Electrical theory 1.6a - AC/ DC Power

In this section we'll discuss alternating current (AC) and direct current (DC) power. This is the most difficult topic to understand so there's pictures and an analogy for you to visualise what's happening. I'll give you the 'bare bones' needed for light shows and some detail for people wanting it.

Bare bones:

AC: AC voltage and current periodically change their amplitudes from positive to negative in a cyclic order. This is shown in the graph.

If a LED is connected to AC voltage, it will emit light for first positive amplitude then blow up on first negative amplitude because it doesn't like voltage applied in wrong direction (see section on LEDs).

AC is converted to pulsating DC using a diode bridge (see next section - link is below).

DC: DC voltage and current have constant or varying amplitudes over time as positive amplitudes ONLY (this is called uni-directional).

LEDs love DC of either type ( ie constant or varying).

Detailed explanation:

What is AC:

Over time, AC voltage and current vary their amplitudes which cyclically change from positive to negative. This is shown on the graph. For 50Hz/ 60Hz supply, the change occurs ever 10ms/ 8.33ms.

Analogy: If you could sit on an AC voltage, you would be going up and down like sitting on a horse going around on a carousel (time passes ie Time >>) and the horse going up above (positive height) and going down below (negative height) the mid point!

This is all you need to know about AC because you'll prefer to use a DC supply for LED lights.

What is DC:
DC voltage and current has an amplitude offset from 0 value in one direction only which could be positive or negative. We'll only use positive DC as shown in the graph opposite.

There are 2 basic types of DC - constant or varying - see graph.

• Constant DC has a fixed amplitude, eg a battery.
• Varying DC has a amplitude varying over time. An example is DC obtained from rectifying AC which we'll cover next.

NEXT >> AC/DC Rectification