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Basic AC rectification
The AC (alternating current) we get from a household wall socket is called a sine wave. If one could fold one-half of the sine wave under/over the other, we would have a circle of 360 degrees. This power is generated at a power plant by spinning an electromagnet within a coil of wire. (Same thing as an auto alternator.) The rotor speed is 3600 rotations per minute (RPM) or 60 cycles per second. The 120 volts from the outlet is the RMS value, which basically means the same power value as DC. (Note "peak" is amplitude.)
AC constantly changes voltage and polarity. The voltage starts at zero degrees at zero volts. It goes to the positive peak at 90 degrees, back to zero volts at 180 degrees, goes to the negative peak at 270 degrees, and back to zero at 360 degrees, and starts again. That is one cycle. Every cycle the sine wave passes what is called the zero-crossing point at 180 and 360 degrees. That is important to know as we will see below. Another way to think of RMS is an average.
In the above example at 120 Volts AC RMS the peak would be 170 volts (120 X 1.414), peak-to-peak would be 340 volts, and the period (1/60) equals 16.7 milliseconds.
Half-wave rectification
Many devices, in particular electronics, must use DC or direct current. A diode is a solid-state device that conducts in one direction only. When the anode (A) is positive and the cathode (K) is negative (though the load) current (I'm assuming electron flow from negative to positive) will flow through the load, through the diode and back to the power supply. Thus current will flow only for the positive half-cycle (0 to 180 degrees) and the diode will shut-off during the negative half-cycle from 180 degrees to 360 degrees.
What is power? Voltage (in volts) is the "push" and the current (in Amperes) is what is being pushed. (Electric charges) Power is voltage times current. Power is measured in watts. So one amp at one volt equals one watt. (I'm not going into all of Ohm's Law here.) We must have voltage and current together to get power, so an open switch, broken wire, or a shut-off diode delivers no power. In the case above, we get very poor power transfer with the diode off half the cycle and the positive voltage level changing constantly.
Let's say the AC in is 12 volts RMS. To get peak we multiply 12 by 1.414, which equals about 17 volts. But the average voltage DC is peak times .3185 equals about 5.4 volts. This is what is called pulsating half-wave DC. Pure DC, such as from a 12 volt auto battery, has none of the "ripple" and will be a real 12 volts. Put a DC voltmeter across the load above, one will read 5.4 volts. Switch the meter to AC, one will still read a voltage of some value. This is normal as one is reading the "ripple" riding the DC. Connect the same AC voltmeter across a clean DC source such as a car battery, one will read zero volts AC.
Full-wave rectification
Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. This is due to each output polarity requiring two rectifiers each. Four rectifiers arranged this way are called a full-wave diode bridge or bridge rectifier.
Note the frequency has doubled! 60 hertz in will be 120 hertz out.
Power delivered here is much greater than half-wave rectification because we are using both half-cycles. Using 12 volts AC again, we have 12 X 1.414 or 17 volts peak. But now to get the average we multiply peak (17 volts) by 0.637 which equals 10.83 volts, double that of half-wave.
In the above example the capacitor (C) acts as a voltage storage device and partially discharges between half-cycles. In this case we have a pulsating DC with a much smaller ripple component. Note that because the capacitor never discharges back to zero, we can't use this in a triac or SCR circuit. See Basic Triacs and SCRs.
In our final example we use a center-tapped transformer for full-wave output. In this case the voltage will be half. So a 24 VAC secondary will produce produce 12 volts X .9 = 10.83 volts pulsating DC. T in this case would be half of 16.7 milliseconds or about 8.35 milliseconds. The frequency (1/T) is 120 Hertz.
Pictures:
3 phase rectifier
Bridge Rectifiers
Diodes
"Stud" rectifier (high current)
Transformers, four salvaged from microwave ovens (PC board mount), one from Radio Shack
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ATMEGA168 Arduino Micro Controller
For more technical details on this see ATMEGA168 Arduino Micro Controller Projects- Interfacing the ATMEGA168/Arduino to the MCP23016 I/O Expander
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- Microcontroller AC Power Control Using Interrupts
PDF files and spec sheets
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