AC-DC Powersupply voltage drop
You just need to add a discharge resistor to the power supply output to provide a controlled discharge path for the resevoir capacitors. In most cases the load itself will discharge the power supply for you when you disconnect the mains power.

Figure 1. RC discharge curve. Image source: School Physics.
The graph shows that the discharge time is a function of RLOAD × CRESEVOIR and this parameter is called the time-constant and uses the Greek symbol τ, &quot;tau&quot;.
Some numbers worth remembering:

At τ the capacitor will have discharged by 63%.
At 3τ the capacitor will have discharged by 95%.
At 5τ the capacitor will have discharged by 99%.

If you do add a discharge resistor then make sure that it doesn't increase the load on the PSU too much and that it can dissipate the continuous power safely.