Electrical theory 1.6b - AC/ DC Rectification

In this section we'll discuss converting AC to DC power to power your lights. This is the most difficult topic to understand so there's plenty of pictures for you to visualise what's happening. I'll give you the 'bare bones' needed for light shows and some detail for interested people.

Bare bones:



A bridge rectifier converts AC to pulsating DC of value = Vac * 0.9 (approx). Connect the diode so that current flows in direction of arrowhead (same as a LED).

Cathode end    Anode end

More detail:

Super critical point to understand:

A diode is a polarised electronic device which conducts current one way when the voltage between anode TO cathode is positive. This is called 'forward biased' and is shown in diagrams for AC half cycle cases. The diode is connected in series in the circuit.

Now when the  voltage between anode TO cathode is negative, the diode becomes practically an open circuit - this is called 'reverse biased'.

 

Diagram for each half cycle of AC supply showing a forward biased condition (ie current will flow)

 Case 1                                                 Case 2

A bridge rectifier consists of 4 diodes arranged so that:

• for positive cycle AC, 2 diodes conduct voltage/ current ( case 1) to output as shown below;
• for negative cycle AC (this is half a cycle later), the other 2 diodes conduct voltage/current (case 2) to output with same polarity as shown below. Note their direction agrees with above;
• in respective half cycle, the grey diodes do not conduct because they are 'reverse biased'.



In the diagram, I've used an analogous battery to show the voltage according to the half cycle shown in the graph. This battery makes it very easy to  see what's happening and when. The output which is shown with a meter, the 2 half cycle outputs are consecutive in time, so the pulsating DC waveform in bottom graph is the resultant output.

 

If you don't believe my analogy, take the TEST!! Get a 9V battery and a bridge rectifier. Connect the battery to the AC input terminals of a bridge rectifier. Measure the polarity of DC voltage against the rectifier's + - terminal markings. Now reverse the battery connections to AC terminals and measure the polarity again. It will not change and it will agree to + - markings!

  

If you don't understand above, please email me and I'll give more explanation or demo an experiment.

For more info. go to http://en.wikipedia.org/wiki/Bridge_rectifier

 

Approximate equation for ‘full wave’ rectification:

Vdc= Vac * 0.9

eg 27Vac = 24.3Vdc

eg 110Vac= 99Vdc

Here's a photo of a bridge rectifier (4 separate diodes) located in a damaged light controller.

Note: for simplicity, filtering capacitors are not discussed or usually needed in Christmas lights.

END