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We need to straighten things out… We need to rectify this situation.

A half-wave rectifier

A half-wave rectifier

Most electronics and some other things require DC electricity, but AC is what we usually have available.  AC is great for the power companies because they can use a transformer and step up the voltage and transmit the power long distances and then use another transformer to step it back down to a safer voltage for use in homes, offices, and factories.  But, more and more of the world runs on low voltage electronics.  We already know how to get the voltage down to a low value but how do we get it to DC?

The first part of that answer is is the “magic crystal” diode described in the last two posts.  Since the diode only conducts in one direction we get DC by putting one in series with our load.  It is DC, but not smooth DC.   We will get to smoothing it out very soon.  In all of the diagrams I used the same transformer.  It is called a 12.6 Vac center tapped transformer because in the middle of the secondary there is a tap (wire) that can be used for our reference instead of one of the end wires.   If it is our reference point we have two 6.3 Vac voltages and those are 180 degrees out of phase to each other.   This is exactly what happens when the power company sets a transformer for you house and provides 240/120 V to your house.  (If you live in USA, I am not sure the voltages in other countries.)

In the first picture, I show a diode in series with the load.  This is call a half-wave rectifier because only 1/2 of the AC cycle makes it to the load.   The maximum peak voltage to the load in this case is (12.6 X 1.414) – 0.65 V.   The 1.414, √2, is because the voltage is expressed in rms volts.   The 0.65 V is from the voltage drop due to the diode built-in voltage.   The actual voltage will probably be a little less because of the internal resistance of the diode.

A positive and negative half-wave rectifier circuit.

A positive and negative half-wave rectifier circuit.

The negative half-cycle is feeling dejected and rejected. If we add one more diode and reverse it in comparison to the first diode we can put the negative half-cycle to work and we now have a power supply able to provide both a positive and negative voltage.  (Think op-amp power supply!).  There is a price to pay for doing this.  The transformer is heated each cycle due to the I2R losses but if only 1/2 of a cycle is used the windings get to cool of each 1/2 cycle.  Since we are now using both halves of the cycle we are only able to draw 1/2 the current we could before without overheating the transformer.  The peak voltage on each of the circuits is exactly the same as the calculation given above, except of course one is a negative voltage.

full wave center tapped rectifier.

full wave center tapped rectifier.

There is another way we can use the negative half cycle.  We could wire the secondary of the transformer as shown in the diagram to the left.  This is called a full-wave rectifier because both halves of the AC waveform appear at the DC side and there is no space between the bumps.  It is still bumpy but more like cobblestones and less like speed bumps.   The price we are going to pay?   The peak voltage out of this one is a little less than 1/2 of the voltage of the half-wave rectifier and the maximum it can be is 6.3 X 1.414 – 0.65.

A full wave bridge rectifier.

A full wave bridge rectifier.

We are not done yet. We want our cake and eat it too.  The circuit to the right uses four diodes in a bridge arrangement to provide a higher voltage and full wave rectification.  During each half-cycle 2 diodes are conducting.  One of these pairs is conducting to the common path and one is conducting to the positive side of the load.  The price we pay for this is we have two diode voltage drops for our peak output voltage.   The maximum peak voltage is 12.6 X 1.414 – 0.65 – 0.65.

A filter capacitor to smooth a half-wave rectifier.

A filter capacitor to smooth a half-wave rectifier.

So far we have changed our AC to DC, but it still is a bumpy ride. We either have speed bumps or cobble stones.   We have just discovered a very good use for a capacitor.  The energy storage will allow us to ride through the bumps.
All this talk about speed bumps and cobble stones reminded me of an old commercial and luckily it is archived on youtube.  The commercial I remembered was the light-bulb F-150 commercial but this one is even better because the waveform produced by the spray gun looks very similar to the one I drew above.
1966 – F-150 commercial.

In this post I only talked general concepts.   Soon I will talk about the sizing of the capacitor.  The obvious answer, “Bigger is better”, is probably not the correct answer for several reasons one of which is you might need to have an F-150 to haul your power-supply around.   This is not exactly like the story of the three bears, there probably is not “just right”, but there is more right answers.

I hope you found this worth your time and hopefully got a few snickers on what could have been a very dry subject.

Gary

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