Electronic – Smoothing a Full-Wave rectifier voltage

I'm going to solve this by placing a 30V zener diode across the main filter capacitor, between Vgnd and Vout, to keep it at 30V maximum. In case more current is drawn from the auxiliary negative supply than from the main positive supply and the voltage starts to rise as described in the question, the zener diode will kick in and compensate by equalizing the difference in currents. I have just tested it on a breadboard and it seems to work well.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. (a) Dual supply from centre-tapped transformer. (b) Dual supply from single secondary transformer. (c) Re-using your bridge rectifier to create the (b) circuit.

If you don't have a centre-tapped transformer you could modify your circuit as shown in Figure 1b. Note that C3 will now only get topped-up every positive half-cycle and C4 every negative half-cycle. You may need to almost double the C values to hold enough charge between cycles.

Since you already have a bridge rectifier you can use it to form D1 and D2. Draw it out using diodes instead of the bridge and you should see that you can double-up the diodes by addding the grey wire.