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What's the Difference between A.C. and D.C.?

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Everone knows that a battery cell gives "d.c." or "direct current" which means that a steady voltage is available to drive your radio or whatever. Less well understood is "a.c." which stands for "alternating current". However, it's important to understand the difference because it could cost you money! You wouldn't dream of connecting your 12 volt radio directly to a mains power plug because you know that it gives at least 230 volts. But do you know that a 12 volt a.c. transformer can do almost as much damage? The reason is that electronic equipment needs not only LOW voltage but low D.C. voltage. Let's take a quick look at the method of making electricity.

sine wave voltage

In a power station, electricity can be made most easily and efficiently by using a motor to spin magnetic wire coils. The resultant voltage is always "alternating" by virtue of the motor's rotation. Fig.1 indicates how the voltage goes first positive then negative - rather like turning a battery cell continually backwards and forwards in its clip.

Now, alternating voltage can be carried around the country in cables far more efficiently than direct current where the voltage is fixed. So the electricity that arrives at your house is still alternating voltage. Electric light bulbs and fires can run quite happily from 230 volt a.c. Other equipment such as televisions have an internal power supply which converts the 230 volts a.c. to a low d.c. voltage that is safe and acceptable to the electronic circuits. How is this done?

bridge rectifier circuit

There are several ways but the simplest is to use a transformer to reduce the voltage to, say 12 volts a.c. (Fig.2) This lower voltage can be fed through a "rectifier" which combines the negative and positive alternating cycles so that only positive cycles emerge. This "rectified" voltage (Fig.3) is suitable for running things like filament bulbs and electric trains but it is still no good for electronic circuits. So a "12 volt dc transformer" is no good for electronic devices. What you need is "regulated d.c." which truly simulates the steady voltage that you get from a battery.

full wave rectification half sine waves

Fig. 3 above

The first step is to connect a large value capacitor to the output of the rectifier. A capacitor acts as a voltage reservoir and has the effect of smoothing the "ripples". This is still not the same as a battery produces but it's often good enough for charging batteries in mobile phones, personal stereo equipment and similar (Fig.4).

full wave rectified voltage with reservoir capacitor

Fig.4 above

The final step is to pass this "rippling d.c." through a regulator unit. This effectively chops off the ripple to leave almost pure "regulated d.c." at a steady voltage.

So, to provide a suitable voltage for electronic circuits you need a power supply which gives a "regulated d.c." output. If your power supply doesn't use those words then it may not be suitable for use with electronic circuits. In this case, saving cost might lead to expensive smoke!

The high-quality Regulated Power supply from SatCure (order code 02-7343) is capable of supplying up to 1.5 Amps (1500 milliamps) of current. A rotary switch provides selection of 3, 4.5, 6, 7.5, 9 or 12 volts "regulated d.c." It has a multitude of uses, including speed control for a mini drill.

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