basic transformer question?

[new guy]

What is the purpose of a "center tap" of a transformer? Is this just to create two smaller secondaries? And why does the center tap always go to ground? Is the ground connection just the return path for the loads on that circuit?

[Chris Smith]

The center tap “allowsâ€

[philba]

it's a bit of a throwback to the days when diodes were expensive. With a center tap, you can use 2 diodes for full wave rectified AC. without the CT you need a bridge rectifier which has 4 recitifiers in it to get full wave. the output voltage level will be 1.414 times the 1/2 half the rating of the secondary.

The CT goes to ground since that's how it works for a 2 diode full wave setup.

you can also use it with a full bridge to get a bipolar supply. CT goes to ground in that case as well.

Here's a fairly decent treatment of this topic: http://www.play-hookey.com/ac_theory/ps_rectifiers.html

[haklesup]

You are correct about a center tap signifying two equal secondaries.

Transformers can have any number of secondary windings and the taps don't have to be in the center or be grounded. In my product we use a custom transformer with two primaries (so it can be switched 110V/220V and 3 secondaries 8V, 25V and 135V usded to make a variety of DC outputs.

Center tapping just so happens to be the most common and probably most useful configuration because it encourages the use of full wave rectification or bipolar (+ and - WRT groud) supply and dual supply configurations. It cost the manufacturer almost nothing to add a tap so if it sells more units, what the heck.

The CT does not have to be grounded but many configurations would require it. Consider a 24V CT transformer. (A and B are the outputs and C is the center tap) If you ground C you get two 12V outputs but if you ground either A or B you get a 12V and a 24V output. Depending on the reguilator circuit you probably need to assign ground to one of the AC terminals but it is not always required either.

[Tommy volts]

new guy,

In addition to what the other gentlemen stated:

Center tapped transformers with 240V AC primary and two 120V AC secondaries are used for nearly all residential service in the United States. This system allows for the "Split Phase Distribution" of electrical current through your household. Black is "hot" and white is "neutral". The "neutral" is tied to the center tap of the transformer which is grounded.

The split phase distribution system significantly reduces the amount of current in the copper conductor system and therefore requires less copper.

[Robert Reed]

"The split phase distribution system significantly reduces the amount of current in the copper conductor system and therefore requires less copper."

With 240 VAC loads, there is no reduction in current and corresponding copper required. With a 'perfectly balanced 'split phase system driving 120 VAC loads, the current and copper required is cut in half from what it normally would be in a two wire only feed and the neutral carrys no current at all. Athough the three wire system does reduce the copper required for 120VAC loads, the bulk of the heavy loads will be 240 VAC, and no savings is obtained here. Also three wires are required for this system as opposed to two.Still, the three wire system offers some cost savings in copper. Probably the biggest advantage is to have 120 VAC and 240 VAC on a common circuit-simplicity.

[russlk]

I believe that the most correct answer to "why have a center tap" is that full wave rectification with 2 diodes has AC flowing in the secondary so less iron can be used and still avoid saturation. Full wave bridge rectification works that way also but back in the day of selenium rectifiers, a center tap was much cheaper.