Country: United States
Date: March 2007
I was opening an alarm clock the other day to
examine a transformer. I notice that it has 2 wires on one side
but 4 wire on the other side. I look at a diagram of a transformer
and saw that it should have only 2 wires on each side. What are
the other 2 extra wire for and how do you test which one of the 4
wire is negative and positive for the output voltage?
There are a two possibilities - your secondary may have taps or you
may have two secondaries. If you check for continuity between the
four secondary leads you can tell which ones are connected. If you
have continuity between A and B and between C and D... but not
between A and C... you have two separate secondaries. They will be
A-B and C-D.
More likely, you will have two taps in the center of the
secondary. If so, you will have continuity between all of the secondary leads.
Once you know which of the above you have, you can input a known a/c
voltage into the primary and measure the output from your secondary
or secondaries. The ratio in the voltages will be the ratio between
the turns of the transformer.
If you find that you have a tapped secondary, the voltages from A to
B plus B to C plus C to D will equal the voltage from A to D
Coming from an alarm clock is it likely that you have a step-down
transformer - the voltage output is lower than the voltage
input. Yet, you would want to be careful when testing the
voltages. A step-up transformer can could put out a voltage high
enough to be harmful. Use caution.
By the way, there is no positive and negative, because you are
dealing with alternating current. A transformer depends on the
change in current flow to operate.
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Update: June 2012