Bridge Rectifiers
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In the electronics industry, one of the most popular applications of semiconductor diodes is to convert alternating current (AC) signal of any frequency, which is typically 60 or 50 Hz, to a direct current (DC) signal. This DC signal can be used for powering electronic devices, rather than batteries. The circuit which converts the AC into DC signal commonly consists of a particular arrangement of interlocked diodes and is known as a rectifier. In power supply circuits, two types of rectifier circuits are commonly used &#; half-wave and full-wave. Half-wave rectifiers only permit one-half of the cycle through, whereas full-wave rectifiers permit both the top half and bottom half of the cycle through, while converting the bottom half to the same polarity as the top. This difference between them is shown in Figure 1.
Between the two types, the full-wave rectifier is more efficient as it uses the full cycle of the incoming waveform. There are two types of full-wave rectifiers &#; the center-tapped full-wave rectifier, which requires a center-tapped transformer, and the bridge rectifier, which does not need a center-tapped transformer. The bridge rectifier will be discussed in this article as it is the most popular and usually comes in preassembled modules, making them easier to use.
Bridge Rectifiers use four diodes that are arranged cleverly to convert the AC supply voltage to a DC supply voltage. The output signal of such a circuit is always of the same polarity regardless of the polarities of the input AC signal. Figure 2 depicts the circuit of a bridge rectifier with diodes interlocked in a bridge configuration. The AC signal is applied at the input terminals a and b, and the output is observed across the load resistor R1.
Let&#;s see how this rectifier circuit responds to an AC signal with changing polarities at every cycle:
Nonetheless, this output voltage of single polarity is not pure DC voltage, as it is pulsating and not a straight line in nature. This problem is quickly solved by connecting a capacitor in parallel with the load resistor as shown in Figure 3. In this new design, the positive half cycle will charge the capacitor via the diodes D2 and D3. And, during the negative half cycle, the capacitor will stop charging and will begin to discharge itself through the load resistor.
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This process is known as filtration, and the capacitor acts as a filter. The capacitor has improved the pulsating nature of the output voltage, and it will now only have ripples. This waveform shape is now much closer to a pure DC voltage waveform. The waveform can be further improved by using other types of filters such as an L-C filter and pie filter.
The bridge rectifier just discussed is a single-phase type, however, it can also be extended to a three-phase rectifier. These two types can be further classified into full controlled, half controlled, or uncontrolled bridge rectifiers. The circuit that we just discussed is uncontrolled since we cannot control the biasing of the diode, but if all the four diodes are replaced with a thyristor, its biasing can be controlled by controlling its firing angle via its gate signal. It results in a fully controlled bridge rectifier. In a half controlled bridge rectifier, half of the circuit contains diodes, and the other half has thyristors.
You never see these in tube guitar amps, (I do use them where thought to be needed or useful) MLR to the tube recto. Older tube manuals will often list them. Some say all tube rectifiers should have them. Tube rectifiers can only deliver so much current, the limiting resistor keeps it from being swamped for current, it will also lower the voltage by ohms law. I'm not sure why no one uses them on guitar amps (besides me that is), but they will extend rectifier tube life and give voltage dropping that can be useful. They will get hot and need big ratings and installed where the heat will not destroy things, maybe that is why they are often eliminated or ignored, still, they can be very helpful in dropping an excess 10 or 15 volts or more and the rectifier tube will love you for it.
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