Electrical – Using Switching Regulators to Make Split Supply

1. The load currents do not need to be balanced. If they're not, it just means that there will be increased current in the center tap (ground return).

2. The transformer needs to be symmetrical. Both halves of the transformer supply power to both regulators, on alternate half-cycles. However, it is not necessary for the regulators to have the same DC output voltages. They can indeed be different, but just be aware that the one with the lower output voltage will be dissipating more power for any given load current, since the input voltage is the same going into both regulators.

If you take the input for the 7805 from the 7809 output, you must include the current drawn from the 7805 into account when determining the power dissipation in the 7809.

You should not use a voltage divider to make 3.3 volts - the voltage on your 3.3 volt line will vary depending on the current drawn from the 3.3 volt line, as that current will have to flow through the "top" resistor of the divider, increasing the voltage drop across it. Look for a dedicated 3.3 volt regulator, or use an LM317 or similar adjustable regulator.

Depending on the currents you need from each supply, it may be best to use a switching regulator to get from the 15 volt supply to 5 volts.

Make sure you have adequate heatsinks on the linear regulators!!