Why extra Diode?

  • Thread starter Watson A.Name - \"Watt Su
  • Start date
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Watson A.Name - \"Watt Su

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I was examining a defunct ATX PS that had a Really Bad Day; two of the
electrolytics exploded and unwound the aluminum foil and sprayed
dielectric and other guts all over the resr of the components.

Anyway, I found this arrangement:


Main o------|<-------+
xformer |
of an |
ATX PS +-----|<-----> to inductor
| and filter caps
|
o------|<-------+

All diodes are FR104, Fast Recovery 1 Amp, 400V or something like that.

Why would they use such an arrangement? Maybe to get a bit of voltage
drop? Just curious.


--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
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Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
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@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@
 
ROFL!!!!

That reminds me of an incident that happened in my first job. I was a
trainee building prototypes of power supplies, and had just finished
building the first of this new huge switch mode thing with 48v at loads
of amps output.

Anyhow, so I proudly plugged it in and turned it on - there was small
pause as the lights flickered in the lab, then BANG - the mains
rectifier electrolytics deposited their entire contents over me, the
ceiling and anything within range - turned out I put them in backwards :)

I never did that again.

Yours, Mark.

Watson A.Name - "Watt Sun, the Dark Remover" wrote:

I was examining a defunct ATX PS that had a Really Bad Day; two of the
electrolytics exploded and unwound the aluminum foil and sprayed
dielectric and other guts all over the resr of the components.

Anyway, I found this arrangement:


Main o------|<-------+
xformer |
of an |
ATX PS +-----|<-----> to inductor
| and filter caps
|
o------|<-------+

All diodes are FR104, Fast Recovery 1 Amp, 400V or something like that.

Why would they use such an arrangement? Maybe to get a bit of voltage
drop? Just curious.


--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@
Click to expand...
 
start up voltage.
some switchers require a min voltage to allow for its feed back
voltage in the coil to get started. the inrush current on the caps
will not allow for enough voltage to be present so a Diode is a
good idea to use since it will give you a approximately .6 volts of a
drop.
putting both together you would end up in the range of 1.2 volts
on the average of at least 1.2 volts to be present at the coils on
start up when the caps are in initial inrush.
anything below that would constitute a short in the diodes which
maybe the reason why your caps exploded it the start up didn't
have a time delay shut down from the voltage not reaching at least its
50% mark.



Watson A.Name - "Watt Sun, the Dark Remover" wrote:

I was examining a defunct ATX PS that had a Really Bad Day; two of the
electrolytics exploded and unwound the aluminum foil and sprayed
dielectric and other guts all over the resr of the components.

Anyway, I found this arrangement:


Main o------|<-------+
xformer |
of an |
ATX PS +-----|<-----> to inductor
| and filter caps
|
o------|<-------+

All diodes are FR104, Fast Recovery 1 Amp, 400V or something like that.

Why would they use such an arrangement? Maybe to get a bit of voltage
drop? Just curious.


--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@
Click to expand...
 
Mark (UK) top-posted:

Anyhow, so I proudly plugged it in and turned it on - there was small
pause as the lights flickered in the lab, then BANG - the mains
rectifier electrolytics deposited their entire contents over me, the
ceiling and anything within range - turned out I put them in backwards :)

I never did that again.
Watson A.Name - "Watt Sun, the Dark Remover" wrote:

I was examining a defunct ATX PS that had a Really Bad Day; two of the
electrolytics exploded and unwound the aluminum foil and sprayed
dielectric and other guts all over the resr of the components.

Anyway, I found this arrangement:
[etc.]
Click to expand...

I was trying to learn how to build a switcher from some chip's data
sheet. I was working with ferrite vendors as well as the chip vendor,
and decided on a core, bobbin, turns, and all that. So, I've got this
cup core or pot core, I'm not sure which it's called - it's the one
with two cylindrical halves and a hole through the middle.

So I've got this carefully committee-designed transformer, about to
fire up this unit at 1MHz, everybody's nervous - I'm cranking up the
variac (this is when I learned that it's hard to ramp up the input
of a switcher and have it behave predictably), and realized, too late,
that I had forgotten you're not supposed to fasten a cup core
transformer down with a steel screw. FzzzzzPow!

I never did get a switcher designed, or even built. After about three
months, and I don't know how many thousands of the client's dollars,
we gave up. We concluded, in 1990, that it's impossible to fit a 300W
power supply into 75 cubic inches.

Some things have changed, but they probably still say you're not
supposed to use a steel screw to fasten down a pot core. ;-)

Ironically, the guy had a brass screw exactly the right size on hand.

Cheers!
Rich
 
"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote:

I was examining a defunct ATX PS that had a Really Bad Day; two of the
electrolytics exploded and unwound the aluminum foil and sprayed
dielectric and other guts all over the resr of the components.

Anyway, I found this arrangement:

Main o------|<-------+
xformer |
of an |
ATX PS +-----|<-----> to inductor
| and filter caps
|
o------|<-------+

All diodes are FR104, Fast Recovery 1 Amp, 400V or something like that.

Why would they use such an arrangement? Maybe to get a bit of voltage
drop?
Click to expand...
Quite possible. You can only really regulate the DC voltages from one output
meaningfully - or maybe cross-couple 2 perhaps.

The output voltages on other windings will then be set by turns ratios ( and
diode drop voltages).

As it's just a 1 amp output it's probably simplest to drop a bit of voltage
this way.

I've seen designs that selectively use Schottkies with their lower voltage
drop to get accurate voltage outputs too.

Graham
 
On Mon, 16 Aug 2004 23:30:37 +0100, the renowned Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:

"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote:

I was examining a defunct ATX PS that had a Really Bad Day; two of the
electrolytics exploded and unwound the aluminum foil and sprayed
dielectric and other guts all over the resr of the components.

Anyway, I found this arrangement:

Main o------|<-------+
xformer |
of an |
ATX PS +-----|<-----> to inductor
| and filter caps
|
o------|<-------+

All diodes are FR104, Fast Recovery 1 Amp, 400V or something like that.

Why would they use such an arrangement? Maybe to get a bit of voltage
drop?

Quite possible. You can only really regulate the DC voltages from one output
meaningfully - or maybe cross-couple 2 perhaps.

The output voltages on other windings will then be set by turns ratios ( and
diode drop voltages).

As it's just a 1 amp output it's probably simplest to drop a bit of voltage
this way.

I've seen designs that selectively use Schottkies with their lower voltage
drop to get accurate voltage outputs too.

Graham
Click to expand...
SMPS supplies can have quite a high voltage/turn and fractional turns
are a hassle.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
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