the opposite, when it
has a value which is the double of the set value, it takes just few seconds to reach the nominal value.
On the board it' s possible to activate an alarm signal which takes place when the modulation is
lower than 10KHz for a period of time longer than two minutes (the level 10KHz has been selected
because is a little higher than the value due to the stereo subcarrier). When the modulation absence
alarm is on, the output power is removed and the transmitter remains in stand-by until the
modulation will be restored.
5.5
PWN board
Power supply for the transmitter does not come from a conventional source (line or
switching with a mains supply transformer). A new approach to power source has been created for:
a) elevated efficiency
b) significantly smaller size
c) light weight
d) consumption of sine current
e) significantly lower reactive power, and therefore less absorbed current
f)
conformity with emission regulations regarding mains supply' s harmonics (EN-61000-3-2 and EN-
61000-3-3)
The negative side to all of these advantages is that active components are not insulated from the
mains supply. However, in each position there should be a transformer for line insulation, thus
confining the problem to weak positions, which sooner or later will require installation of this
transformer in order to protect against high-intensity electrical shocks.
All traditional power supplies are comprised of a transformer, a rectifier bridge, a large capacity,
and a circuit regulating output voltage which can be linear or switching. In both cases, the absorbed
current from the transformer' s primary circuit, which turns out to be the line current, is not
sinusoidal but rather impulsive, with rising edges due to the switching of the rectifier bridge diode
conduction. This is learned in school, though the consequences of a similar absorption are never
noted: in this case a virtual reactive power exists that is caused by the harmonics of the current
(Fourier transformation of impulses) with all of the disadvantages of this power. In this case, it is
preferable to focus on power factor rather than phase angle. It is possible to reach such high values
that it becomes necessary to re-size the transformer with almost double the power compared to that
of the active power. The current harmonics, with a null medium value, do not cause consumption
of the user' s active energy, but uselessly heats the network cables, causing a loss of energy to the
electrical network provider.
transformer absorb a sinusoidal current in phase with a line voltage; it is necessary to create a
"POWER FACTOR CONTROL".
The PWN circuit operates on this principle, having mains supply voltage as the direct input, and
voltage output of 48 Vdc with a current above 22A. The first part is made of a large mains supply
filter which rejects the switching spurious; the second part is comprised of a circuit which generates
a continuous voltage of 380 Vdc with a control of phase and amplitude of the absorbed current in
relation to the line voltage. The third part is a traditional forward-converter which reduces voltage
from 380 Vdc to 48 Vdc using a high frequency transformer.
The only way to remedy these inconveniences is to make the
Pag. 38
A07B500S manual
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