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I am trying to obtain the hysteresis curve of a iron core transformer.

To integrate the voltage output from the secondary coil, I used an LM741 opamp, 10k ohm resistor, 0.1uF capacitor, and 1M resistor. This is my setup. Later, I actually added a capacitive voltage divider on another breadboard in addition to what is shown (this is mentioned later in the following text.)

0 Initially, I got a largely saturated and almost distorted curve with 1V as the AC power supply. I then realized that 1V is too large.

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The transformer's primary coil is 600 turns 2A, the secondary has 300 turns 4A. The input voltage into the opamp integrator is 0.5V.

After adding a capacitive voltage divider (consists of 0.1uF and 10uF capacitors with 0.5V as the AC power input,) I lowered the voltage to about 0.005 V (as the starting point.)

This picture below is obtained when the AC input is 4V (2V after going through the transformer, and around 0.02V into the opamp integrator, after it passing through the voltage divider.)

This looks much better. However, I observed that it is more saturated in the forward direction than in the positive direction.

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I mean, the loop should theoretically look like this somehow: ideally

The saturation in the opposite direction starts to appear at AC input = 8V (4V after going through the transformer.) This is how the loop looks like when AC voltage = 9V.

What would the problem could be? Are there any theories or effects that I can further read about?

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UPDATE After lowering the capacitance of the second capacitor in the voltage divider, I got a much more symmetric curve. However, a dip appears. Is it because of the quality of my LM741 opamp?

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Juye C
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    I really appreciate the work you've put into this. To be honest, I've not done this kind of experiment and I'm very impressed that you are doing it. Very impressed. I notice on the web this [Chegg site that reflects an Arizona University lab discussion](https://www.chegg.com/homework-help/questions-and-answers/discussion-transformer-core-characteristics-observed-applying-ac-excitation-observing-sign-q31532047) that discusses these measurements. I've not attempted them and I've not reviewed the details on the site. But it seems useful to me, first glance. In any case, +1 to your question!! – jonk Jul 20 '21 at 10:10
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    What AC frequency are you using? Why are you measuring primary voltage rather than current (see @jonk's link)? The asymmetry could be explained by any remnant magnetism in your core, or by exposure to an external field. – Dave Tweed Jul 20 '21 at 10:37
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    @DaveTweed I am using 50Hz AC. I directly connected the primary voltage to the primary coil of the transformer, without adding any resistor in between to get the current, because I thought there would be no difference in terms of measuring current or voltage. Would it make a difference though? – Juye C Jul 20 '21 at 11:57
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    Well, yes, of course! An unloaded transformer is an inductor, which means that the voltage and current are 90 degrees out of phase. – Dave Tweed Jul 20 '21 at 12:00
  • On what technical theory is this experiment based? Not measuring current is a major flaw. – Andy aka Jul 20 '21 at 18:14
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    The link posted by @jonk is rather clear about how it should be set up. – Dave Tweed Jul 20 '21 at 18:49
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    @Andyaka Thank you. I managed to measure the current with an ammeter after updating my setup! I connected the meter with the resistor and the input in series. – Juye C Jul 20 '21 at 20:29
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    @DaveTweed Thank you very much. Your hint was very helpful. – Juye C Jul 20 '21 at 20:30

1 Answers1

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I believe the issue is how you are connecting your power supply to your op-amp. Most of the DC bench power supplies Ive worked with do not supply V_supply on the + terminal relative to ground and -1 * V_supply on the negative terminal relative to ground. Instead the voltage displayed on the supply is the voltage across the + and - terminals.

If the + and - terminals on your power supply are floating relative to ground (this is typical when a supply has +, -, and ground but I would read the manual for your supply rather than make assumptions) you can get positive and negative supply rails using two supplies, as described in this guide by BK Precision https://bkprecision.force.com/desk/s/article/how-do-i-get-a-positive-and-negative-voltage-output-out-of-my-dc-power-supply

Luke Sackash
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  • Thank you. Could you please explain a bit what do you mean by "floating"? – Juye C Sep 24 '21 at 04:13
  • Effectively, there is an infinite resistance between them and ground. One way to think about it might be to imagine a battery being held in the air. There is 1.5 volts between the positive and negative terminals but they can be at any voltage with respect to ground since there is no electrical connection from them to the ground. In this case I'm referring to ground as in the electrical potential of the literal ground. of course, for the voltage supply, this only works for it's rated range but for your experiment your probably staying well within that. – Luke Sackash Nov 10 '21 at 19:21