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I am working on my first power supply design for +12 V and -12 V output. The +12 V is working perfectly. The negative output at the rectifier is only -1.87 V.

I have checked the wiring, replaced some components, and ran simulations on a few different programs. I am wondering if adding another 4700 μF isolation capacitor could solve my problem. I have attached the schematic:

The centre tap of U1 is shown open here but in practice is grounded.

enter image description here

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    \$\begingroup\$ Your centre-tap should be connected to your 0 volts. If this is so then fix your diagram. \$\endgroup\$
    – Andy aka
    Apr 24, 2023 at 7:20
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    \$\begingroup\$ I'd be grateful if the close voters explained their reasons. This seems to me a valid design question AND the problem was a well known one which was advised by an answer. \$\endgroup\$
    – Russell McMahon
    Apr 24, 2023 at 10:06

2 Answers 2

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A common screwup when using the 78xx and 79xx is not noticing (or remembering) that their pinouts are different. Might that be your problem?

enter image description here

A second common mistake is when attaching these parts to heat sinks. Whereas the 78xx tab is conveniently ground and can be attached to any grounded piece of metal (like the chassis), the 79xx tab is connected to its lowest voltage, the negative input. This means you can't heatsink the 79xx to the same heatsink as the 78xx or to the chassis. The 79xx needs either an independent heat sink or a thin insulator between its tab and grounded metal.

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  • \$\begingroup\$ Thank you for this! I did not realize this! I appreciate your help with this. I will post update once corrected! \$\endgroup\$
    – Josher
    Apr 24, 2023 at 4:05
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    \$\begingroup\$ Replaced regulator and connected correctly! Everything works great now!! \$\endgroup\$
    – Josher
    Apr 24, 2023 at 4:40
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    \$\begingroup\$ I came here to comment on this :-). Too late. Which is good. \$\endgroup\$
    – Russell McMahon
    Apr 24, 2023 at 10:04
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In addition to what td127 said, the center tap of your transformer is shown as not connected. Without that connected your supplies will only get ground current through the opposite supply which will not only limit the available current but also any imbalance in the loading of the supplies will make one have more voltage than the other, you will not get good regulation.

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  • \$\begingroup\$ I apologize for that mix up. In my actual design, I have the CT connected to "common" for this. I appreciate your help with this!!! \$\endgroup\$
    – Josher
    Apr 24, 2023 at 4:06

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