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'[PIC]:PICSTART+ Powert Supply problem'
2001\12\02@024420 by Russell McMahon

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Original topic on this May 27 2001 Post by Roman was

               Re:      Re: [PIC]:PICSTART+ Hardware problem

This post and Romans (see below) MAY help people with PICSTART PLUS startup
problems caused by power supply degradation.

Got sick of my PICSTART Plus not starting for about 20 minutes after being
plugged in.
Mine has only two ecaps on secondary side (may be older?)

Replacing both failed to cure the problem.
The first is 470 uF 16V? and is fed by the diode from the switching power
supply. The second smooths the output proper and is fed via an inductor from
the first. The second is 100 uF 25V as Roman says but clearly doesn't NEED
to
be 25V as the first is 16V and the psu is regulated 9v

SO - I replaced the second with 1000 uF at ?16v. Brute force but it does fit
and "almost" sits down on PCB.
This ONLY JUST works. Putting mains on psu first gives 9 volts open cct. But
when PICSTART is plugged in it powers up "most of the time" but not all the
time. If not just unplug secondary and reinsert and it usually goes. Once
going its fine. Clearly the system is marginal. It works for now - I may go
looking for why some time but don't count on it. Mayhaps the cap's ESR is
critical to the feedback loop?

Mayhaps the primary DC supply (about 300 volts) is ripply but I didn't
bother chasing it when this fix (almost) worked. (Small HV caps are less
commonly
to hand here than LV ones).




     Russell McMahon
_____________________________


{Original Message removed}

2001\12\02@032431 by Chris Carr

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Russell, I have not taken this PSU to bits but often the Control IC on a
SMPS
is powered from a tertiary winding which is rectified and smoothed with a
small
electrolytic. For some reason this capacitor seems to be prone to drying
out,
when it does you can get all sorts of strange effects including loss of
regulation in some instances. Replacing this capacitor often works wonders.

My apologies if this has been mentioned already but I haven't
been following this thread.

Regards

Chris Carr

{Original Message removed}

2001\12\02@093253 by mike

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SMPS startup problems are frequently caused by dried out electrolytics
on the PRIMARY side - the first thing I do with these is replace all
the low-voltage electrolytics on the primary side - there are usually
only one or two,  using 105 deg.C types. This often fixes the problem.
Also, the high-value (100K-ish) startup supply resistor going high is
another possibility.
Secondary-side problems seem rare with small SMPSUs.

On Sun, 2 Dec 2001 20:19:37 +1300, you wrote:

{Quote hidden}

>{Original Message removed}

2001\12\02@095315 by Roman Black

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Russell McMahon wrote:
{Quote hidden}

Hi Russell!! :o)
I've been off the PC for a while with back injury
after crashing my bike. Nothing serious, just
very irritating for the last 3 weeks if I tried
to "sit". I've got a big cushion on my PC chair
now and can do a hour here and there.

I'm not sure with your Picstart PSU problem, it
might be that the new 100uF cap you put in already
was old and had some ESR problems. I test new
caps before using them, and have taken some back
to the supplier when the "new" caps had already
exceeded their ESR spec.

With a small circuit like that I would simply
unsolder one leg of each 2-legged part, resistors
diodes etc, and measure their values. It's very
common for resistor values to drift badly when
they get hot in those little smps products.
Check for reverse leaky diodes, or diodes which
change Vf as you are actually seeing it on the
meter. Extra check anything near "heat spots"
on the board or obvious heatsinks. :o)
-Roman

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2001\12\02@095722 by Roman Black

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Mike Harrison wrote:
>
> SMPS startup problems are frequently caused by dried out electrolytics
> on the PRIMARY side - the first thing I do with these is replace all
> the low-voltage electrolytics on the primary side - there are usually
> only one or two,  using 105 deg.C types. This often fixes the problem.
> Also, the high-value (100K-ish) startup supply resistor going high is
> another possibility.
> Secondary-side problems seem rare with small SMPSUs.


Not exactly true. :o)
The caps that cause the problem are small electros
usually 10uF to 47uF and they are the feedback filter
caps in the smps. Usually low volts caps with about a
9v winding on the smps transformer.

They CAN be on the primary or secondary. Primary side
doesn't need an optocoupler. Both are common systems.
The rule I use is any small cap directly connected to
the smps transformer gets a good inspection, or simply
replaced if time is short.
:o)
-Roman

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