Enterprise versus "consumer" grade drives

[Rod Speed]

Rod Speed wrote

I'm glad to hear that it's still that easy. I seem to recall seeing
a circuit board a few years ago that had components with only
part nos. printed on them - no real specs.

You don't see that much with electros and even if you did find
some, you should be able to get the data off the web.

You normally can with semiconductors which can have
reduced part numbers when they are too small for the
full number, even with transistors etc.

 

[Rod Speed]

"Arno" explained:


Ummm, yeah. Time is Money, ka-ching! Do people offer "cap kits"
for popular boards or types of circuit that frequently have cap failure?

Yep.

 

[miso]

Not at all. The capacitor will usually contain a manufacturer name,
a "type" and a voltage, capacity and temperature rating. With the
type and manufacturer, you look into the datasheet. Typically
these will be standard, low ESR or very-low ESR capacitors.
This must match. Temperature must be same or better. Capacity
must match. Voltage must be same or higher. You may also want
to look at diameter and distance between the connecting wires.

See, easy ;-)===)

Arno

Some LDOs will oscillate if the ESR is too low. Most of the time you
will be encountering switchers and this won't be an issue.

 

[Tom Del Rosso]

Some LDOs will oscillate if the ESR is too low. Most of the time you
will be encountering switchers and this won't be an issue.

Low dropout? As in linear regulator?

 

[miso]

Low dropout? As in linear regulator?

Yes. There are quite a few papers on this. Now mostly due to ceramics,
but also some electrolytics are very low.

 

[Tom Del Rosso]

Yes. There are quite a few papers on this. Now mostly due to ceramics,
but also some electrolytics are very low.

I don't doubt they can oscillate, but I didn't know they used linears on
mobos.

Why LDO? Do they have 5V going into the 3.3V or something like that?

 

[Arno]

If you connect the new capacitors with 1 inch leads, you might as well just
leave them out, regardless of securing it and insulating the leads.

Ah, no.

Example: A Rubycon ZL 25v, 2200uF low-ESR capacitor
I pulled out from my stash at random has internal resistance
about 15mR at 20C and 100kHz. The leads are 0.6mm diameter
copper, which is 0.28mm^2. Copper has a resistance of
17mR at 1m and 1 mm^2. Hence these leads have
(17mR / 0.28 mm^2) * 0.0256m = 1.5mR each fpr an "Inch",
altogether 3mR. This is an increase of resistance of just 20%
and will not make any large difference. It will decrease
capacitor lifetime a bit, as it heats up about 45% more.
If you do not cool them well, that may make a difference.
The increased thermal resistance may alsu be beneficial, if
the capacitor is cooler than the PCB as a result. It depends.
But long leads are only a problem in borderline situations.

The thing is that the lead diameters are selected by the
capacitor manufacturer that even using the full length has
only a relatively small impact.

Arno

 

[Arno]

"Tom Del Rosso" replied:

Good to know. That's what I'll look for first if any of my
equipment has a cap failure.

*TimDaniels*

It takes a significantly shorter time selecting new caps than
actually removing the old ones and mounting the new ones. I have
done it several times. But a cap kit is nonetheless a good idea.

Arno

 

[Arno]

Some LDOs will oscillate if the ESR is too low. Most of the time you
will be encountering switchers and this won't be an issue.

Yes. But these will not have low-ESR capacitors in the first place
and these caps are usually not under enough load to pop. But you
are right, there are situations where replacing regular caps
with low-ESR ones could be a problem.

Arno

 

[Arno]

I don't doubt they can oscillate, but I didn't know they used linears on
mobos.

Why LDO? Do they have 5V going into the 3.3V or something like that?

Yes. If high quality is needed, e.g. on a soundchip. There are
newer LDO regulators that do not mind low resistance in their
output caps and can use ceramics (which are always low-ESR
compared to electrolytes). I expect thet you will see mostly the
second variant on mainboards, but YMMV, people some times stick
to old designs without good reason and many circuit designers
are juniro people. Admittedly though, if you do a lot of new
circuitry on something like a mainboard, you will probably
debug it forever.

Arno

 

[Arno]

It is not the resistance of the extra leads that will cause problems.
As you note, the DC resistance is not huge (and you can use thicker
wires if you want to lower it).

At higher frequencies it is the /inductance/ that dominates. The
frequencies going into and out of these caps is not particularly high,
but flying leads an inch long make a rather large loop - enough for the
inductance to limit the effectiveness of the cap. It will also act as
an antenna radiating noise.

You think this is a problem? >100kHz electrolyte caps do not cut it
anymore anyways. And below I expect it is less of an issue.

