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The
Transformer
INDEX
Page 55
A transformer can be designed to do seemingly amazing things.
One amazing thing is the output current can be higher
than the input current! This is
quite a normal feature for a transformer and will be produced by almost any
transformer providing the output winding has less turns than the primary and is
wound with thick wire.
As we reduce the number of turns of the secondary, we can use thicker wire and
the extreme case is a single secondary turn made from very thick copper wire. The
secondary will produce 30, 100, or even 500 amps.
But before you get too amazed, you need to realise how this is possible. Don't
think you are getting "something for nothing" - because you aren't.
A transformer is capable of taking a certain number of watts (this is determined
by the size of the transformer) and delivering the same number of watts to the
secondary. The value of watts is the voltage across the primary multiplied by
the current through the primary. The exact term for this is called the VA rating
(we say VA because the voltage and current is rising and falling,
whereas the term watts refers to DC values or CONSTANT values).
(The output
will be slightly less than the input due to losses).
If we have a transformer with a VA rating of 60, the output can be any combination
of voltage and
current with an answer of 60.
For instance the output can be 30v @2amp, 15v @ 4amp, 10v @ 6amp,
6v @ 10amp, 1v
at 60amp, or 0.5v @ 120amp.
The animation below shows how the output current will increase as the turns on
the secondary are decreased.
Note: The waveform below is different (opposite) to the waveform on the previous
page. It shows the output CURRENT of the secondary as the
turns are decreased. The
diagram on the previous page showed output VOLTAGE.
The output current capability of a
transformer.
The output current can be much higher
than
the input current.
What's the secret to making a transformer with a high output current?
The secret is LOW IMPEDANCE.
The output winding must be very thick so that it has a very low resistance. In
all cases, when you have a high output current, the accompanying voltage will be
fairly low. In most cases the voltage will be just enough to "push" the current
through the load.
If we take the case of a spot welding transformer, the voltage will be as low as
1v - 2v @ 500 amps. The resistance of the external load must be less than
1/500th ohm. In other words the load must be a "DEAD SHORT" to allow
the current to flow. If the load is 1 ohm, only one or two amps will
flow. If the load is 1/10th ohm, only 10 amps will flow. Now you can see why the arms
of a spot-welder are short and thick and the metal to be spot welded is placed
between electrodes at the ends of the two arms.
Why the spot welder?
We described the spot welder to emphasize the fact that transformers are capable of delivering a high
output current.
Now we come to the example of a 1 amp transformer delivering current to a power
supply.
All the theory you have learnt to date has given the impression that the transformer is
constantly supplying 1 amp to a power supply circuit.
But this is not so. The transformer is delivering two short bursts of very
high
current during each cycle. It is working much harder than expected.
One of the concepts you must understand with a power supply is ENERGY FLOW.
If a power supply is delivering 1 amp @ 5v, the FLOW OF ENERGY is volts x amps
or 5 x 1 = 5 watts. This energy is a constant 5v @ 1amp and is called DC.
A power supply generally obtains its power from an alternating source (called
AC) such as
the "mains." The voltage (and current) of the mains is rising and
falling and thus it is only capable of providing PULSES OF ENERGY.
These pulses of energy are delivered to the input section of a power supply via
a diode or set of diodes. On the other side of the diode(s) is an electrolytic
called the STORAGE ELECTROLYTIC where the pulses of energy are stored and the
electrolytic develops a voltage across it called DC. This is basically how AC is
converted to DC.
Here is the NEW theory:
All the theory you have learnt to date relates to the first cycle charging the
electrolytic via a diode (or diodes).
But it's the continuing cycles that have never been covered.
The animation below shows "packets of energy" being continually
delivered to the load but the energy from the transformer is coming in
"bursts." This means the "bursts of energy" must have a high
current.
The "bursts of energy" from the transformer can only be delivered when
the voltage of from the transformer is higher than the voltage on the storage
electrolytic. As can be seen from the animation, this only occurs during the tip
of the waveform.
That's why:
(a) the input voltage must be higher than the voltage on the storage
electrolytic, and
(b) the circuit is delivered short bursts of high current.
The "bursts" of input current.
The important feature
to note is the high current delivered to the circuit during
the period called the CHARGING PERIOD. The electrolytic delivers current
(actually the term is ENERGY) to the load
ALL THE TIME and this includes when the transformer voltage is low.
The animation shows a simple half-wave rectifier. The circuit only accepts
energy during the "positive" half the input waveform (a full wave rectifier accepts both halves of the input
waveform).
CONCLUSION:
The input current (called the charging current) is very high for
a small portion of the cycle.
Rectifier diodes must be designed to withstand high currents for a short period
of time.
During each cycle the reservoir electrolytic is charging and discharging. In
general, it is only charging and discharging a few volts however this action
causes it to get warm. If the electrolytic is not large enough it will get very
hot.
One of the hidden factors when describing an electrolytic is "RIPPLE
FACTOR." Two electrolytics may be rated at 2200u/25v but one will be larger than
the other. The larger electro will accept a higher charge and discharge current.
The smaller electro will get hotter and may deliver a reduced life.
The simple secret is
to feel all the components after 30 minutes of operation. If you cannot hold
your finger on anything, it needs heatsinking or some other form of
attention.
See our discussion
on the Voltage Regulator
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