by Oliver Max, 2/1/01
Voltage Regulators
The easiest way to provide a constant voltage to power circuitry (cpu, op-amps, etc.) is to use a 3 terminal voltage regulator. This is a device which will take a variable input voltage (usually up to 20-30 Volts) and drop it down to a fixed output voltage (5, 12, 15 etc.). It simply dissipates the extra energy, since it draws the same amount of current as it supplies to the load (plus a tiny bit extra to operate itself ~ a few milliamps).
Common Regulators - For currents up to 1A, the 78xx and 79xx families of regulators are
cheap and easy solutions. The 78xx family is for positive voltages while the 79xx
family produces negative voltages (ie: 7805 is a +5 V regulator). One characteristic
of these regulators is that they require an input voltage of approximately 2 volts more
than what you expect to get out of them. This means you'd need 7+ V to create a
steady 5V supply. If you expect to actually draw 1A, you will also need to heat sink
the chips. They typically come in TO220 packages which can dissipate about 1W alone.
Suppose you supply a 7805 w/ 8V and draw one amp - it will drop the voltage 3V and
consequently dissipate 3W (or overheat and turn off if you don't heat sink it so that it
actually can dissipate 3 W).
For a little more money, you can get low-dropout regulators which can
supply the output voltage with an input voltage that is only 0.5 V or so higher than the
desired output. The LM2940 is a handy 5V chip in this category.
If you need some oddball voltage, there are convenient 3 terminal
adjustable voltage regulators such as the LM317.
Also, don't forget to put a capacitor on the output of the regulators.
A 1 uF cap will make it work nicely, although you might want a larger cap depending
on the load. If you forget the capacitor, the output will oscillate since the output
voltage changes so rapidly with small changes in current.
See Horowitz and Hill Chapter 6 for more details on regulators and heat
sinking.
Switching Regulators - If you want to step up a voltage, invert it, or drop it in a more efficient manner than simply dissipating the extra voltage as heat, you can use a switching voltage regulator. The basic idea is to store some energy from the input in an inductor and then switch a circuit element so that this energy is dumped to the the output of the circuit. A capacitor is usually used on both input and output to smooth out the voltages resulting from the pulses of current. You can certainly build your own regulator with garden variety circuit elements like op-amps and oscillators, but fortunately other people have already done this and packaged the result as a cheap IC. Two chips I've used for building switching power supplies are the LM78S40 (national) and the Maxim1771. The Maxim runs at a higher frequency (up to 400kHz, I think) and can supply slightly more power, but unfortunately the suggested circuits from the data sheet don't work so well with out a lot of careful debugging and extra features to kill noise. As for the LM78S40, the suggested step-up converter (in datasheet) worked the first time I built it. Both chips allow you create wide ranges of output currents and voltages by varying a few external components.
The circuit below is copied from the LM78S40 datasheet, and some scope traces for a properly operating circuit are shown below.
The above circuit was built with a 100uH toroidal inductor (the ones we've got in lab), and configured to produce 7 volts at the output. The input voltage was 4 volts. The output was loaded with a 91 ohm resistor. Channel 1 (top) is the voltage at the driver transistor collector. Channel 2 (2nd down) is the output voltage. Channel 3 (3rd down) is a zoomed in version of the output voltage (AC coupled) - notice the effect of ESR (effective series resistance) of the cruddy electrolytic caps. Channel 4 (bottom) is the current measured across the sense resistor (0.5 ohms in my circuit) (also AC coupled).
