How does voltage affect a circuit

By Doug Lowe

The purpose of a power supply is to provide power for an electronic circuit. For a given amount of power, there’s an inverse relationship between voltage and current. Whenever current increases, voltage must decrease, and whenever current decreases, voltage must increase. This simple fact, unfortunately, has an adverse effect on power supply circuits.

When you connect a voltmeter to the output terminals of a power supply, the meter itself draws an almost insignificant amount of current, so the meter reads very close to the voltage you expect to obtain from the power supply.

However, if you connect a circuit that draws significant current from the power supply, the voltage from the power supply will drop in proportion to the current. Depending on the nature of the circuit you’re connecting to the power supply, this voltage drop may or may not be a bad thing.

Some circuits designed for 12 VDC will work fine if only given 9 VDC. But other circuits are sensitive to the input voltage, so the power supply needs to work harder to make sure it delivers the desired voltage.

To maintain a steady voltage level regardless of the amount of current drawn from a power supply, the power supply can incorporate a voltage regulator circuit. The voltage regulator monitors the current drawn by the load and increases or decreases the voltage accordingly to keep the voltage level constant.

A power supply that incorporates a voltage regulator is called a regulated power supply.

You can design your own voltage regulator circuit, but it’s far too easy to buy one of the many available integrated circuit (IC) voltage regulators. Voltage regulator ICs are inexpensive (under two dollars) and, with just three pins to connect, easy to incorporate into your circuits.

The most popular type of voltage regulator IC is the 78XX series, sometimes called the LM78XX series. These voltage regulators combine 17 transistors, three Zener diodes, and a handful of resistors into one handy package with three pins and a heat sink that helps dissipate the excess power consumed by the regulator as it compensates for increases or decreases in current draw to keep the voltage at a constant level.

The last two digits of the 78XX ID number indicate the output voltage regulated by the IC. The most popular models are:


The most common are the 7805 (5 V) and 7812 (12).

To use a 78XX voltage regulator, you just insert it in series on the positive side of the power supply circuit and connect the ground lead to the negative side. As this shows, it’s also a good idea to place a small capacitor (typically 1μF) after the regulator.

You must supply a voltage regulator with about 3 V more than the regulated output voltage. Thus, for a 7805 regulator, you should give it at least 8 V. The maximum input voltage for a 7805 is 30 V.

The diodes in a bridge rectifier will drop about 3 V from the transformer output, so you’ll need a transformer whose secondary delivers at least 11 V to produce 5 V of regulated output. Eleven-volt transformers are rare, but 12 V transformers are readily available.

Thus, a 5 V regulated power supply starts with a 12 VAC transformer that delivers 12 V to the bridge rectifier, which converts the AC to DC and drops the voltage down to about 9 V and then delivers the voltage to the filter circuit, which smoothes out the ripples and passes the voltage on to the 7805 voltage regulator, which holds the output voltage at 5 V.

Another popular voltage regulator IC is the LM317, which is an adjustable voltage regulator. An LM317 regulator works much like a 78XX regulator, except that instead of connecting the middle lead directly to ground, you connect it to a voltage divider built from a pair of resistors. The value of the resistors determines the regulated voltage.