OT: What is a barrier rectifier ?

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Skeleton Man

Hi all,

This is probably a question for you kony, but what is a barrier rectifier
and can I test it with a DMM ?

The part in question is a MOSPEC S60D40C Schottky barrier rectifier, 60A,
40V and it's in my ATX PSU.

I'm trying to elimate parts one by one as an exercise (blown PSU, +5VSB ok,
won't power on, fuse is ok).. I've long since replaced the unit..

Chris
 
Another quick question.. there's a small resistor near one of the caps on
the AC side that's reading something weird like 22 MegOhms (I think it's
meant to be around 220K - red, red, yellow, gold), while all the rest read
between 100 and 250K.. would be this be a startup resistor and a possible
cause of the problem ?

Chris
 
Hi all,

This is probably a question for you kony, but what is a barrier rectifier
and can I test it with a DMM ?

The part in question is a MOSPEC S60D40C Schottky barrier rectifier, 60A,
40V and it's in my ATX PSU.

I'm trying to elimate parts one by one as an exercise (blown PSU, +5VSB ok,
won't power on, fuse is ok).. I've long since replaced the unit..

Chris
Click to expand...

It's just two high current schottky diodes in a (what they
call, but what I'd usually call TO-247) TO-3P package, with
the center leg and back 'sinking tab as a common cathode.

In other words, using your meter diode check function
between the left leg and center, and the right leg and
center, you'd test as if it were two separate diodes.

While it's not impossible for one to fail, IMO it happens a
lot less often than the chopper transistor(s), especially
when they're the big TO-247 package instead of a small
TO-220 or even worse how some generics just crimped together
a couple of 3A diodes.

Also if one of these high current parts fails it will tend
to make a funny smell when the magic smoke was released,
might have a burnt mark on the casing or discoloration on
the PCB (might only be on the bottom).

A usenet group may not be the most effective way to work
through a failed PSU, especially when some groups have more
members with intimate knowledge of specific model's
weaknesses and topology . Try posting a description of the
PSU, the problem, pictures, and what you've done so far to
this website forum:
http://www.badcaps.net/forum/forumdisplay.php?f=32

(but beware, some of the members there take the website
domain name a little too seriously and will encourage you to
replace capacitors that aren't necessarily the problem in
many cases).
 
Another quick question.. there's a small resistor near one of the caps on
the AC side that's reading something weird like 22 MegOhms (I think it's
meant to be around 220K - red, red, yellow, gold), while all the rest read
between 100 and 250K.. would be this be a startup resistor and a possible
cause of the problem ?

Chris
Click to expand...

First, you might need to pull the resistor out to measure it
accurately, BUT measuring resistance directly across it's
lead the value could only be lower than it is, not this much
higher. If the value were continually changing it could be
that the meter is charging a capacitor, but once it reaches
a steady reading the value read should be at lead that of
the resistor or lower so something does seem wrong with the
reading (with the resistor).

Yes it seems wierd there would be a 22 MOhm resistor on the
high side, but I don't know anything about this PSU as you
haven't pictured it, mentioned make or model, etc.

220K is roughly the value I might expect to be a bleeder
resistor across the high side capacitors, if that's where
it's located. A value slightly higher might be used across
hot and neutral on the AC side in the input filter.
 
First, you might need to pull the resistor out to measure it
accurately, BUT measuring resistance directly across it's
lead the value could only be lower than it is, not this much
higher.
Click to expand...

Pulling it looks like the hard part.. it's hidden between a large capacitor
and a heatsink, so I'll have to remove the cap to get at it.. (I'd need
tweezers like 4" long and a couple of mm wide to grab it otherwise).
Yes it seems wierd there would be a 22 MOhm resistor on the
high side, but I don't know anything about this PSU as you
haven't pictured it, mentioned make or model, etc.
Click to expand...

It's meant to be a Wintech 300W, but the PCB says WIN-250PS. I'm pretty sure
the bands on the resitor are red, red, yellow, gold..

I can't find a schematic for any wintech boards to compare mine with..

Chris
 
First, you might need to pull the resistor out to measure it
accurately,
Click to expand...

I'm trying to unsolder a large capacitor that's in my way, but the solder
won't take to the solder wick.. The iron will melt the solder if I press it
directly against it, but if I try with the wire, it just heats the wire and
nothing else..

Any ideas ?

Chris
 
Skeleton Man said:
I'm trying to unsolder a large capacitor that's in my way, but the solder
won't take to the solder wick.. The iron will melt the solder if I press
it
directly against it, but if I try with the wire, it just heats the wire
and
nothing else..
Any ideas ?
Click to expand...
Larger (wattage) soldering iron ?.
 
Larger (wattage) soldering iron ?.

I got it now.. I didn't realise you had to cut it first.. I was trying to
use the solder wick while it was still on the roll.. (figured I could cut
off the used part later)

Chris
 
Update: the resitor is not the issue.. I managed to get it out and measure
and it reads 217.5K (which is well within tollerance right ?)

Back to the drawing board..

Chris
 
I got it now.. I didn't realise you had to cut it first.. I was trying to
use the solder wick while it was still on the roll.. (figured I could cut
off the used part later)

Chris
Click to expand...

A higher wattage soldering iron would allow using wick while
still on the roll w/o cutting but some areas with lots of
copper and solder make using wick more of a pain, slower
than using a solder sucker, either separate pump-only type
or one with the iron built in.

Well, I wrote higher wattage above but a stout iron tip with
a point that puts a lot of surface area against the solder
or braid, and that this tip be clean and well-tinned, helps
a lot too. A good 30W iron/tip combo should do that, but
something like a Radio Shack iron with the long conical tip
might have trouble getting the solder hot enough.
 
