Controlling the Voltage from a Battery

CHAPTER 5: Batteries and Power 95

Controlling the Voltage from a Battery

The thing with batteries is that even though they may say 1.5V, 3.7V, or 9V on the package, their voltage will drop as they discharge—often by quite a high percentage.

For example, a 1.5V alkaline AA battery when brand new will be about 1.5V and will quickly fall to about 1.3V under load but still deliver useful amounts of power down to about 1V. This means that in a pack of four AA batteries, the voltage could be anything between 6V and 4V. Most types of battery, whether single-use or rechargeable, exhibit a similar voltage drop.

This may not matter much; it just depends on what the battery is powering. If it is powering a motor or an LED, then the motor will just go a bit more slowly, or the LED will be a little dimmer as the battery discharges. However, some ICs have a very narrow voltage tolerance. There are ICs designed to work at 3.3V that specify a maximum working voltage of 3.6V. Similarly, if the voltage drops too low, the device will also stop working.

In fact, many digital chips such as microcontrollers are designed to work at a standard voltage of 3.3V or 5V.

To ensure a steady voltage, we need to use something called a voltage regulator. Fortunately for us, voltage regulators come as convenient three-pin, low-cost chips that are very easy to use. In fact, the packages just look like transistors, and the bigger the package, the more current they can control.

Figure 5-6 shows how you would use the most common of voltage regulators, called the 7805.

Using just a voltage regulator IC and two capacitors, any input voltage between 7V and 25V can be regulated to a constant 5V. The capacitors provide little reservoirs of charge that keep the regulator IC operating in a stable manner.

Figure 5-6 A voltage regulator schematic