# Weird AC dip after center-tap transformator

Discussion in 'Electronics Homework Help' started by bennethos, Jan 8, 2013.

1. ### bennethos

15
0
Jan 6, 2013
Gents,

Would you be able to help me out in determining why I have this weird dip in my AC waveform.

I noticed that if I just put a 150ohm resistor just after rectification and leave everything else off (5v regulator circuit + 2200uF cap). I have a pure and beatiful sine wave +- 18v AC RMS.

Also the dip in the AC waveform starts progressively as I start the simulation and gets worse until it reaches the state of the attached picture.

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2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,491
2,833
Jan 21, 2010
It might pay to measure the current in the diode (or even the voltage across the diode, or the voltage across the filter capacitor) and compare that to the transformer voltage.

Measuring the current through the diode is the most direct way of illustrating why this is happening.

3. ### Harald KappModeratorModerator

11,446
2,628
Nov 17, 2011
I can imagine 2 reasons:

1) a simulation artefact:
Depending on the accuracy of the models used the math used by the simulator can under certain circumstances produce weird, even unrealistic results. Try putting a resistor (e.g. 1kOhm) across the transformer's output pins to allow a continuous current flow.

2) an artefact caused by your simulation setup:
Instead of using a center-tapped transformer you use two separate transformers. What may look like an equivalent circuit has one drawback: each transformer sees only DC current on the secondary. This may lead to saturation of the core and (again depending on the quality of the models used) may lead to artefacts.
If you use a single transformer with a split secondary instead (your schematic suggests the model is capable of that), the currents through each part of the secondary have inverse direction thus avoiding saturation of the core.

Harald

4. ### bennethos

15
0
Jan 6, 2013
Well, I replaced the transfo in multisim by an AC_SOURCE and the artifact is gone.

So it was indeed a simulation artifact.

thx guys