# Transformer puzzle

Discussion in 'General Electronics Chat' started by The Electrician, Jan 20, 2010.

1. ### The Electrician Thread Starter AAC Fanatic!

Oct 9, 2007
2,301
339
Here's an interesting transformer related puzzle.

If you *know* the answer as a result of wiring it up and making the
measurements, please wait a couple of days before posting the answers (any or all of them).

If you simulate it, go ahead and tell us what simulator you used, what
magnetic model was used, and the results.

If you just think about it, tell us what you think, and why.

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I have a small filament-type transformer rated for 120 VAC @ 60Hz input, 28
volts @ 3 amps output (I measured the turns ratio as 4:1). This transformer has a laminated iron core. I have a 6 ohm resistive load and I have two thermocouple (correctly responding to AC+DC) ammeters to measure primary and secondary current.

I have a variac to reduce the applied primary voltage to no more than about 45 VAC for reasons that will become apparent.

Everybody has seen such a transformer used in a half-wave rectifier circuit by putting a diode in series with the output winding and the resistive load (I won't be using any reservoir capacitor in this puzzle).

Determine the dotted ends of each winding (in the attached schematics, the plus sign stands in for a dot). For those who don't know what this means, let me explain (also see http://en.wikipedia.org/wiki/Dot_convention). Using an isolation transformer, apply 120 VAC to the primary of the unloaded transformer, and with an oscilloscope look at the input voltage on one trace (be safe) and the output voltage with another trace. One end of each winding will be connected to a ground clip of the scope probe, the other to the probe tip. If the waveforms appear out of phase on the scope, reverse the connections of the output winding and its probe (if they appear *in* phase, don't reverse the connections). Now the leads connected to the probe tips are the dotted ends of the windings, primary and secondary. In other words, when the voltage is positive going on the dotted end of the primary, it's also positive going on the dotted end of the secondary. Mark them.
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Circuit 1: Connect the primary to the variac (preset to zero output), but with a rectifier diode of at least 200 volt and 1 amp rating in series with the primary, and with the diode's arrow pointing toward the dotted end of the primary winding. Put a 6 ohm load on the secondary, and turn up the variac until you get half an amp in the primary (true RMS AC+DC); the applied voltage is about 35 VAC to give .5 amp.

What will be the voltage waveshape applied to the primary (after the diode)? What will be the primary current wave shape? What will be the secondary waveshapes? Will there be any secondary current? IF so, what will be the magnitude of the current (true RMS AC+DC) in the 6 ohm resistor?
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Circuit 2: Leave the variac setting as it was in Circuit 1. With the same diode left in the primary circuit, now put a diode (3 amp rating) in series with the secondary winding and its 6 ohm load with the arrow of the diode pointing away from the dotted end of the secondary. In this topology, it would seem that when the primary diode conducts, the secondary diode also conducts, and there would be a current in the secondary load resistor.

What will be the voltage waveshape applied to the primary (after the diode)? What will be the primary current wave shape? What will be the secondary waveshapes? What will be the magnitude of the current (true RMS AC+DC) in the 6 ohm resistor?
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Circuit 3; Leave the variac setting as it was in Circuit 1. With the same diode left in the primary circuit reverse the diode in the secondary ciruit so that the arrow is pointing toward the dotted end of the secondary winding. This is the interesting case.

What will be the voltage waveshape applied to the primary (after the diode)? What will be the primary current wave shape? What will be the secondary waveshapes? What will be the magnitude of the current (true RMS AC+DC) in the 6 ohm resistor, if there is a current?

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2. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
If you have to ask these questions, obviously you have not ventured into the world of SMPS.

Your transformer with a series diode in the primary will block AC current for the negative cycle so will basically becomes a SMPS design, albeit with only 50Hz as the switching frequency and thus will shown characteristic and current waveform of such nature.

Depending on the polarity of the secondary diode, it will then operate in either forward or flyback mode.