At 09:43 AM 7/19/2005, Gran Clark wrote:
>Joe
>
>Carefully consider expanding into switchers. The designs are quite
>varied. To repair them you really need a good high frequency scope,
>shielded isolation transformer, 0.18 ohm 1000 watt resistor load,
>and most of all a good schematic. Switcher parts are hard to find
>especially the low ESR capacitors (a common failure). Fixing an
>analog supply is a slam dunk compared to a switcher. I would not put
>one at a repeater site unless the site happens to be my home.
On the other hand, a well designed switcher can easily outlast a linear.
Heat kills, and a switcher dissipates a LOT less heat on any given
load, than a linear supply.
Jim Williams of Analog Devices, published a formula for predicting
capacitor life in a given design. There are a number of factors that
go into it, but the temperature term decreases the projected life of
the caps by half, for every 10C rise in ambient temperature. This is
true wether the cap is used in a switcher, or a linear supply.
Without the formula, you can simply predict a cap's life in a
moderate application, by using it's rated lifetime and the
application temperature.
If you look at Digi-Key's site, you'll see that each type of cap has
a rating in so many hours, and so many degrees C.
A cap rated for 2000 hours at 85C is going to fail roughly 4x faster
than one rated at 2000 hours at 105C.
Both will last twice as long if you run them 10C under their ratings.
For the large electrolytics in a switcher, you can estimate ripple
current requirements by the output current and the number of caps in parallel.
Say an output cap in my IOTA died. I would replace the output caps,
output diode, and probably the switching transistor as well, before
even turning on the supply.
To pick a new cap, I would look up the ratings on the existing ones
in Digi-Key, and pick a similar, but higher lifetime unit.
But, let's say that I couldn't find the ratings.. Ok, so it's a 55A
output, and maybe there are three caps in parallel across the output.
(I haven't opened it up)
I divide 55A by 2 ( N-1 to be conservative) and look for caps rated
at 25V and 27A, of roughly the same uF rating. Then I take what I
find, and use the lowest ESR and longest lifetime ratings that will
fit the case. If I can't satisfy that, then I use 55A/3 and try
again. It's very unlikely that you can't find a similar or better cap
to replace it with. If the original design used 16V parts on the
output, I could stick with that, but I prefer more margin
there. Always remember, the original manufacturer was cost
constrained, and you really aren't. Adding another $1 to the cost is
really not an issue to you, but adding another $0.05 may have been a
real battle for the designer.
The waste heat may also be affecting other devices in your system,
depending on how you deal with removing it.
Replacement caps do need to be chosen properly, but as time goes on,
caps get better and better. Today's "so-so" caps have roughly the
same ratings for ripple current and ESR as the "exceptionally good"
caps of a few years ago. Similar for transistors and diodes. By the
time yours fail, odds are that if they used state-of-the-art parts in
the design, those parts are now "good" or "average".
Switcher parts are generally available through Digi-Key, or other
similar sources.
As to what you need to repair them, I disagree. Just plain common
sense, and an understanding of how switchers work is enough.
If you see a failed component, there is a reason that it failed, and
there are other things that this failure will have overstressed.
Fix all three, if possible. