How to Build an AC to DC Converter

By Dan Keen

Many electronic devices, from computers to TVs to cell phone chargers, require various DC (direct current) voltages to supply power to circuitry. For those appliances and devices that get their power by plugging them into an electrical wall outlet, a circuit must be designed to convert the 120 volt AC power to a desired DC voltage.

Things You'll Need

  • Step-down transformer
  • Four semiconductor diodes
  • Eectrolytic capacitor (optional)
  • Wire
  • Breadboard
  • Soldering gun/penci and solder
  • AC line cord and plug

Converting AC To DC

Step 1

Many electronic devices, from computers to TVs to cell phone chargers, require various DC (direct current) voltages to supply power to their circuitry. For those appliances and devices that get their power by plugging them into an electrical wall outlet, a circuit must be designed to convert the 120 volt AC power to a desired DC voltage.

Step 2

Alternating current, as is found in your home's electric outlets, changes polarity 60 times a second, referred to as "60 Hertz," or "60 cycles". The voltage increases from zero to its maximum positive voltage and then swings below zero to its maximum negative voltage, in a smooth sine wave transition. In a DC voltage supply, the polarity remains constant; plus (+) and minus (-) polarity points do not change, as with a flashlight battery.

Step 3

Use one semiconductor diode to obtain a DC voltage from an AC source. Simply place the diode in one of the two legs of the incoming AC source. One side of a diode is the "anode" or positive side, and the other is the "cathode" or negative side. When the leg of the AC source is connected to the anode of the diode goes positive, the diode allows current to flow through. Placing a volt meter on the cathode side will register the positive voltage present. As the leg connected to the diode's anode turns negative during the AC cycle, the diode acts as a block, and does not let the negative voltage through. Thus, the "output" on the cathode side of the diode will always be positive. While this simple "rectifier" or "AC to DC converter" circuit helps explain how a diode works as a rectifier, the circuit only recovers half of the AC voltage cycle. Also, although the circuit's output is only positive, there is no output during the negative half of the input cycle.

Step 4

Connect two diodes to the output of a transformer that has a "center tap" such that both the positive and negative part of the AC cycle are converted. Often, a step-down transformer is used to change the 120 volts from the wall outlet down to a voltage needed by the device. Transformers and diode combinations are used in "wall warts" or power adaptors, many of which are probably around your home for cell phone chargers and phone answer machines. In a two-diode configuration, connect the anode end of a diode on one leg of the transformer and also connect the anode end of a second diode to the other transformer leg. The transformer must have a "center tap" connection. This will be the "ground" or negative connection. Connect the cathode end of both diodes together. This will be the positive DC output connection.

Step 5

Place an electrolytic capacitor across the DC output of the rectifier circuit --using either the two-diode or four-diode configuration--to further smooth out the DC voltage created by the full wave rectifier. Observe the polarity of the capacitor, connecting the positive end to the positive output of the rectifier circuit, and the negative end to the ground, or minus, connection--which is the transformer's center tap in the case of a bridge rectifier. The voltage rating on the capacitor must be higher than the DC output voltage--with no "load" connected. Formulas have been devised to calculate the best capacitance value, but generally, a large capacitor value will reduce ripple significantly. Start by experimenting with a value of 100 microfarads for a circuit with a 12 volt output. An oscilloscope can be used to see the effect of a capacitor on ripple smoothing.