We Got Power Now... Baby

This section will introduce Power Supplies. In the circuits in prior sections we relied on a battery as the source of power. Batteries are good for portable circuits but not very good for long lasting power. If we can harness a power source that is sustainable then we can consume even more power. We might even be able to recharge our batteries using a sustainable power source. So where do we begin?

Halfwave Rectifier Power Supply

We will start this chapter by introducing the HalfWave Rectifier power supply. First we will introduce the parts then we will break apart the meaning of it name.

Looking at the left side of the illustration, we see the symbol with a sine wave inside a circle. This represents any alternating current generator. Along the top is a diode, also knows as a rectifier. On the right side is a resistor representing the load.

Note the sine wave graph directly below the words AC voltage source. You should recall from an eariler section that a diode is a one-way gate only allowing electrons to flow in one direction. You should also recall the graphs from the AC section. They showed that the voltage starts at the zero line and increases over time to its positive peak voltage, then moves back to the zero line over more time. This is half the sine wave. Next the voltage moves in the negative direction to that peak voltage and then again back to the zero line after some more time. This is the second half of the sine wave.

Now look at the name Halfwave rectifier power supply. During the first half of the sine wave cycle, current is flowing through the rectifier (diode). Electrons are flowing out the lower generator terminal, which is negative voltage, through the load resistor, through the forward biased diode and back into the positive terminal of the generator.

During the second half of the sine wave cycle the diode is reversed biased thus no electrons (current) is flowing.

Fullwave Rectifier Power Supply

This is a fullwave rectifier power supply. This supply takes advantage of both half's of the sine wave, thus gaining more available power and provides a smoother output voltage.

The left side of this illustration has the same AC voltage source as in the prior example. The next item is an iron core transformer. This transformer has a center tapped secondary coil. Want that means is that the coils are wound on an iron core. The secondary coil had two end terminals and a center-tapped terminal. There are two diodes and a load resistor.

Look at the fullwave rectifier circuit for the first half of the sine wave cycle. Recall that the changing fields from the AC voltage source connected to the primary coil of the transformer will induce energy into the iron core, which will induce voltage to the secondary coil. During this first halfcycle the top terminal of the transformer secondary is the most positive and the bottom terminal of the transformer is the most negative. The current can not flow out the bottom terminal due to the lower diodes reverse biased state.
Electrons will flow out the center tapped terminal of the transformer, which is negative voltage compared to the top terminal. Electron flows through the load resistor, through the top diode, which is forward biased and back into the positive (top) terminal of the transformer.

Look at the circuit for the second half of the sine wave cycle. During this cycle the bottom terminal of the transformer secondary is the most positive and the top terminal of the transformer is the most negative. The current can not flow out the top terminal due to the upper diodes reverse biased state.
Electrons will flow out the center tapped terminal of the transformer, which is negative voltage compared to the bottom terminal. Electron flows through the load resistor, through the lower diode which is forward biased and back into the positive (bottom) terminal of the transformer.

Fullwave Bridge Rectifier Power Supply

This is a fullwave bridge rectifier power supply. This supply can be used with or without a transformer. This 4-component arrangement of the diodes is called a bridge.

During the first half of the sine wave cycle the lower portion of the bridge is negative portion coming from the AC source. The current flows through D3, then through the load resistor, then through D1 and back into the positive portion of the AC source. Diodes D2 and D4 are reverse biased so no current flows.

During the second half of the sine wave cycle the upper portion of the bridge is negative, coming from the AC Source. The current flows through D4, then through the load resistor, then through D2 and back into the positive source. Diodes D1 and D3 are reverse biased so no current flows.

Ripple

Next we will look at the output ripple. Ripple is the fluxuation in the output voltage. In most cases this is undesirable. An example of ripple in an audio circuit is a low pitched buzzing or humming sound. Ripple can be reduces buy filtering it. In this illustration, the halfwave rectifier output is A and the fullwave rectifier output is B.

Recall from the capacitor section that a capacitor resists a change in voltage. If the supply voltage is less then the stored voltage in the capacitor (cap), the cap will want to give back electrons. If the supply voltage is higher, the cap will tent to take on electrons.

By adding a filter capacitor, the power supply can stores energy in the capacitor. During the portion of the cycle when the input voltage is less then the capacitor voltage, the capacitor will give energy back to the circuit. This will level- out the ripples. The more filtering, the less ripples. If the supply LOAD is very small, the capacitor can remove all noticeable ripples.