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