Pulling it looks like the hard part.. it's hidden between a large capacitor
and a heatsink, so I'll have to remove the cap to get at it.. (I'd need
tweezers like 4" long and a couple of mm wide to grab it otherwise).


It's meant to be a Wintech 300W, but the PCB says WIN-250PS. I'm pretty sure
the bands on the resitor are red, red, yellow, gold..

I can't find a schematic for any wintech boards to compare mine with..
Click to expand...

I'm a little impressed they actually used the aforementioned
schottky diode pack, it must have been one of their *higher*
end PSU meant to run for more than 90 days.

Is this PSU really worth your time? A 250-300W rated
(probably worth closer to 150-200W) Wintech is of little
value even when brand new and working. Unless the failure
was caused by a random AC power surge, it may fail again in
the same area if the replacement of failed part isn't an
upgrade of the original, or other areas of the PSU might
also be aging towards failure sooner rather than later,
including the capacitors.
 
Skeleton said:
This is probably a question for you kony, but what is a barrier rectifier
and can I test it with a DMM ?

The part in question is a MOSPEC S60D40C Schottky barrier rectifier, 60A,
40V and it's in my ATX PSU.
Click to expand...

There's a really good FAQ about switching-mode power supply repair at:

www.repairfaq.org

Computer power supplies use Schottky diodes because they drop only
about half as much voltage as other diodes do, and they can switch on
and off a lot faster than regular diodes. I once tried replacing a
Schottky with a regular diode, and the power supply squealed loudly
and the diode got really hot.
I'm trying to elimate parts one by one as an exercise (blown PSU, +5VSB ok,
won't power on, fuse is ok).. I've long since replaced the unit..
Click to expand...

If only the +5Vsb is working, it's possible that a diode on the output
has shorted out, but another possibility is that a transistor in the
high voltage section has shorted, although the latter usually blows
the fuse.

Wintech PSUs are often free after rebate when sold under the Ultra
brand, but last Friday Fry's had 500W Antec Basiqs for $7 after
rebate, and I'd trust a Basiq a lot more than a Wintech because
they're made by Fortron-Source/Sparkle and have good capacitors
(CapXon), unlike the SmartPower and TruePower Antecs (but Seasonic-
made Antecs, like NeoHE, Earthwatts, Trio, and Quattro, are fine).
 
If only the +5Vsb is working, it's possible that a diode on the output
has shorted out, but another possibility is that a transistor in the
high voltage section has shorted, although the latter usually blows
the fuse.
Click to expand...

Update: I never found out what the original problem was, because when I went
to power it up again to test (been in my atic for months) the fuse blew,
taking both the high voltage transistors with it! (and god knows what else)
I'm wondering if sticking it in a box in a hot dry attic for a few months
caused already failing caps (and other components) to let go entirely..

I don't know if I'll continue with repairs or not.. the transistors are not
something I can find at the local radioshack.. I'd have to order them from
ebay (at like $10/each when you include shipping).. might see if I can
cannibalise this one to fix something else..

Chris
 
Skeleton said:
Update: I never found out what the original problem was, because
when I went to power it up again to test (been in my atic for
months) the fuse blew, taking both the high voltage transistors
with it! (and god knows what else) I'm wondering if sticking it
in a box in a hot dry attic for a few months caused already
failing caps (and other components) to let go entirely..

I don't know if I'll continue with repairs or not.. the
transistors are not something I can find at the local radioshack.
I'd have to order them from ebay (at like $10/each when you
include shipping).. might see if I can cannibalise this one to
fix something else..
Click to expand...

Very few transistor and diode parameters are critical. The
important ones are usually reverse voltages, maximum Vcc, and
possibly Beta. Speed also shows up. So you can almost always find
an intelligent replacement at a reasonable price.
 
Skeleton said:
I got it now.. I didn't realise you had to cut it first.. I was trying to
use the solder wick while it was still on the roll.. (figured I could cut
off the used part later)

Chris
Click to expand...

If youre struggling to get the joint hot enough you need more solder
on it, not less, to maximise heat conduction from tip to pad. Clear
the hole after the component's out.

However, as said this might be a discharge R, which is an optional
part not needed for psu function, so it might be sensible to say
where its connected first. 'On the high side' is far too vague.


NT
 
If youre struggling to get the joint hot enough you need more solder
on it, not less, to maximise heat conduction from tip to pad. Clear
the hole after the component's out.

However, as said this might be a discharge R, which is an optional
part not needed for psu function, so it might be sensible to say
where its connected first. 'On the high side' is far too vague.
Click to expand...


Adding solder won't work so well when the purpose was to
remove the solder with wick, because ultimately the wick
area heated becomes saturated with solder and then the
remaining solder has to be removed with fresh wick without
adding solder (else you're back to square one again).

Adding flux instead of solder is generally helpful, as well
as a hotter iron. Clearing the hole on this cheap PSU is
unnecessary as it's going to be a single sided board, no
plated holes, meaning the hole will practically clear itself
or if it didn't, only a very thin layer of solder would
remain that can be pierced quite easily with a needle (from
the solder-side not the component side to minimize the risk
of pulling up copper traces.
 
Skeleton said:
I don't know if I'll continue with repairs or not.. the transistors are not
something I can find at the local radioshack.. I'd have to order them from
ebay (at like $10/each when you include shipping).. might see if I can
cannibalise this one to fix something else..
Click to expand...

Mouser.com is cheap, but you'll probably have to find substitutes to
match the brands they sell (STmicro is one).

Oddly, the components in ancient 200W PSUs are often just as hefty as
those in newer 300-500W ones. BTW if you have to replace a diode pack
or transistor connected in parallel, be sure to either get the exact
same part number or replace both parts in the pair so the load is
shared equally.
 
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