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Power Supplies

When dealing with electronic circuits, we have to meet the basic requirement of providing electrical power for them to work. Without that power, your circuit is no more useful than a single raindrop in a hurricane.

The basic purpose of a power supply is to provide a fixed voltage to the working circuit, with sufficient current-handling capacity to maintain the operating conditions of the circuit. The power source doesn't have to be fancy; the typical hand-held transistor radio uses a 9-volt battery as its power source. A flashlight uses cells that are physically much larger, but provide a lower voltage. Major electronic appliances such as television sets, VCRs, and microwave ovens have electronic circuits built in that take power from a wall socket and convert it to the form and voltages required by the other internal circuits of the appliance.

Such an electronic power supply circuit is imperative if you plan on doing long-term experimenting with electronic circuits. The alternative is to spend a considerable amount of money on replacement batteries at regular intervals.



The requirements for a power supply depend primarily on what you intend to do with it. A power supply for digital IC circuitry must have an output voltage as required by the ICs of the logic family you're using. The most common modern requirement is +5 volts dc, as required by TTL devices and quite suitable for CMOS devices. However, RTL ICs will want +3.6 volts, while ECL ICs are designed to work with a -5.2 volt supply (-5 volts will work, and so will +5 volts if you remain consistent).

On the other hand, a power supply for analog circuits such as radio receivers and audio amplifiers will depend on the design requirements of the circuitry. A pocket transistor radio is specifically designed to operate at 9 volts, so it can be powered by a single 9-volt battery. However, any circuit employing operational amplifiers (op amps) will require both a positive and negative power supply, in the range of ±12 to ±15 volts. Such voltages are also quite satisfactory for experimental analog circuits, and so are practical values to have for a breadboarding system.



As a starting point for the early analog experiments on these pages, you can use a 9-volt battery. This will also work for digital experiments using CMOS ICs (which can operate from any voltage in the range +5 to +15 volts). As you continue with these experiments, however, we will introduce working power supply circuits that you can build and readily use for the rest of the experiments and for your own investigations.


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