Specifications

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In order to transform the input voltage from 24V AC to 3.3V DC several steps
must be taken. First, the signal needs to be converted from AC to DC. To
accomplish this we have decided to use full wave rectification. This is also called
bridge rectification and is accomplished through the implementation of four
diodes. The bridge diode is an arrangement of four diodes in a diamond
configuration so that no matter the polarity of the input, the polarity of output is
the same no matter the polarity of the input. In other words, whenever the AC
signal would go below the x-axis, the signal is reflected about the x-axis so that
the entire signal is above the x-axis. A bridge rectifier that would be applicable to
our design is a KBPC8005.
At this point, the entire signal is of the same polarity, but the magnitude of the
signal is still not constant enough to be considered DC. The signal is still
continuously varying, or “pulsating” in magnitude, which is commonly referred to
as “ripple”. In order to smooth out the ripple a capacitor, known as the reservoir,
or smoothing capacitor is used. The value of the smoothing capacitor is
dependent upon the voltage regulator implemented.
Once the signal is smoothed, it is considered close enough to a true DC signal to
be sent through a voltage regulator. A voltage regulator is designed to take a DC
input voltage and output a specified DC voltage. For our design the desired DC
output voltage is 3.3V. This will allow us to use one voltage regulator to deliver
the necessary power to the main microcontroller, the temperature and humidity
sensor, the Zigbee wireless chip, and the 802.11b wireless chip.
The voltage regulator we have decided to implement into our design is the
LM2937-3.3 from National Semiconductor. It has a maximum input voltage of
26V, which makes it eligible for our design because our input voltage will be 24V.
The output of the LM2937 is 3.3V DC. When placing this voltage regulator in a
circuit, a 0.1uF capacitor should be placed between the regulator and the input,
and a 10uF capacitor should be placed between the regulator and the load. The
placement of these capacitors is shown in Figure 53 shown below.
Figure 53 Capacitors Associated with LM2937-3.3 Voltage Regulator
(Permission Requested)
Overall, the 24V AC signal must be manipulated in two stages. First, it must be
changed from AC to DC. To accomplish AC to DC transformation, bridge
rectification is used. The bridge rectifier does not completely smooth out the