Electronic – Different voltage sources in parallel

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.

If you connected the 1V @ 100mA power supply in SERIES with the 2V @ 200mA power supply, you would have a total of 3 volts. However the current is limited by the power supply with the least capacity, so you could expect only 100mA.

OTOH, connecting two power supplies of DIFFERENT voltages in parallel is an extremely bad idea. The two power supplies will fight with each other and may actually kill each other. NEVER RECOMMENDED.

Even connecting two power supplies of the SAME voltage together is not typically recommended because here in the Real World, they are never EXACTLY the same voltage. Ways around this include using diodes to isolate the power supplies from each other. Or using low-value resistors to provide a "buffer" between each supply and the load.