Hi, if I connect two or more power ATX power supplies in series I have to make sure only one of them has the secondary grounded, right?
I intend to connect 3 in series.

 

Yes, only one can be grounded.
Preferably the one used closest to the -ve side of the total supply.
Max.

 

Hi, if I connect two or more power ATX power supplies in series I have to make sure only one of them has the secondary grounded, right?
I intend to connect 3 in series.

Yes, no problem as long as you have a death wish.

 

Somebody is going to find out about ATX PSU's the hard way.
If you don't understand the danger involved, don't do it.

 

But what's the difference between isolating the pcb from the case or simply unearthing the chassis? Isn't the earth connection just a path for the electricity to flow in case a live wire burns out and touches the case?

 

Hmmm. !
Can't you see those blue capacitors grounding the negative sides ?
Can't you see those blue capacitors connecting the primary and secondary grounds ?
Try removing those and see what happens to PSU when you power it up.

 

But what's the difference between isolating the pcb from the case or simply unearthing the chassis? Isn't the earth connection just a path for the electricity to flow in case a live wire burns out and touches the case?

You can remove the Earth link to the Negative output, and leave the case connected to earth easily.

 

Now I'm even more confused with the mixed replies.

 

The input AC side is generally totally isolated from the DC output side, often there is just a short foil trace that goes from the DC neg to case earth.
If this is removed the DC side is isolated from earth GND. and the supply will continue to operate.
At least this has how it has been done on the older style (pre ATX) version.
I think this is the link @Dodgydave is referring to.
Max.

 

Still...only one PSU should be earthed.
Earth more than one and you are in for trouble

 

These are very old ATX power supplies that still have -5V and -12V.
What would happen if I simply remove the earth wire from the case?

 

Can anyone tell me what this thing is? Looks like a ferrite transformer but there are only two wires.

 

I expect it is an iron cored inductor and it is used to correct the power factor of the PSU input.

 

This one does say Passive PFC on the case, thank you.

 

This psu with pfc won't turn on, shouldn't the purple 5Vsb always put out 5V even when off? The green wire won't turn the psu on and I can measure 0.2~0.25V on it.

 

If it has no standby power then it won't turn on. The standby supply is a separate small switch mode supply which is always on when mains power is connected. Clearly that standby supply is defunct.

 

What I wanted to hear, one less piece of trash in my bedroom.
Now, I unearthed one of the psu's and tied them in series and got 24V and I don't get shocked when touching the case. The unearthed supply was the closest to the "negative".
Would I need to unearth the second psu if I wanted to use the -12V to create 48V?

 

Actually, when I measured the 24V across the tied supplies the unearthed was the closest to the "positive".

 

I don't get shocked when touching the case

Long may it continue but what happens when a fault occurs?

 

The service power earth ground is still connected to the case, it is just the low voltage side that is isolated.
Max.