2.5cm of wire have about 25nH, I do not see that causing issues
at these frequencies.

A simple way to mitigate these effects are to twist the two leads round
each other (make sure they are properly insulated first!). The
inductance comes from the /loop area/ traversed by the current passing
through the cap, so keeping the leads tightly together minimises that.

Ah, no. Twisting reduces the antenna effect down to basically
nothing, but twisting a wire onto itself (as you do here) just
increases the inductance. You are thinking of a differential
pair. That is different as both wires are not carrying the
same signal.

Arno

 

[Grant]

I would replace bad caps for the satisfaction of knowing that I had
"fixed" or "improved" the board, but having the board down while I
searched for appropriate replacement capacitors could be a waste
of my time. As for a replacement board, I'd just be getting more caps
that are likely to go bad. The best option balance (for me) would be
to install better caps on the same board as soon as possible.

Replacement caps can be higher voltage, higher value, higher temperature
rating, if they physically fit. From what I've seen, manufacturers put
too few caps in place, running too much ripple I through under-sized caps.
This is besides the old 'bad cap' issue. More a design issue with a
"let's make it last out the warranty period" mindset. Or the production
engineers downgraded cap specs to save a few cents up front.

Grant.

 

[GMAN]

"Arno" explained:


Ummm, yeah. Time is Money, ka-ching! Do people offer "cap kits"
for popular boards or types of circuit that frequently have cap failure?

*TimDaniels*

www.badcaps.net

Direct link here for premade kits
http://www.badcaps.net/pages.php?vid=21

 

[miso]

I would replace bad caps for the satisfaction of knowing that I had
"fixed" or "improved" the board, but having the board down while I
searched for appropriate replacement capacitors could be a waste
of my time. As for a replacement board, I'd just be getting more caps
that are likely to go bad. The best option balance (for me) would be
to install better caps on the same board as soon as possible.

*TimDaniels*

Actually, you can't get a replacement board. By the time the caps fail,
the mobo will be long obsoleted. In fact, you will be lucky if a mobo
exists that will accept your CPU.

Depending where you live and your system duty cycle, running the old
mobo, even if fixed, may not make sense since newer PCs are lower power.

 

[miso]

LDOs are often used when you don't need much current. They are a lot
smaller and cheaper than switchmode regulators, but obviously they are
much less efficient. They are also "quieter", so they are a good choice
for more sensitive supplies. So you'll find them around sound chips (as
Arno mentioned), oscillator supplies, and for various ad-hoc supplies
around the board. But you won't find them on the processor supply!

Often they use a LDO to remove the ripple from a switcher. This does
work well if the pass device is a P-fet.

 

[miso]

Are "bad caps" always electrolytics and meant for filtering ripple
current?
In such a scenario, I can see the value in higher capacitance, but is
that usually
the case?

(I once collected 0.5 farad worth of surplus 350VAC electrolytics to
build
the power supply for an "ultimate" stereo amp, but the price and weight and
shipping charges for the transformer put a halt to the project. The amp
would
have been the center of mass of the living room, and I would have needed a
halogen fire extinguisher nearby for safety, but the power available for
transients would have been monumental.)

*TimDaniels*

Tantalum caps can also fail.

Lots of cap videos on the net.

Surface mount caps are usually what people end up exploding by accident,
especially a certain type of capacitor where the polarity is determined
by a little nib sticking out. They literally jump off the PCB and land
on the bench still smoldering if not flaming.

http://www.newark.com/vishay-spragu...lum-capacitor/dp/53K6051&ICID=Vishay_Passives

This is a decent cap except for the violent failure mode. Due to the
density of the device, these are used where space is a premium. Also due
to the low ESR, they are used in high end devices. Just spec them
conservatively.

In linear supply design, you reach a point where it makes more sense to
actively regulate than to brute force filter. A transistor, if
maintained in a safe operating area, is very reliable. Big ass
electrolytics will fail eventually. There is a trend to increase
switching frequencies in SMPS simply to be able to use ceramic caps.
SMPS in excess of a MHz isn't all that odd.

 

[Arno]

I haven't done the calculations (I've never had a good feel for
inductances), so I'll take your word on the inductance of the wire.

Each new generation of motherboard point-of-load switchers gets a higher
switching frequency - the idea of very high currents (peaks of hundreds
of amps) at hundreds of KHz along a two-inch round trip of wire sounds
very wrong to me. But maybe I am worrying too much here.

No, you _are_ right in principle. It is just that electrolythe caps
do not do well on high frequencies. If the switcher hits, say, 1MHz,
then filter caps will likely be ceramics, both because the
electrolythe caps do not cut it and the ceramics can be very small.

