and switch mode power transformer. The transformers design is
different.
Sometimes it is just referred as ac transformer. Linear transformer
mostly used in radio, project kits, VCD and etc while switch mode
transformer can be found in Monitor, Tv, Fax and so on.
A transformer consists of two or more coils wound on the same core.
For power transformers, the core material is usually iron. For a
radio-frequency transformer, the core material is usually ferrite or
air. The basic property of a transformer is to change AC voltage. A transformer cannot change direct current voltage. A step down
transformer has a lower AC output voltage at its secondary winding
than the AC input voltage to its primary winding. Conversely, a step
up transformer has a higher secondary than primary voltage.
The transformer laminations or coating of shellac, enamel or varnish
is to insulate adjacent turns from shorts between winding.
I will not go too details about how transformer work and transformer calculation as you can read more details from most electrical and
electronic transformers books.
How to test transformers:
Transformer failures are relatively rare in most electronic
equipment, compared with other components. If too much current goes
through a coil or winding, the winding heats up and can either open
up completely, or the insulation between turns of wire can break
down, causing the coil to be partly or completely shorted.
Just like a coil, you can check continuity of any primary or
secondary transformer winding. The actual value of DC resistance you
read is rather meaningless, but at least you will know whether a
winding is open. Checking linear transformer is quite straight
forward. Apply the ac voltage to the primary winding and expect
output voltage at the secondary side. No or low output voltage means
the transformer
winding has open or shorted winding. Look for any evidence of
overheating, such as darkened or blackened areas or a burnt smell.
About how to test a power transformer-it is a little bit complicated.
You cannot apply ac voltage to the equipment and expect the desire
output at the secondary side without connect to load otherwise over
voltage will happen (because you have removed all the secondary load)
and will blow the power section which consist of power fet, pwm ic
and etc. Swith mode power supply are designed to run with load while
linear
transformer can be test without load.
The best way to test a switch mode power transformer is first remove
it
from the board. Check only the primary winding as secondary winding
failures are exceedingly rare. How do we know which one is the
primary winding? Trace from the big filter cap the positive pin to
see where the circuit goes. It will go to one of the pin of the
transformer. Then look for the power fet middle pin which is the
Drain and it will lead to another
pin of the transformer. This two pins is the primary winding of the
transformer. Checking the resistance of the primary winding only can
reveal whether it is open but it cannot check if it has developed a
shorted winding. Only by using a ringer test you will know if it
shorted or not. A shorted turn in the primary winding can cause the
power section to blow.
The cheapest ringer tester that i came across is Dick Smith LOPT METER or flyback tester. Just connect the probes to the primary winding and
the result are instantly seen from the Led bar graph. 4-8 bar
represent
a good winding, 1-2 bar represent partial short and if the led goes
off means there is a heavy short in the winding. Besides checking
the transformer it can be use to check B+ COIL, horizontal yoke coil and flyback primary winding and ballast (choke).
The expensive ringer tester is from sencore equipment. Sencore LC102
and LC103C besides checking capacitor it also have the capability to
check all kind of winding. Whether it is a low voltage transformer,
high voltage transformer, current transformer, pulse transformer,
electric transformer, custom transformer this equipment will be able to check and test it.
If you do not wish to invest on this tester you may use a light bulb
to check it. Remove one of the primary winding pin and connect a 60
watt light bulb. One end to the circuit while the other to the
transformer pin. If the bulb light and didn't goes off suspect a
shorted winding and if the light goes off, the winding is ok.
Transformer repair is not easy and it require a good skill to do it. Most of the transformer repairer preferred to rewind bigger winding
such as fan, car alternator and starter. Swith mode power transformer
is a little bit difficult to rewind because of the small size and low
price.