Transformer Center Tap
Name: W. J. G.
Date: August 2004
What exactly is meant by "Center Tap" in relation to a
transformer? i.e. 400-0-400?
Your transformer has one winding with a center tap. From the center tap to
either side is 400 volts. Across the entire winding is 800 volts.
I don't know what your application is, but often this is done when there is
a requirement for greater amperage at a given voltage. Part of the
application is attached to one side and another part to the other side.
Sometimes a center tap is used to power a full wave rectifier providing DC.
The center tap is simply another wire which is typically connected
somewhere in the middle of the secondary coil. For example, suppose you
had a transformer with 120VAC primary (2wires) and a 24VAC secondary (2
Wires at each end of the secondary coil). If you placed a center tap on
the coil, using the center tap and either end you would get 12 VAC if
the center tap is truly in the center. This concept is used in
electronics to generate positive and negative voltages using the center
tap as the ground.
A conventional transformer has two output wires. The rated voltage is
produced across those wires. It is the "RMS" or root-mean-square voltage.
In the 400-0-400 example, assume the three wires are A, B and C. You would
find 400V between A and B. 400V between B and C, and 800V between A and C.
A transformer consists of two coils of wire that magnetically interact with
each other. The "center tap" or center connection is simply a wire that
attaches at the halfway point on the output coil.
Industrial machinery may contain transformers with many input and output
taps. For example, the transformer in a power supply that I have contains
input taps for 208, 220, and 240V. The user would connect to different
taps, depending on the voltage coming into the building,
In the example, one wire would be marked 400
Suppose you wind 1000 turns of wire for the secondary (output) coil of a
If you then find the middle, at turn #500, cut through the wire's
insulation and attach another wire leading to the outside world,
that is a "center-tap".
You can see it's in the center of the coil or transformer-winding.
The word "tap" is used in the same sense a "tapping a keg", or a well, or
It means making a connection by cutting into the object at hand.
It is electrically the same as having two separate secondary coils, of 500
turns each, wired in series so their AC voltages add up.
Often the transformer is used with the center-tap connected to the
"ground" or "common" or otherwise zero voltage point in the circuit being
So, when listing the voltages on each of several taps on a long coil, one
of the taps will be labeled "0",
and each other will be labeled with the AC voltage it has with respect to
the "0" tap.
In your example, if this is a power transformer (120VAC applied to the
it's likely that "400-0-400" means that this secondary coil produces 800
VAC total. Look out!
If the center tap is considered "zero-volts", then each end-tap will have
But their polarities are opposite each other, +/-.
Being 60Hz sine waves, they are 180 degrees out of phase.
You cannot connect them directly together to use in parallel, that would
be an 800V short circuit.
A fuse will blow, or the transformer will burn out.
You can, however, rectify each end tap with a single diode, both oriented
for positive output, and hook those together.
That works well (has less ripple than one winding and one diode),
because the two windings have their positive surges at complementary times.
60 times a second they take turns driving the output positive.
Be real careful and study your circuit well before you mess with voltages
There are procedures to learn, too.
For example, each of the diodes in the above example must be rated at over
Any less it will probably burn out instantly. Hope you had a fuse in the
"400" may mean 400v peak: V(t)= 400 sin(2pi60*time),
or 400vrms, which is more like 560v peak: V(t)=560 sin(2pi60*time).
When the sine-wave on an end-tap goes to -560v, and the capacitor after
the diodes is still charged up to +400v,
the diode experiences -960v.
And it could get higher if the capacitor charges higher.
For reasons like this, well-designed vacuum tube supplies often use two
800v or 1000v diodes in series as one single diode.
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Update: June 2012