Hot caps

[Elitist]

Here's an alternative to PTH's symptoms, that may raise a titter.
Several boards have been turning up that would not POST, but the fans spin, some regulators get warm, cpu gets warm, FDD & HDD seeks, but no beep (don't own a POST card).
None of the caps are bulging or leaking.
So, leave the board 'running' for five minutes and stick tip of tongue onto top of biggest/suspect caps - the ones that are warm are the duds! Caps should not be much above ambient - they are energy stores, not energy consumers.
Not wishing to revisit any previous discussion, but the best way to test whether a soldering iron is up to temperature is to raise it towards your lips. Burnt fingers, arms, hands but never lips.....

So, PTH, you may be Canadian, but please edit your spelling mistake.

[PTH]

Re: Hot caps

So, PTH, you may be Canadian, but please edit your spelling mistake.

Huh ?

Apparently I'm not just Canadian, but also a tad dim today...

[PeteS in CA]

Re: Hot caps

Caps should not be much above ambient - they are energy stores, not energy consumers.

I have several caps undergoing test at present that are between 100C and 120C in a 25C environment. How? The same way O/P caps in a MB VRM or a P/S get significantly warmer than ambient - the power dissipated due to the ripple current being conducted by the cap. P=(I^2)(R) remains true, whether it's a DC current through a resistor or the ripple current through a cap's ESR. Now, a cap that is much hotter than those in parallel with it might be a bad part.

[gonzo0815]

Re: Hot caps

And not forget those large cooper traces in uuall VRM desing. Those conduct the heat from rectifier, coils and mosfets directly to the caps.
But ok, i know some very capable TV repair guy`s which go the same route (but not with the tongue...could be bit shoking (in a elektric manner ). But ok, today an ESR meter will do the same with a little more confidence and acuracy.

[Elitist]

Re: Hot caps

OK! I(squaredR) in free air. But as anyone who has done recap-ing on a motherboard knows, the can is connected to the ground plane of the interlayer which behaves as a massive heatsink, so ripple, etc. dissipation doesn't tend to raise the can temp. by much, normally. Unless there's something wrong. If you've got G-Luxon or OST written on the sleeve, they might just as well have printed QED on it!
The tongue-test has served me well over the years in all sorts of situations...

SOLDER - PTH! I guess my previous little epistle has passed you by. And probably the better for it. But Canadian aint no Yankee. You still qualify for the sword on the shoulder and a free packet of tea. Can I recommend the Catherine Zeta Jones School of Beautiful Diction. [And you can wipe that smirk off your face that boy at the back trying to spell double entendre].

[tiresias]

Re: Hot caps

Can I recommend the Catherine Zeta Jones School of Beautiful Diction. [And you can wipe that smirk off your face that boy at the back trying to spell double entendre].

Hehe... Fantastic!

On topic: as Gonzo mentioned, indeed, often motherboard designs are such that heat generated by the VRM's FETs, which may be considerable, does end up warming up the caps in question. This could, of course, also be partly responsible for increased rates of capacitor failure, particularly when those slightly dubious brands are used.

[PeteS in CA]

Re: Hot caps

But as anyone who has done recap-ing on a motherboard knows, the can is connected to the ground plane of the interlayer which behaves as a massive heatsink, so ripple, etc. dissipation doesn't tend to raise the can temp.

While some heat will be conducted through the leads, which are pretty thin, the can itself is not electrically connected to either electrode in modern low-Z caps. And between the can and any copper ground or voltage plane will be sleeve and solder-mask (and possibly one or more layers of FR-4). In my tests, with caps with thicker-gauge wire, the cans are 10C-20C higher in temp than the wires, so core-can-ambient seems to be a lower thermal resistance path that core-leads-PCB planes.

[Elitist]

Re: Hot caps

What you say is most interesting. Unfortunately it flies in the face of all my field experience with bad caps. Removing and refitting caps provides definitive thermal evidence that one electrode is strongly coupled to the can and that one (the same one) is strongly thermally coupled to a copper interlayer. This is especially evident if using an underpowered soldering iron which will fail to desolder one terminal every time. If you review the plethora of posts here, it seems that most folks find the same effect. It had never occurred to me to check the electrical connection (would be pointless on a dud unit) but I will now do so - you may well be correct. As for the tongue-test, whatever the theory, it works every time in practice. Normally the cans of my caps which are in tact are more or less at ambient; those that are failing seem to be ~35-40C for an ambient of 15-20C. I guess there will be some variation across different board designs. As you know, in servicing, it's always best to rely on what you see, feel, measure than what the book wants to tell you! Most small shop and personal repair operations are conducted by block substitution and observation rather than reference to the manuals and test gear which takes longer/costs more time, resources and effort! What happens in the lab may be rather different.

[arneson]

Re: Hot caps

Every tongue is different.
For example, a forked tongue could test two caps at once.

[PeteS in CA]

Re: Hot caps

... one electrode is strongly coupled to the can ...

I've taken apart scores of caps (12.5mm, 16mm, and 18mm diameter) from Nichicon, Panasonic, NIC, Rubycon, UCC, Lelon, and Jianghai. I've not seen any with an electrode connected to the can.

[Elitist]

Re: Hot caps

Yup - that seems to be correct. I checked quite a few yesterday. In the good old days twernt like that. Any road up, the negative terminal and the can take up the thermal load disproportionately - I checked that also, yesterday. I am now using the electric hot air paint stripper to heat up the underside of boards prior to removing caps. They seem to tolerate this kind of abuse surprisingly well, and it certainly speeds things up and leaves a much tidier result!

[PeteS in CA]

Re: Hot caps

In the good old days twernt like that.

Very true. The "-" terminal used to be connected to the can, and might still be in some GP types (e.g. axial). With the old/ancient multi-section lytics used in 50s (and 60s?) TVs, the can was the negative terminal for all the sections. Those were fun.

[Elitist]

Re: Hot caps

If you want to become really confused, then there's nothing better than to get tangled up with battery terminal conventions. In rechargeable cells, the negative terminal may not be the anode - depends whether it's being charged or discharged! [I guess that an electrolytic cap is a special type of battery]. Only recourse is to determine which direction the electrons are moving at the interface.

[PeteS in CA]

Re: Hot caps

My brain won't do electron flow anymore. It's stuck in conventional current mode.

[kc8adu]

Re: Hot caps

Every tongue is different.
For example, a forked tongue could test two caps at once.

i have some hot caps here you can tongue test...in a drake l4 !!!!!!!

[tazwegion]

Re: Hot caps

Might be safer for the tongue to use a cheap digital thermometer



Just a thought

[PeteS in CA]

Re: Hot caps

Interesting idea, but what is the measurement range of those things? Body temp is in the 35C range. If the in-case ambient is in that range, and the thermometer won't read much higher, it won't be very informative.

[Rainbow]

Re: Hot caps

IR thermometer is the way to go. There are some cheap ones - I have this one:
http://hh17.vip.sina.com/product/1322.htm
(about 35 euro here).

[PeteS in CA]

Re: Hot caps

Be aware of that focus cone. If I understand the drawing properly, the tightest spec'ed focal spot is 25mm at a distance of 150mm (~1" spot at ~6" distance). I'm not sure how that works out trying to measure the temp of a 10mm or 8mm diameter cap. Does it do OK if you get really close to the cap?

[kc8adu]

Re: Hot caps

harbor freight has one on sale for $10
cen-tech.
no model visible but its a little grey keyfob like unit.even has pics in the manual of it being used to check cpu temp.
found the link
http://www.harborfreight.com/cpi/cta...emnumber=93983