I was trying to simulate a 3 phase AC full wave rectifier, and wanted a 5V output.

I connected an LM2596 module to turn the rectified voltage into the desired 5V output and ran the simulation in LTSpice.

To my disappointment, I found that the voltage was not only higher than the expected output (it was about 6.7V at first), but was also constantly decaying (in 210 ms it went from 6.7 to 6 V.)

The rectified voltage was almost totally constant but the output voltage (that is, the voltage produced after being passed through the regulator module) was not, this suggests that there maybe some problem with the module.

Is there anything wrong that I'm doing that you can point out to fix this problem?

At the same time, suggestions are also welcome for building a 3 phase AC full wave rectifier, with 12 V input and 5V regulated output, without this module in a better way as well.

P.S. - Also can someone clear up why my rectified voltage (11.4V) is slightly lower than the input voltage?

  • 1
    \$\begingroup\$ You need to post a schematic of your circuits with component values for people to speculate on what is going wrong. \$\endgroup\$
    – K H
    Jan 28 '21 at 8:31
  • 3
    \$\begingroup\$ The rectifier diodes drop around 0.6V. That might explain the difference. The LM2596 might not appreciate having no load on the output and thus can't regulate. \$\endgroup\$
    – Kartman
    Jan 28 '21 at 8:41
  • \$\begingroup\$ @KH Yes I did that (took some time) \$\endgroup\$
    – Areen
    Jan 28 '21 at 8:42
  • \$\begingroup\$ @KH done, thanks for the help \$\endgroup\$
    – Areen
    Jan 28 '21 at 8:54
  • \$\begingroup\$ Hmm you appear to have connected everything correctly based on the reference circuit, which is bad because it means an amature like me can't help you. \$\endgroup\$
    – K H
    Jan 28 '21 at 9:06

Just think about how you have shorted out the diodes in the rectifier stage: -

enter image description here

Consider V1 (red box) for one minute. On one half cycle it will produce a voltage polarity as shown. This will drive current into the ground connection and that current will re-emerge into the main part of the circuit just below U1. Just follow the purple arrows round and you will see that basically V1 voltage is shorted on each half cycle. Ditto V2 and V3.

The problem is that your star point is earthed. Remove it and re-check. Or remove D1, D3 and D5 and live with it being a half wave 3 phase rectifier.

I'm not saying this is the only problem but it's the big glaring problem.

  • \$\begingroup\$ Yes I am aware of that and have actually heard that while building 3 phase connections you shouldn't short the star point. But the problem that I encountered with no ground on the star point was that I was not getting an exact sine wave voltage output from the voltage source, it was shaped like a sine curve but actually had two successive peaks instead of one. Can you explain why this happens? ![](i.imgur.com/hXnVbNr.png) \$\endgroup\$
    – Areen
    Jan 28 '21 at 10:00
  • \$\begingroup\$ If you need to display it, then give the star-point a node name and display the voltage with respect to that node name. \$\endgroup\$
    – Andy aka
    Jan 28 '21 at 10:04
  • \$\begingroup\$ I managed to do that and it worked! Its perfectly sine in nature! But after removing the ground the rectified voltage is 19.6 V instead of 11.4V, any help on this as well? \$\endgroup\$
    – Areen
    Jan 28 '21 at 10:09
  • \$\begingroup\$ The peak DC voltage that you will get is due to line voltages and, for 3 phase with a 12 volt peak per phase it is \$\sqrt3\$ higher hence a 12 volt peak becomes 20.78 volts line peak voltage then, a couple of diode drops later and it'll be about 19.5 volts DC after rectification. \$\endgroup\$
    – Andy aka
    Jan 28 '21 at 10:12
  • \$\begingroup\$ Thank you so much for the help, this solved everything! \$\endgroup\$
    – Areen
    Jan 28 '21 at 10:13

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