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Variable DC Power Supply


Half wave rectifier circuit

Half wave rectifier circuit

We’ve spent most of our time in lab working on AC circuits. But DC power is an efficient and portable source of electricity. Many of the things you use in your day to day life run on DC power supplies. We have primarily described DC circuits as being powered by batteries. Today we will learn about a circuit that turns AC into DC called a rectifier. A rectifier is at the heart of a DC power supply. Many gadgets, chargers, and computers run on these DC power supply circuits.

DC power supplies function in place of a batteries.

Small DC power supply for charging portable electronics

Small DC power supply for charging portable electronics

Larger DC power supply for laptop

Larger DC power supply for laptop


We are going to split the half wave rectifier circuit into functional blocks and describe each part in order.

  1. Rectifier- uses a diode - Changes AC into DC

  2. Filter - uses a capacitor - Smooths voltage output

  3. Scaling - uses a potentiometer - Adjusts voltage to proper level


Diode

a semiconductor device that blocks current flow in one direction

Symbol and actual diode below. Notice how the terminals are polarized and identified. The line in the symbol matches with line on the actual diode.

Symbol and actual diode below. Notice how the terminals are polarized and identified. The line in the symbol matches with line on the actual diode.


Different-types-of-Diodes.jpg

Diodes come in many forms. We have already talked about LEDs (light emitting diodes) .

The diodes used in rectifier circuits are very different. The diodes used in rectifier circuits are typically called rectifier diodes or power diodes.

Communication circuits use special high speed or high frequency diodes called signal diodes.


Diodes in a Dc circuit

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When the Anode is on the positive side of the circuit, and the Cathode is on the negative side of the circuit The diode is Forward Biased and current flows normally.

When the Anode is on the negative side of the circuit, and the Cathode is on the positive side of the circuit The diode is Reverse Biased and the diode acts like an open, with no current flow.


Diodes in an AC Circuit (rectifier)

In the simple rectifier circuit above, the red AC waveform has been rectified by the diode giving us the blue waveform on the OUT. The diode is only allowing the current to pass in one direction, and is cutting out the negative portion of the input. We just made DC. The blue wave is only present in the positive portion. Although this is technically DC, we still need to clean it up. This power supply is OFF half the time currently.


Capacitor

a device used to store an electric charge

Symbols for Capacitors

Symbols for Capacitors

Variety of Capacitors

Variety of Capacitors

In a DC power supply a capacitor acts as a filter. In the rectifier, we had an output that was ON half the time and OFF half the time. The capacitor will help us fill in the OFF parts. The capacitor is charged up on the positive cycles, and when the voltage starts to drop off towards zero, the capacitor dumps its charge back in to the circuit. When a capacitor is matched to the circuit it should be able to provide a steady DC output as shown below. You see the large “hump-wave” signal from the diode, and the filter capacitor is doing its best to filter that hump into a steady DC voltage.

The filter capacitor charges and discharges filling in the gaps of the rectified signal

The filter capacitor charges and discharges filling in the gaps of the rectified signal