Electronic – Voltage of Different Voltage Sources in Parallel with Diodes

First some theory:

In general, a PC power supply isn't expected to operate in a redundant mode (i.e. with outputs tied together).

In industry parlance, this function is called OR-ing (not O-ring). If a power supply is designed with OR-ing in mind, there will be several additions to the circuitry:

1. Some means of isolation (diodes or MOSFETs)
2. Some means of maintaining regulation at absolute zero load (anti-rollback)
3. Some means of load balancing (forced or droop)

These factors allow you to connect identical voltage rails together to provide load current beyond what a single supply can do, and allow for the rail to stay up (if the load can be delivered by N-1 units) if a single unit goes down. It also gives you some measure of protection if you accidentally connect a higher voltage to a lower voltage.

Also, most PC power supply returns are all tied to each other. There typically isn't an isolated output (independent return) available.

Now, the practical ramifications of your experiments:

1. Connecting the +12V and +5V rails together is only "safe" if there is some means of OR-ing on the +5V that can withstand the +12 applied to it. The +5V will not be delivering any current to the load, as it will be blocked by the OR-ing device.

Most likely you've back-biased the +5V and put 12V on some electrolytic capacitors that are probably only rated at 10V.

1. Connecting the +12V and +5V in series is only "safe" if one of those rails has a return independent of the other. If the returns are common, all you're doing is short-circuiting the rail that's on the "bottom" (the rail that has a return connected to the high side).

Batteries != power supplies. Energy going into a battery charges it. Energy going into the output of a power supply usually smokes it.

Putting the two dissimilar panes in parallel is not a good idea. Mostly, the higher voltage panel will deliver most of the current and the low voltage panel does little.

If the two panels have similar current ratings but different voltage rating, you can put them in series to get the sum of the voltages at the single current rating. Then use a switching power supply to buck the higher voltage down to what you want. For maximum voltages under 30 V, it's actually not hard to make your own buck converter. There are many off the shelf chips that you only need to add a inductor, diode, and a few other parts too. You can even make your own with a microcontroller if you want to learn more about buck converters.