Home › Builds › 8. Voltage regulator: a 12 V supply and a steady 5 V

8. Voltage regulator: a 12 V supply and a steady 5 V

Bench build · Fundamentals · Beginner · about 30 minutes

Step 8 of the automatic light: The build outgrows a 9 V battery, so it moves to a 12 V adapter (D1 still guards it). U1 7805 gives 5 V for the light sensor, so the dusk level no longer drifts with the supply. R1 becomes 680 Ω for the higher voltage, and D1 becomes an SR560 Schottky (5 A, 150 A surge, about 0.4 V drop): the adapter can push tens of amps into C1 for an instant at switch-on, past a 1N4007’s 30 A surge rating, the lamp will draw 0.75 A later, and every volt lost in D1 is light lost in the lamp. Component 8 of 14: the voltage regulator. A linear regulator holds its output fixed whatever the input does (as long as the input stays about 2 V above it). The 7805 gives 5 V. The difference times the current is burnt as heat: (12 − 5) V × 10 mA = 70 mW here, nothing, but at 1 A it would be 7 W and need a heatsink.

8. Voltage regulator: a 12 V supply and a steady 5 V: the finished build running on the simulator's bench

Build it in the labGuided build, test sheet and fault finding, free in your browser.

Parts list

Test procedure

Each test has an expected value and a pass range, like a factory test sheet:

  1. 5 V rail
  2. Dark: LED on from the 12 V supply
  3. Daylight: LED off

How it works

The voltage regulator. A linear regulator holds its output fixed whatever the input does (as long as the input stays about 2 V above it). The 7805 gives 5 V. The difference times the current is burnt as heat: (12 − 5) V × 10 mA = 70 mW here, nothing, but at 1 A it would be 7 W and need a heatsink.

Reading it. 7805 in TO-220, pins IN-GND-OUT (facing the print, legs down). It wants a small capacitor on each side (0.33 µF in, 0.1 µF out) close to its legs, or it can oscillate. 78xx: last two digits = voltage (7812 = 12 V).

Testing it. Power it and measure OUT to GND: 4.8–5.2 V. Input below 7 V and the output sags: that is dropout, not a fault.

In the build. The build outgrows a 9 V battery, so it moves to a 12 V adapter (D1 still guards it). U1 7805 gives 5 V for the light sensor, so the dusk level no longer drifts with the supply. R1 becomes 680 Ω for the higher voltage, and D1 becomes an SR560 Schottky (5 A, 150 A surge, about 0.4 V drop): the adapter can push tens of amps into C1 for an instant at switch-on, past a 1N4007’s 30 A surge rating, the lamp will draw 0.75 A later, and every volt lost in D1 is light lost in the lamp.

Builds on lesson 7: everything from before is still there.

Next: lesson 9, comparator (lm339).

Things to try

  1. Measure IN, GND and OUT of U1.
  2. Set the adapter to 9 V: the 5 V rail does not move.

Background

One circuit, built up one part at a time: by lesson 14 it is a solar-powered, dusk-sensing, motion-triggered timed lamp with an alarm.

More builds: fundamentals