I am not twisting the wire onto itself - I am assuming there are two
wires, one for ground and one for positive, of roughly equal lengths.

These carry the same signal.

Twisting these together (or just tying them together in some way) will
reduce the inductance that the current through the cap sees.

Hmm. Tying together should work. Twisting, I don't think so.

But you could well be right that the inductance is not going to be an
issue - as I say, I haven't done the calculations.

Doe not matter that much. I have done "long lead" cap replacements
(no space) and never saw any problems. The one problem I did saw was
when I approximated total capacity of two capacitor banks and got
one 20% lower and one 20% higher (with a filter-coil in between).
That did cause oscillations.

Arno

 

[Arno]

Are "bad caps" always electrolytics and meant for filtering ripple current?
In such a scenario, I can see the value in higher capacitance, but is that usually
the case?

(I once collected 0.5 farad worth of surplus 350VAC electrolytics to build
the power supply for an "ultimate" stereo amp, but the price and weight and
shipping charges for the transformer put a halt to the project. The amp would
have been the center of mass of the living room, and I would have needed a
halogen fire extinguisher nearby for safety, but the power available for
transients would have been monumental.)

Don't use higher values! Switching regulators need to be carefully
tuned to voltage drop, frequency, inductor and capacitor values.
In the worst case, higher output filter caps can cause oscillations
and destroy your curcuit. I have seen oscillations as low as 0.25Hz
from this effect before I understood it. (I once build a PSU for a
computer myself, fed from a 200W transformer at 24V. That adventure
took a significant part of my spare time for 3 months, but I learned
a lot and it did work reliably at the end.)

And yes, "bad caps" are low-ESR capacitors used as output filter
capacitor in switching regulators. But the capacitor is only half
of the filter, the other half is the inductor. LC circuits are
tricky. They do have a main resonance frequency and more at
multiples of that main one. Switching regulators have to be
carefully designed to deal with that, do not change either L or
C values on an existing design without very good reasons.

Arno

 

[Arno]

On 10/17/2012 9:17 AM, Timothy Daniels wrote: [...]
Actually, you can't get a replacement board. By the time the caps fail,
the mobo will be long obsoleted. In fact, you will be lucky if a mobo
exists that will accept your CPU.

Yery true. I now have resorted to use the same board in my desktop
and file-server and to keep a spare on hand. That does not solve
the problem completely, but it gives me time to find a suitable new
board if one fails and it does somewhat protect the investment in
CPU and memory. One of the reasons I use AMD CPUs, they have a much
longer lifetime as they do not have the saocket-chaos Intel has.
(They also still have a far superiour memory interface.)

Depending where you live and your system duty cycle, running the old
mobo, even if fixed, may not make sense since newer PCs are lower power.

I advise to think about this beforehand and have a strategy in
place. For me that is if the board fails Saturday evening, I do
not want to be without computer over the weekend, hence I have
spares for everything. I also want some time to think about
what replacement I am going to get, hence the identical mainboard
spare. A factor is how long it takes to get the replacement you
want. If you can just pick it up same day locally, no problem.
If you are more picky (I am) it may take a week or two and you
need to be prepared for that.

That said, OS used to be a factor, but it has gotten much better.
Linux was always pretty benign (exception: higher-end graphics card)
and will basically boot one installation on any other hardware. But
XP used to be a real pain to move to a new mainboard. Fortunately
MS finally caught up a bit, I have moved my Win7 installation
(used oly for critical things, i.e. gaming ;-) between two boards
with only minor issues.

Arno

 

[Grant]

Are "bad caps" always electrolytics and meant for filtering ripple current?
In such a scenario, I can see the value in higher capacitance, but is that usually
the case?

Not so much as having suitable ripple current rating, I've replaced caps
with lower capacitance, but higher voltage and ripple current rating, with
excellent result. Consider electro tolerance might be +/- 20% or -50, +100%.
So using 820/35 for 1000/25 is not so evil, if the caps were on hand.

I see higher capacitance as harmless, I've not had a switchmode regulator
fail loop stability due to additional load capacitance. LDO regulators
are a special case, they have a defined band of stable cap loading, less
than some value, or greater than dome other higher value.

(I once collected 0.5 farad worth of surplus 350VAC electrolytics to build
the power supply for an "ultimate" stereo amp, but the price and weight and
shipping charges for the transformer put a halt to the project. The amp would
have been the center of mass of the living room, and I would have needed a
halogen fire extinguisher nearby for safety, but the power available for
transients would have been monumental.)

As would've been the startup inrush current! Active PFC for such a monster
would be a better solution, if you could the switching noise out of the audio
side

Grant.