23 Jun
1997
23 Jun
'97
5:20 a.m.
Tony,
Ok that is no problem with PCjr matter. :)
Too bad that happened!
That is pretty rare for me.
Yep, so do I. Most SMPS faults are pretty simple (at
least if the
transformer has not got shorted turns...). And it's certainly worth doing
for the more obscure supplies (i.e. not generic PC ones...)
Shorted turns is very,
very rare in SMPS that I came across.
Long run of lucky I guess.
> > as the reference. It's the only monitor
that _requires_ a green power-on
> > LED. When my blew up (chopper failed, etc), I replaced the entire mess
> > with a little torroidal transformer. It was cheaper than a new chopper
> > transistor and a lot simpler.
> >
Using LED as a reference? That is poor idea to do that! It should
have put the sense line on that heavy current line to work.
Hmmm your hack to feed 20vac there looked better! :)
Actually, the 50Hz 30VA transformer was no larger than
the original
bits...
Cool!
Are you talking about this beige tiny 12"
green or amber monitor with
That's the one.
angular back? In this, chopper trans pops and
kills one resistor.
Well, I had shorted turns in the chopper transformer (tested on a homebrew
'ring tester'). It didn't seem worth rewinding. I still have mine on a much-hacked PC/AT
Oh yea,
it looked nice and impossibly compact.
I prefer TTL there than composite unless it is for color.
It may as well go here - it's just about on-topic....
The basic circuit design is :
1) Rectify/double the AC mains input to give 340V DC
2) Feed that into a non-isolated 'flyback' converter to step it down to
150V.
hi frenquency ac or dc at that point?
3) Feed the output of that into a free-running
push-pull oscillator using
a couple of big power transistors.
Well, I can't imagine that would able to
work if the power is ac to
that push/pull trans?
4) That oscillator drives the primary of the main
transformer
Snip! loads of worthless protection info... :)
drive to (2) if it overcurrents.
Now, here's what goes wrong.
1) The chopper (2) shorts. The output of that stage now leaps to 340V
(since there's a DC path through the shorted transistor and the inductor)
Ah! Not a transformer just a giant inductor driven by that chopper?
:) that would be a biggest OUCHIE! Why not jerryjig a design to
clamp down based on voltage level when exceeding 150v at this point
when chopper went so it will blow the fuse instead?
2) The oscillator (3) continues running, so the 5V
line leaps to about 12V
3) The crowbar trips, shorting the output to ground
4) The overcurrent trip tries to work, but as the chopper (2) is shorted,
it doesn't do a darn thing.
Other option, Why not use this output of this crowbar to trip other
clamper at the 340vdc output to blow the fuse instantly?
You want to blow the fuse first before the last 2 trans dies
resulting in expensive heap of smelly stuff. :)
5) The 2 oscillator transistors short as well. The
total load on the 340V
line is now the 3 shorted transistors, a few low-resistance inductors, and
the 0.12 Ohm sense resistor. The latter explodes, feeding 340V into the
sense circuitry, which dies expensively, often taking some PCB tracks with
it.
6) The fuse fails...
What a load of smelly results and worthless fuse there! :(
I got mine from IBM about 2 months ago. I'll try
and find the part
number/form number to order it if you like.
Thank you, I will find out how it costs when I have that part
number.
Jason D.