Yousuf said:
I recently had to replace my 600W PS because it was causing my optical
drives to fail: I had a couple of DVD burners which both failed at the
same time, I then bought a BR burner, which was showing signs of failing
just like the other two. It was a Zalman 600W, which on-the-face-of-it
is a pretty good brandname. After doing the calculations on the online
PSU calculator, I found it was slightly underpowered, not by much but
still underpowered. So when I went to look for a replacement for it, I
found that the modern PSU's have changed quite a bit since my old one
(bought only in 2008). Back in the old days, PSU's were advertised with
multi-rail 12V outputs. That meant that there were multiple separate 12V
rails, and some went into the motherboard, others went into the video
cards, while others went to everything else. So some rails might be
underpowered while others are just fine. These days all of the better
PSU's seem to be advertising single-rail 12V. The difference is that the
single rail now runs with a higher current than each of the separate
rails of the old PSUs. I think the old PSU's attempted to keep the
individual rail voltages below 19A, but now you can regularly see one
12V rail running over 50A. One rail has less chance of being
underpowered for specific types of connections.
Yousuf Khan
This is true in some ways, but not others.
A modern supply can be rated for 12V @ 50A. This would be the rating
at the transformer or rectifiers.
But when that ampacity gets to the wiring harness, it might be
dangerous to have that much available. Current limiters
may still be in place.
If you grabbed an arbitrary yellow (12V) wire on the 50A PSU, you'd hope
the design would not allow 49A to be continuously drawn through the
yellow wire. The wire would probably heat to the point of melting and
burning the plastic on the wire.
Instead, you'd hope there is a current limiter, a circuit that monitors
current flow, and limits the current to a value safe for a single yellow
wire (shuts off the supply, if more than that value is drawn).
On an old supply, you might have seen three "12V @ 18A" outputs. They
could have been implemented as a single transformer with three current
limiters. When you see a power supply with a single rail "12V @ 50A",
there may still be current limiters involved, which means the label
on the new supply is hiding a detail. The limit may not be 18A, it
could be a higher number, but there should be a limiter there to
prevent the whole 50A going through one wire. It really all depends on
how much current you can push through a cheap 22 gauge wire.
*True* independent outputs, require separate transformers. When a
few supplies were made like that (there weren't that many of them),
the reviewers took pictures, and you could see completely independent
"islands" on the PCB of the power supply. Such supplies were 1" to 1.5"
longer than regular supplies. That was a hint you were getting the
real deal. When a supply claimed to have independent rails, and yet
managed to do it in a regular sized box, then you knew they were
cheating (because there wouldn't be room for that many transformers).
When it comes to power supplies, deception is all part of the marketing.
This kind of thing, raises the same question about some older supplies.
They made power supplies, where the label said the 5V rail could provide
40 amps. Now, what if one wire on a Molex cable, decided to draw that
40 amps. What does molten plastic smell like, anyway ?
Too high a
current rating, isn't always such a good idea, if the harness can't
take it. And back in those days, I doubt they would have had current
limiters or monitors in the 5V section.
Paul
