Page 236 - Robot Builder's Bonanza

REGULATING VOLTAGE 205

Voltage regulator
Positive
from battery IN OUT
(Example: 7805) 7805
2 volts or higher
than voltage 10 μF 1 μF 1 Input
regulator output tantalum Ground tantalum 2 Ground
3 Output
Negative 3
from battery 1 2
Decoupling/filtering (typical for
capacitors V+ regulator)
Figure 19- 11 A linear voltage regular requires no external parts, though it is customary
(as good design) to include capacitors to help stabilize the voltage provided by the regulator.

More current is provided by regulators in the TO- 220 style “power transistor” package.
Use these when powering circuits that consume less than 500 milliamps. Other variations of
the TO- 220 style regulator can handle 1 amp or more. Check the datasheet that comes with
the part you order.
Pinouts of voltage regulators can vary. Negative voltage regulators often have different pin
assignments than positive voltage regulators. Reversing the connections to a voltage regulator
will burn it out, so always double- check the datasheet for the device you are using. Always.

If you need more current, linear regulators are provided in larger TO- 3 style transistor
packages. They provide current outputs to several amps, depending on the exact device.
You can also get adjustable voltage regulators. These are some of the more common ones:

• The LM317T is an adjustable regulator in a TO- 220 package. With some external parts
(see its datasheet), you can adjust it to deliver from 1.2 volts to 37 volts, at a maximum of
1.5 amps.
• The LM317L offers the same voltage span, but is designed for use on circuits demanding
100 milliamps or less.
G To enjoy the full current capacity of a voltage regulator you need to mount it on, or attach it to,
a heat sink. Expect somewhat less current- carrying ability when a heat sink is not used.

Linear regulators require that the input be at least 1 volt, and usually 2 volts, over the
expected output voltage. For example, for a 5- volt regulator, the unregulated supply should be
about 2 volts higher, or 7 volts. By the same token, avoid applying too much voltage in rela-
tion to the output. The regulator throws off the extra voltage as heat, which is not only waste-
ful of energy but potentially harmful to the regulator. For that 5- volt regulator, the input
voltage should be between 7 and 12 volts.

SWITCHING VOLTAGE REGULATION
Linear regulators take an incoming voltage and clamp it to some specific value. Linear regula-
tion isn’t very efficient; as the voltage is stepped down to its desired level, excess energy (the
difference between the input and output voltages) is wasted in the form of heat.
In AC- operated circuits this inefficiency is marginal, but it’s particularly notable in battery-
powered systems, where battery life is limited.

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