Exercise: Linear Voltage Regulator

Objective

Test different types of voltage regulation components.

Power supplies are the heart of any electronics project, as every circuit needs a source of energy, and usually with very specific electrical characteristics. Most of our circuits will require regulated DC supplies, with energy coming either from a battery or the building electrical system. Unless otherwise specified, regulated means the voltage is controlled to a constant level despite changes in the load.

In this exercise we will look at linear voltage regulators, and in a later exercise we will look at switching regulators.

In brief, a linear regulator acts as a variable resistor governed by feedback to keep the output voltage at a fixed level. Linear regulators can only step voltages down, and the excess energy is dissipated as heat. So for example, a 5V linear regulator providing a 500 mA current from a 12V supply creates a 7V voltage drop. That represents 3.5 Watts of heat dissipated in the package, which typically means the package needs a good heatsink to carry that heat away.

Steps and observations

1. Make sure you use the correct voltage regulator for this circuit. The required chip is the L7805CV. This is written in small white text on the surface of the chip.
2. Wire up the circuit, being very careful to observe the correct input polarity (especially on those capacitors. The minus signs on the can marks the negative lead). The capcitors are optional, but help keep the power stable. When making circuits for a project, it is a good idea to include them.
3. Measure the unloaded outputs.
4. Find a power resistor or component representing a reasonable load. E.g., dissipating 100 mA from a 5V regulator can be accomplished using a 0.5 Watt 50 ohm resistor, or a lamp or motor with similar properties. (Hint: a half-watt 50 ohm resistor can be formed from two quarter-watt 100 ohm resistors in parallel.)
5. Measure the loaded output.

Comments

In the example of a linear regulator, the the load is only consuming 2.5 Watts of energy while the regulator dissipates 3.5 Watts, so this circuit is operating at only 42% efficiency. Linear regulator circuits are simple to use and inexpensive, so they are applicable for low-power applications or where efficiency and heating are not a problem. The primary advantage is they are potentially low-noise compared to switching regulators, so they often appear in high-fidelity audio applications.

A good debugging heuristic for any project is to begin solving problems by checking the power supply. There are many, many possible problems, so for example:

1. overloading a power supply can cause the voltage to drop or current limits to trip
2. power supplies connect across every circuit subsystem, so signal problems can be linked in subtle ways
3. high-current signals from motors can create voltages in the ground wires (“ground bounce”)
4. digital supply noise can couple into analog circuits and inject noise
5. etc., etc.

The diagram shows a 7805 in the common TO-220 leaded package, which has the following connections:

1. Input
2. Ground (also connected to heatsink tab)
3. Output

Other Files

1. voltage-regulation.fzz