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How to Use Positive Regulators for Negative Outputs

A regulator is an electronic device that takes in an unregulated power supply voltage and delivers a voltage that holds steady despite changing currents. Fixed, positive regulators come in a numerical series beginning with the numerals ̶0;78̶1; and ending with the regulated voltage; for example, a 7812 has a positive 12-volt output. The 79-series regulators produce a negative voltage output. If you need a negative regulated voltage, yet do not have a 79-series regulator, you can use a circuit trick that reverses the roles of the regulator̵7;s common and output leads, producing a negative voltage from a 78-series component.

Things You'll Need

Step-down transformer, 15-volt secondary
Two-prong standard line cord with bare wire ends
15- to 30-watt soldering iron
Electronics solder
Electrical tape
4 12-inch pieces of solid 22-gauge wire
2-amp bridge rectifier
Solderless breadboard
470-microfarad, 35-volt electrolytic capacitor
7812 voltage regulator
.1-microfarad, 35-volt capacitor
Digital multimeter

Instructions

- 1
  Solder the bare wire ends of the line cord to the transformer̵7;s primary solder lugs. Wrap each connection with a few inches of electrical tape. Solder one end of a 12-inch piece of 22-gauge wire to each of the transformer̵7;s secondary solder lugs. Each secondary solder lug should have one wire soldered to it. Connect the free ends of the wires in another step.
- 2
  Insert the leads of the 2-amp bridge rectifier into the solderless breadboard. Make sure each lead goes into a separate column in the breadboard. The holes in the breadboard are electrically connected in columns, but the columns are insulated from each other. To make a connection in the breadboard, insert wires or leads so they share a column.
- 3
  Insert the free end of one of the 12-inch transformer wires so it connects to a rectifier lead marked with the AC symbol, ̶0;~.̶1; Insert the free end of the other 12-inch wire so it connects to the remaining rectifier AC input.
- 4
  Set the negative lead of the 470-microfarad capacitor into the breadboard so it connects to the rectifier̵7;s positive lead. Set the positive capacitor lead so it connects to the rectifier̵7;s negative lead. Insert the LM7812̵7;s input lead so it connects to the rectifier̵7;s negative lead.
- 5
  Insert the LM7812 regulator̵7;s common lead so it connects to the 470-microfarad capacitor̵7;s positive lead. Insert the .1-microfarad capacitor so one lead connects to the regulator̵7;s output lead and the other connects to the regulator̵7;s common lead.
- 6
  Connect one end of a 12-inch wire to the regulator̵7;s output lead. This wire serves as the regulator circuit̵7;s common connection. Connect one end of the remaining 12-inch wire to the regulator̵7;s common lead. This wire is the regulator circuit̵7;s negative voltage output.
- 7
  Double-check your connections. Turn the digital multimeter on and set its selector knob to DC volts. Touch the multimeter̵7;s positive probe to the free end of the regulator circuit̵7;s negative output wire. Touch the multimeter̵7;s negative probe to the free end of the regulator̵7;s common wire. Plug the line cord into a 110-volt socket. The multimeter should display a reading of -12 volts.

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