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How to Increase a 78 Series Fixed Regulator Output Voltage

The 78 series fixed voltage regulators are three-terminal electronic devices that provide a tightly-controlled voltage output from an unregulated input. They hold the output voltage to within 4 percent of their specified rating. The device number reflects the voltage: a 7805 regulates to 5 volts, a 7812 to 12 and so forth. You may want to regulate a voltage for which no 78 series device exists; 7 volts, for example. You can accommodate nonstandard voltages by shifting the device̵7;s ground connection with a pair of resistors. The output then becomes a function of the proportion of the resistors.

Things You'll Need

78 series voltage regulator
Solderless breadboard
.33-microfarad, 50-volt capacitor
.1-microfarad, 50-volt capacitor
1K-ohm, 1/2-watt resistor
1/2-watt resistor, ohm value determined in procedure
4 12-inch pieces of 22-gauge solid wire
Unregulated voltage supply

Instructions

- 1
  Select a 78 series voltage regulator device by choosing the first one with a voltage just lower than what you need. For example, to regulate 11.5 volts, select a 10-volt device.
- 2
  Select a 1/2 watt resistor by calculating its value. Use the formula:
  Vo = Vxx * (1+( R/1000)) + (.005 * R), where Vo is the output voltage, Vxx is the regulator̵7;s rated voltage, and R is resistance in ohms. For an 11.5-volt output, use a resistance of 100 ohms.
- 3
  Insert the regulator into main part of the breadboard. Each pin must go into a separate column in the board; holes in each column are electrically connected to each other, but adjacent columns are separate.
- 4
  Insert the .33-microfarad capacitor so one lead connects to pin 1 of the regulator and the second lead connects to pin 2. Connect the .1-microfarad capacitor between the regulator pins 2 and 3. Likewise, connect the 1K-ohm resistor between pins 2 and 3.
- 5
  Insert one lead of the second resistor so it connects to the regulator̵7;s pin 2. Set its other lead into a free column. Insert one end of two 12-inch wires into this column. These wires serve as ground connections for unregulated input and regulated output. It does not matter which of the two you use for input or output.
- 6
  Connect one end of a third wire to the regulator̵7;s pin 1. This is the unregulated voltage input. Connect one end of the fourth wire to pin 3. This is the regulated output.
- 7
  Connect the input wire and one ground wire to a source of unregulated voltage. Connect the output wire and the remaining ground wire to a circuit for which you want to provide regulated power.

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