You are assuming a power supply controller will see all
failures and then shutdown accordingly. What happens when
controller's feedback fails? A feedback circuit that will be
discussed again below. Power supply controller outputs more
power trying to raise a voltage that never rises. IOW power
supply has output excessive and destructive voltages. Just
another reason why "separate and distinct" overvoltage
protection has been required 30 years ago as it still is
required today.

Neither you nor I care how a particular power supply meets
this 'well proven to be necessary' OVP function. We don't
even care if the function uses a simple and easily constructed
crowbar or uses something different. The point remains a power
supply must provide that defined function. Schematic from
electro-tech.narod.ru quite obviously violates the industry
requirement. It has no overvoltage protection. It
demonstrates how power supplies are sold for well under $60
retail. They forget to include essential functions such as
OVP. That supply from http://electro-tech.narod.ru is
designed to be dumped into a market driven by 'bean counter'
engineering.

Provided were numeric specs for a supply that does provide
the OVP circuit. Back to the original point. Properly
designed supply does not and can not sell for $25 retail. So
how do others sell power supplies for $25 retail? They forget
to include required and necessary functions such as
Overvoltage Protection.

Is this done by a crowbar circuit or by some other means?
Neither you nor I care. Industry standards demand that OVP
function exist for 'long proven' necessary reasons. Some
supplies do provide such functions. But these minimally
acceptable supplies cannot sell for $25 retail - again
repeating the bottom line point.

In the meantime, that DTK PSU from www.pavouk.comp.cz
violates another essential function. The power supply must
provide galvanic isolation of at least 1000 volts. Therefore
the "Feedback" circuit must contain an optocoupler or
something equivalent. The DTK PSU has no such isolation. It
violates another industry standard. And so another essential
function would be 'forgotten'.

Earlier a power supply controller would output overvoltage
because the feedback circuit failed. Controller never knew it
was outputting excessive voltage. What feedback components?
Same optocoupler that is required with galvanic isolation.
Just another missing specification to sell at $25. Again the
point. Power supplies missing essential functions and
routinely dumped into the market because so many computer
assemblers don't even have basic electrical knowledge; never
learned about galvanic isolation, overvoltage protection,
feedback current limiting, or overpower protection.

When a computer assembler looks only at power and price,
then a game of specmanship is afoot. For example, a Dell or
HP power supply may claim only 250 watts. The 'bean counting'
computer assembler then claims that is woefully too small.
And yet if the same power supply was being marketed by others
to computer assemblers, then suddenly the same supply is rated
at 375 watts. Why? They don't list the output power. Rated
is maximum power that a power supply might consume. The 'bean
counting' computer assembler then declares the HP and Dell
supplies are undersized.

It gets even more interesting. A power supply must be
completely shorted out and still must not be damaged. And yet
here are power supplies, designed for a 'bean counter' market,
that self destruct even before reaching 100% load:
http://www6.tomshardware.com/howto/0...021/index.html

First thing to look for in any power supply: if the supply
manufacturer does not provide a long list of numerical specs,
then bet it is a scam. More responsible power supply
manufacturers provide numerous numerical specs. One need not
even know what those specs mean. Just having written numerical
specs is a first requirement. Manufacturer commits; says to
the 1% who know technology that this supply does provide these
functions. However a manufacturer who is dumping inferior
supplies into a market of computer assemblers must disempower
the 1%. He must provide no written specifications. Then a
knowledgeable 1% cannot warn the other 99% of a defective
product.

Welcome to a world where so many power supplies don't
provide specs and are then recommended based only on the price
and watts.

#1 requirement for a power supply: it must provide a long
list of numeric specs. If that power supply does not
specifically state overvoltage protection, then another
essential function is missing.

We care less how he provides overvoltage protection. We
care more that he claims to provide OVP. That missing
function demonstrates why so many supplies sell for only $25
and $40 full retail.

Franc Zabkar wrote:
On Mon, 11 Apr 2005 19:35:21 -0400, w_tom put
finger to keyboard and composed:
...

I understand that separating the two functions (OVS and OVP) gives
added security, but I can't see the need for a brute-force approach to
OVP. By this I mean that it is not necessary to clamp the output with
an expensive high-current zener or SCR - one can much more elegantly
achieve the same end, and still comply with Intel's spec, by turning
off the PWM controller, or by turning off its drive transistors. See
this DTK PSU which implements OVS, OVP and OPP with about $2 worth of
parts:

http://www.pavouk.comp.cz/hw/en_atxps.html

I have a 1000W minicomputer SMPS whose control module senses the
output and shuts off the oscillator in the event of an OV fault. There
are no expensive OVP parts, and this is in an SMPS that has 4 or 5
screw terminal capacitors the size of soft drink cans, stud mounted
diodes on massive heatsinks, four TO3 chopper transistors, and an AC
fan. The +5V cables (+5V @ 150A) are thick enough to start my car.
...

Some TV sets use 130V protection zeners (eg R2M, R2KY, $1.85 retail)
on their 100-115V supply rails. These designs have no OVS. Ironically
it appears that the designers have chosen this sledgehammer approach
because it costs *less* than OVS. I'm not really comfortable with such
a design because its success depends on the failure mode of the
protection device. IME the zener always fails SC, and therefore
protects the TV, but if it fails OC (unlikely, but possible), then
there is no protection at all. Killing the oscillator would be much
safer. In fact, HV protection and beam current limiting is usually
implemented by shutting down the horizontal oscillator.

I guess a comparable analogy may be MOVs in "surge protected" power
boards. They may sacrificially absorb the first surge, but thereafter
they are useless.
...

Sorry, I don't see it. You state that the original IBM supplies had an
OVP circuit, but you don't elaborate. How exactly did they do this? I
have the original IBM AT Tech Ref Manual but it doesn't adequately
spell out the PSU spec. Can you recommend one upmarket PSU that
handles OVP by brute-forcing the output(s)?
...

True. I witnessed a discussion at aus.electronics where an SMPS failed
in such a manner that AC leaked into the DC side of the switchmode
transformer, causing catastrophic damage to the PC. I suggested that
an external crowbar circuit could provide protection against such
disasters but the responses were negative.
...

What I find hard to comprehend is how even the upmarket ATX PSUs can
deliver their claimed power given the relative size of the components
in my 1000W SMPS. For example, when comparing the mains filter caps,
one is the size of a Coke can, the other is smaller than a C size
battery. Has technology really improved that much?

- Franc Zabkar