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Rectifiers Most electronic equipment makes use of a steady DC Voltage to control all of its electronic components. The problem is that the power that is delivered to our houses or places of business comes in the form of AC Voltage from the power company. So we need to be able to convert Full wave AC power to Straight line DC power. To do this we use a device called a Rectifier. Basically a rectifier can be as simple as 1 diode. As you know a diode will only pass current in one direction and that is when the Cathode is biased in a more negative condition compared to its Anode. So if a diode is connected to one side of a AC power supply, the other end is connected to one leg of a resistor (load) with the other leg of the resistor connected to the other side of the power supply the voltage would only be present over the resistor when the diode is forward biased. If you were to measure the voltage over the resistor with an oscilloscope the wave would look like the diagram below. The above wave represents the output of what is called a half-wave rectifier. Notice that the signal is only positive and the negative part of the wave is not being used at all. Therefore the output is now called “halfwave varying DC” Now this would be great if we wanted our equipment to be turned on and off on the cycle of the AC power. As you can guess we need to have a device that will give us constant power (DC). This is accomplished by using 4 diodes connected in a certain way to make use of both the negative and positive parts of the AC wave. This device is called a Bridge Rectifier and is configured as in the picture below It is very important to make sure the diodes are connected in the right configuration for a Bridge Rectifier to work properly. A Bridge Rectifier provides a what is called a “Full-wave varying DC” output and would look like the wave below. Now before we go any further what is important to remember is that diodes use up .7 Volts each out of the supplied voltage when they are forward biased. Therefore as voltage has to pass through 2 diodes to complete the circuit on each half cycle the Bridge Rectifier will use up 1.4 Volts of the power provided. Bridge Rectifiers are rated in the following way Bridge rectifiers are rated by the maximum current they can pass Also they are rated by the maximum reverse voltage they can withstand (this must be at least three times the supply RMS voltage so the rectifier can withstand the peak voltages). Now that we have a pulsating full-wave signal we now need to flatten or smooth out the signal (straighten to represent a DC signal) To do this we use a capacitor across the load.