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Capacitor replacement

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Reply 40 of 54, by PCBONEZ

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gdjacobs wrote:

It's the same for tuned LC band stop filters, but that wasn't what they were using. More like a second order RLC filter, but split into two sections.

I was looking through my cache of old ATX schematics (trying to find typical values for the Pi filter inductors) and it suddenly occurred to me what this RLC configuration probably is.
Many PSUs have load resistors somewhere near the outputs. They are basically small "dummy loads".
They provide a minimum load on the given rail to ensure it regulates properly if the external load is too small.
Some PSUs without them will not stay running if they don't have an external load.
I believe their use is fading away because they reduce a PSU's overall efficiency.
.
Some schematics show them on every rail and others only on some rails.
Some show them before the series inductor and others after.
.
I also came across a few that have the load resistor but there is no filter inductor at all.
They count on just the toroidal transformer and a cap being enough filtering.
.

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Reply 41 of 54, by TELVM

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PCBONEZ wrote:

... They count on just the toroidal transformer and a cap being enough filtering ...

And thus they achieve "spectacular" results in the ripple "suppression" chapter. 🤣

- Gutless Wonder Exhibit 1 - No PI coils, just one single crappy cap per main rail:

UuWNLm5C.jpg . . . TQ542Yep.gif
http://www.clubedohardware.com.br/artigos/tes … x-de-230-w/2876

^ 148.5mV of peak to peak ripple on +12V rail (ATX spec puts the limit at 120mV). 😵

But we can go even cheaper (copper is expensive after all) ...

- Gutless Wonder Exhibit 2 - As above but with just one single toroid inductor for all three main rails (no separate inductor for +3.3V):

YmamZOCc.jpg . . . dxsXMaHb.gif
http://www.clubedohardware.com.br/artigos/tes … t-500-p42s/2790

^ 155.4mV of peak to peak ripple on +12V rail. 😵 😵

^ And those two ticking bombs were brand new when tested. After some time the crappy caps will go south, and ripple off the scale. 😵 😵 😵

Some cheap PSUs manage in principle to barely keep ripple inside ATX spec without PI coil and just a couple of crappy caps in parallel per main rail, like this one, but as time passes by and crappy caps go south they'll quickly go off spec again.

As a rule, for classic group-regulated PC SMPSs (like vintage and cheap modern) I'd stay away from anything that doesn't come with two decently sized & reasonable quality caps flanking one PI coil per each main and +5VSB rails (-5 and -12 rails can usually do OK with just one or two smaller caps, though a PI coil wouldn't harm).

Output-filter.jpg
http://www.hardwareinsights.com/wp/how-a-swit … r-supply-works/

Let the air flow!

Reply 42 of 54, by gdjacobs

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Considering the fact that they problem have no mains filtering either, I'd hate to see what noise is flowing back to the power socket.

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Reply 43 of 54, by TELVM

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"Input filtering??? That'd cost a whole bunch of pennies!!!" 🤣

BEsJfc0U.jpg

PT9DOwkq.jpg

Let the air flow!

Reply 44 of 54, by alexanrs

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Well... they are trying to improve airflow!

Reply 45 of 54, by PCBONEZ

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One of the other old farts over at BCN came across some that had dummy weights glued inside - to make them seem heavier.
.

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Reply 46 of 54, by gdjacobs

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I've heard of dummy passive PFC coils made of iron, lead, or cement!

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Reply 47 of 54, by TELVM

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Yep fake pPFC coils have been found inside some gutless wonders.

000701027.jpg 000701028.jpg

000701083.jpg 000701084.jpg

Let the air flow!

Reply 48 of 54, by PCBONEZ

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And I thought I had seen it all...

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Reply 49 of 54, by gdjacobs

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Like the "Russian doll" capacitors we all have to watch out for.
tumblr_inline_mx35vlebii1rf01e3.jpg

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Reply 50 of 54, by PCBONEZ

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Another example. This pic is a classic. Been around the web a while. IIRC it was an eBay purchase.

The attachment FakeRuby_12.jpg is no longer available

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Reply 51 of 54, by gdjacobs

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I love how the real cap inside the fake is a knock-off anyway. Rulycon, indeed!

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Reply 52 of 54, by PCBONEZ

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Well, the Mod that moved this here decided the title should be "Capacitor Replacement" so I suppose we should try to get back to that.
Maybe tomorrow. It's close to the end of my day today.

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Reply 53 of 54, by PCBONEZ

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I put this in another thread but it should be in this topic too.

PCBONEZ wrote:
One big deal thing to check on ESR meters is that they use a 100kHz test signal. Might be square wave, saw tooth, sine wave - th […]
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One big deal thing to check on ESR meters is that they use a 100kHz test signal.
Might be square wave, saw tooth, sine wave - that doesn't matter too much but the frequency does.
.
The datasheets for low ESR caps give the ESR at 100kHz.
If the test signal is not 100kHz you have to do math for each cap to know what it is at 100kHz before you know if it's in spec.
(There is a conversion chart in most datasheets for this purpose.)
Huge PITA.

Most combined LCR-ESR meters don't have a 100kHz test signal.
Many cheap ESR (only) meters don't either.
.

Link below is a thread regarding needed equipment. (So we don't need to duplicate all of it here.)
Recapping gear!

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Reply 54 of 54, by gdjacobs

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Found a good article describing non-ideal capacitor behavior. I've included the graphic of the equivalent component model to help those who wish to understand.
capacitor_lumped_model.png?version=1&modificationDate=1377037140490&api=v2
https://eewiki.net/display/Motley/Non-Ideal+P … s+of+Capacitors

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