Why the waveform of a transformer is distorted when drawing current? - Page 1
Forum section: electronics / projects

author: Sainteus
author_rank: Contributor
timestamp: May 02, 2017, 09:04:57 am
content: We are doing a project on a dual-channel power supply for current measurement. But we are curious about some phenomenon when the system is drawing current which is the input of transformer AC voltage will be distorted and when no current draw the waveform is perfect.Hoping anyone can guide us on this matter.CH1(Yellow) - AC Input of RectifierCH2(Cyan) - Output of RectifierCH3(Magenta) - Output of Linear Rectifier

author: Benta
author_rank: Super Contributor
timestamp: May 02, 2017, 09:32:24 am
content: It's quite easy. The distortion happens when the rectifier diodes turn on and start charging the filter capacitors. This only happens at the top of the sine.The slope at the top is the caps charging, leading to increased voltage and reduced current draw.You'll see this waveform in all 50/60 Hz transformer/bridge/cap power supplies.

author: Zero999
author_rank: Super Contributor
timestamp: May 02, 2017, 01:22:03 pm
content: Yes, it's because the current waveform is not sinusoidal. The current is only drawn on the peak of the waveform, when the diodes in the rectifier turn on.

author: Seekonk
author_rank: Super Contributor
timestamp: May 02, 2017, 02:34:20 pm
content: Transformer resistance is dropping the voltage with increased current. If you had an oversized transformer this would be less obvious. Then you may need surge limiting to protect diodes and cap. So not all bad.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2017, 04:39:19 pm
content: What the heck is that cyan waveform?

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 02, 2017, 04:41:29 pm
content: Quote from: T3sl4co1l on May 02, 2017, 04:39:19 pmIt's current from V1, which is a voltage source configured for AC output and series resistance of 1 ohm.

author: Zero999
author_rank: Super Contributor
timestamp: May 02, 2017, 04:49:57 pm
content: Quote from: MagicSmoker on May 02, 2017, 04:41:29 pmYes, that's true for my simulation but I thought he was talking about the original post, which appears to be the ripple on the DC side.

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 02, 2017, 05:09:21 pm
content: Quote from: Hero999 on May 02, 2017, 04:49:57 pmAh, that makes more sense...My guess is the cyan trace is the AC ripple on the output, even though it is a bit more sinusoidal than is usually the case with a capacitor input filter. Perhaps the transformer has really high leakage inductance? That would contribute to more severe clipping of the AC voltage waveform, too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2017, 05:50:57 pm
content: Quote from: MagicSmoker on May 02, 2017, 05:09:21 pmYeah, that makes sense, the LL is rounding it off pretty hard.Guessing OP's transformer is a split bobbin type -- they're pretty bad like thisTim

author: Sainteus
author_rank: Contributor
timestamp: May 03, 2017, 11:05:39 am
content: Quote from: Hero999 on May 02, 2017, 01:22:03 pmThank you very much for your answer, it really helping us to understand. May I know which software you are using? Is it LTSpice?

author: Zero999
author_rank: Super Contributor
timestamp: May 03, 2017, 12:31:25 pm
content: Quote from: Sainteus on May 03, 2017, 11:05:39 amYes, I used LTSpice. If you download it, you can simulate the .asc file I posted above, for yourself.

author: Sainteus
author_rank: Contributor
timestamp: May 03, 2017, 12:36:12 pm
content: Quote from: T3sl4co1l on May 02, 2017, 04:39:19 pmVery sorry for my poor explanation, the cyan waveform is actually the output of our power supply after linear regulator.

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 03, 2017, 12:39:56 pm
content: Quote from: Sainteus on May 03, 2017, 12:36:12 pmThen your regulator is not working, or set to an output voltage higher than the input, because there should be almost no 100/120Hz ripple present.

author: Sainteus
author_rank: Contributor
timestamp: May 03, 2017, 12:45:35 pm
content: Quote from: Hero999 on May 03, 2017, 12:31:25 pmThank you very much. I would love to try it.

author: Sainteus
author_rank: Contributor
timestamp: May 03, 2017, 12:56:49 pm
content: Quote from: MagicSmoker on May 03, 2017, 12:39:56 pmThank you very much for your explanation. But I still not getting what you trying to say, can your further explain?

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 03, 2017, 01:37:39 pm
content: Quote from: Sainteus on May 03, 2017, 12:56:49 pmThe cyan waveform shows relatively high ripple - 2.8V peak-to-peak - while the mean voltage is not much lower than the peak of the AC input...Wait, I just looked at the screenshot again and it seems what you call blue is what we are calling cyan, and what you are calling cyan is what we would call magenta.So if magenta is the output of the regulator then it looks fine (assuming it is set to 15V, that is). The cyan trace also looks fine, though the ripple is a bit high and still very sinusoidal, suggesting there isn't much filter capacitance present.

author: Zero999
author_rank: Super Contributor
timestamp: May 03, 2017, 05:51:24 pm
content: He's updated the original post.Quote from: Sainteus on May 02, 2017, 09:04:57 amYes, that is correct. Going from what you've got there, the regulator is working properly.