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I have simulated a circuit which gives +12V and -12VDC output. I am planning to obtain input signals from a transformer. I have some questions related to it, if you share your ideas i will be very happy. I used LM7812, LM7912 regulators and S1M diodes.

  • There is a delay at the beginning of the output2 signal (-12V). What can be the reason of it? Is simulation the reason of this delay or will it still there when the cirucit is set in real life? How can this be prevented? I intend to use this negative supply for SN75188 VCC- pin. Can this delay affect IC's operating?

  • If a special IC like MAX764 is used, does this delay still happen?

This is my circuit;

enter image description here

Output Voltage Signals:

enter image description here

Output Current Signals (Reversed because of LTSpice's display of current, both signals should be inverted):

enter image description here

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  • \$\begingroup\$ Woaw, I first thought that you had uploaded two black images. But no, it's dark blue on black background. We're not owl's, we're old people with old eyes. \$\endgroup\$ Commented Jul 20, 2017 at 11:16
  • \$\begingroup\$ No, you could easily realize:) \$\endgroup\$
    – layout789
    Commented Jul 20, 2017 at 11:29

2 Answers 2

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You are driving your simulated bridge with two sources ​that are in-phase. Presumably, these are supposed to represent the output of a center-tapped transformer. This is incorrect — the sources should be out of phase. Either reverse the connections to the lower source, or change the sign of its amplitude parameter.

With them in-phase, they both reach their positive peak during the first half-cycle, and their negative peak during the second half-cycle, which explains your results. This is why you should always graph both the inputs as well as the outputs in a simulation — this issue would have been immediately obvious.

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  • \$\begingroup\$ Actually i will obtain the AC signal from a transformer like this circuit in real life. Do you think the delay also will happen when i establish the real circuit, because i have not a chance to change the lower source while i was taken from transformer. imgim.com/12-volt-dual-power-supply-550x299_555154286.jpg \$\endgroup\$
    – layout789
    Commented Jul 20, 2017 at 12:37
  • \$\begingroup\$ No, that's my whole point -- a real center-tapped transformer WILL give you out-of-phase voltages when the center tap is grounded! If you don't believe me, you should try adding an actual transformer to your simulation. \$\endgroup\$
    – Dave Tweed
    Commented Jul 20, 2017 at 12:46
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There is nothing in your circuit that suggests the -12V signal should lag the +12V when powering up.

The problem is (as pointed out by Dave Tweed) that your sine sources are in phase which would not be the case in real life.

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  • \$\begingroup\$ This is incorrect. There is a reason for the delay, and the simulator is not wrong. \$\endgroup\$
    – Dave Tweed
    Commented Jul 20, 2017 at 11:17
  • \$\begingroup\$ @DaveTweed. Good catch - will modify my answer. \$\endgroup\$
    – Glenn W9IQ
    Commented Jul 20, 2017 at 11:21

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