Plus and minus 5 V
Two 12 V sources make a split supply, and a 7805 and a 7905 turn it into +5 V and -5 V, each with a load on it. There is no microcontroller in it: the simulator solves the 6 parts as a circuit, so it runs the moment you press Run, and nothing needs compiling.
Teaching? Assign it to your class in one click.
Beginner About 10 minutes to build and run. Runs in your browser. Free, and no account needed.
The parts retail for about $4.10. On Mokxi they're free.
What you'll learn
- Building a split supply from two sources in series
- How a fixed linear regulator holds a steady output
- Why a negative regulator has a different pin order
- Dropout voltage: how much input a regulator needs above its output
- Difficulty: Beginner
- Time: About 10 minutes
- Parts: 7
How it works
Two 12 V sources are joined in series, with ground where they meet. That makes +12 V on one side and -12 V on the other, with 0 V in the middle. This is a split supply, the kind an op-amp circuit runs on.
A 7805 regulator takes the +12 V and holds its output at about +5 V. A 7905 takes the -12 V and holds about -5 V. It is a mirror of the 7805, so its readout is negative.
Each output feeds a 470 ohm resistor to ground. That load draws about 10.6 mA, in opposite directions on the two sides. There is no microcontroller. The simulator just solves the circuit.
Parts list
7 parts, plus the jumper wires. They retail for about $4.10; on Mokxi they're free.
- 1 × Half breadboard
- 2 × DC voltage source
- 2 × Voltage regulator
- 2 × Resistor, 470 ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
Wiring table
Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.
| Part and pin | Connects to |
|---|---|
| Voltage regulator (1) pin IN | DC voltage source (1) pin V+ |
| DC voltage source (1) pin V- | Ground |
| DC voltage source (2) pin V+ | Ground |
| Voltage regulator (1) pin GND | Ground |
| Voltage regulator (2) pin GND | Ground |
| Resistor, 470 Ω (1) pin 2 | Ground |
| Resistor, 470 Ω (2) pin 2 | Ground |
| Voltage regulator (2) pin IN | DC voltage source (2) pin V- |
| Resistor, 470 Ω (1) pin 1 | Voltage regulator (1) pin OUT |
| Resistor, 470 Ω (2) pin 1 | Voltage regulator (2) pin OUT |
The notes that come with it
There is no microcontroller in this one, so nothing is compiled: the simulator solves the circuit itself. The file in the editor is the notes that come with it.
// Plus and minus 5 V from a split supply.
//
// There is no microcontroller here. Press Run and the simulator solves the
// circuit.
//
// THE SUPPLY. Two 12 V DC sources in series, with ground where they meet:
// +12 V on the top red rail, -12 V on the bottom blue one, and both other
// rails at 0 V. That is a split supply, what an op-amp lab runs on.
//
// THE REGULATORS. A 7805 from +12 V holds +5.01 V at this light load (its
// sheet allows 4.8 to 5.2). A 7905 from -12 V holds -5.01 V: it is the same
// regulator in a mirror, and its readout is negative.
// Its legs are not the 7805's either: GND, IN, OUT from the left, so its
// middle leg goes to the negative supply, not to ground.
//
// THE LOADS. 470 ohms on each output, about 10.6 mA each way. The 7805's
// bias, 4.2 mA, comes out of its ground leg; the 7905's, 1 mA, goes into
// its ground leg and out of IN. Either way the input current is the output
// current plus a little.
//
// THINGS TO TRY. Turn the bottom source down to 6 V: the 7905 needs about
// 1.1 V across itself and holds -5 V down to about 6.1 V in. The 7805 needs
// 2 V: turn the top source down to 6.5 V and watch it drop out.
Troubleshooting
The usual mistakes with this circuit, and the fix for each.
- The negative regulator gives no output, or the wrong one This project
The 7905 pins run GND, IN, OUT from the left, which differs from the 7805. Its input comes from the -12 V side, not from ground. - An output sags well below 5 V This project
A regulator needs some voltage across itself. The 7805 needs about 2 V and the 7905 about 1.1 V. If you turn a source down, the output drops with it. - Both rails read 0 V This project
Join the first source's V- and the second source's V+ to ground, along with both regulators' ground legs and the resistors' lower ends. - The circuit sees no voltage or the wrong amount. DC voltage source
Make sure V+ and V- go to the right nodes. The source floats, so tie one side to your circuit ground. Check the set voltage matches what your parts allow. - A part gets too much voltage. DC voltage source
Check the rating of every part before raising the supply. Use series resistors with LEDs, and do not connect the source straight across a short. - The output is lower than the label because the input is too close to the output. A 7805 needs about 7 V in, and an AMS1117-3.3 needs about 4.5 V. Voltage regulator
Check the dropout in the datasheet and feed at least that much above the output. Do not feed more than the maximum input, around 35 V for a 78xx. - The output oscillates or is noisy, or the part gets very hot. There are no capacitors, and the heat is input minus output times current. Voltage regulator
Add 0.1 uF to 10 uF capacitors on the input and output, as the datasheet says. Cut the voltage drop, add a heatsink, or use a switching module for large currents. - The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure.
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