Electrical – How to design CMOS bridge rectifier

Either can work correctly if designed properly. If you have a dumb rectifier supply feeding a 7805, then all the rectifier part needs to do is guarantee the minimum input voltage to the 7805 is met.

The problem is that such a power supply only charges up the input cap at the line cycle peaks, then the 7805 will drain it between the peaks. This means the cap needs to be big enough to still supply the minimum 7805 input voltage at the worst case current drain for the maximum time between the peaks.

The advantage of a full wave rectifier is that both the positive and negative peaks are used. This means the cap is charged up twice as often. Since the maximum time since the last peak is less, the cap can be less to support the same maximum current draw. The downside of a full wave rectifier is that it takes 4 diodes instead of 1, and one more diode drop of voltage is lost. Diodes are cheap and small, so most of the time a full wave rectifier makes more sense. Another way to make a full wave rectifier is with a center tapped transformer secondary. The center is connected to ground and there is one diode from each end to the raw positive supply. This full wave rectifies with only one diode drop in the path, but requires a heavier and more expensive transformer.

A advantage of a half wave rectifier is that one side of the AC input can be directly connected to the same ground as the DC output. That doesn't matter when the AC input is a transformer secondary, but it can be a issue if the AC is already ground-referenced.

The Raspberry Pi is generally powered from a 5V wall-wart type of DC power supply. The secondary is galvanically isolated from the mains voltage for reasons of personal safety (a fault will not expose the end user to the mains voltage).

The DC return of your bridge rectifier circuit is most certainly not isolated from the mains. The Raspberry Pi ground is 'floating' with respect to the bridge rectifier ground - there is no galvanic connection between them, hence your voltage measurement.

If you were to connect the DC return of the bridge rectifier circuit to the Raspberry Pi ground, you bypass the galvanic isolation that the DC power supply gives you. This means your Raspberry PI is now mains-referenced, and any fault could potentially expose you you to lethal voltages. I wouldn't do this.

A further complication comes if you also hook up an earth-referenced return to the Raspberry Pi, like a connection to a PC, with the mains-reference return connected. When you mix a mains-referenced return like your rectifier circuit with earth, things are going to explode (you essentially short out your bridge through the earthed return, which is often a flimsy wire that gets really hot and melts/catches fire while blowing up everything connected to it). Another reason not to do this.

You would be much better off with a small line frequency transformer to (1) step down the mains voltage to a lower level ahead of your resistive attenuator, and (2) provide galvanic isolation from the mains. Put your bridge and attenuator in the secondary of the transformer. With this, you can safely connect the low voltage isolated rectifier return to the Raspberry Pi return.

(You also must include a fuse in the line to isolate the rectifier circuit from the mains if there is a severe fault like a transformer fault or a short circuit.)