Introduction

Regulators

Other Options

Introduction

Building a power supply for most electronic projects is not difficult and rather basic in application. The most common and easily available DC supply is the battery. It is ideal for portable systems but, in the long run, constant replacement or recharging can be expensive and time consuming.

For workbench testing and prototypes a power supply unit is more suitable. It should deliver -- under load -- a DC voltage comparable to a battery that is as smooth as possible and with a minimum ripple. All at a reasonable cost. Let's cover the basics without the pains of formulaes but respecting the conventions.

The Transformer

We select a transformer for ( a ) Voltage output and ( b ) Current output required. We must also consider the physical size of the tranformer if we need only a few miliamps of current. Choosing a transformer the size of truck will naturally be much more expensive but will also be bulky, unless of course you want brute force.

If you need to build a supply for only one system that requires a fixed voltage and current, then choose a transformer that is rated at a voltage slightly above that voltage and rated at approximately 25% above the needed current. If your needs are for a dual polarity supply (+ 0 - ) then you will need a center-tapped transformer. The end taps (usually same colour wires) being the total output voltage and the center tap used as the common point or Ground sometimes referred to as zero (0) V.

AC/DC Power Adaptors

Wall plug-in transformer adapters are a good economical source for transformers. Beware: most are only halfwave rectified but that is no problem if you output into a fullwave bridge. You can find them for little cost at swapmeets, garage sales, surplus stores etc. Some things to look for on the transformer case are voltage, current, and if it's DC or AC, and if it's a positive or negative ground -- usually indicated on the ouside ring of the plug symbol. AC is our first choice.

When measuring the output voltage with no load applied the voltage will always read as much as 40% higher than its rating. To determine the true voltage and current rating of the unit apply a resistance load at the ouput and measure the voltage.

For example, to find the resistance for a voltage rating of 12VDC @ 300mA. Looking at Ohms Law with , E/I , E=12V, divided by I=.300A = .040k ohms or 40 ohms and the wattage of the resistor should be E X I or 12V X .300A = 3.6Watts. That's the wattage required for a permanent application, but for a fast voltage measurement a one watt resistor could be used.

The Rectifier

The rectifier diode is simply a one-way conductor. Applied in an AC circuit it will pass only the positive or negative phase of a sine wave. Rectifier diodes are rated for operating voltage and maximum current.

A rectifier bridge is composed of two or four separate diodes. Its rating is the total of all diode ratings on the bridge. Bridges are useful as they are already internally connected. A full bridge has four connections; two AC inputs and, two rectified voltage outputs marked " + " and " - " .

Half wave or Full Wave ?

Fullwave rectification is where both the positive and negative phases of a sine wave are rectified. Halfwave rectification is where only the positive or the negative phase is being rectified. Halfwave rectification is used where the DC voltage need not be smooth and for economy.

The Filtering Capacitor " C "

The capacitor is used as the primary filtering element of the rectified voltage and as an added bonus serves as a tank to help maintain a constant voltage level as the load demands. For production units the value of " C " is usually calculated by the design engineer to meet the minimum demand and costs necessary for function requirements. I have my own formula make them as large as you can fit them in capacitance wise and double the voltage of the rectified output. If the capacitor is too large to fit you may use several smaller ones connected in parallel, the total capacitance will be the sum of each element's capacitance rating.

NOTE: I've found that with a variable supply hooked to a very large capacitor, adjusting the voltage without a load may be slow and lagging. This can be overcome by adding a 1K resistor across the output as a permanent load. This technique is also useful in maintaining good regulation under a light load.

On the next page we review the application of regulators.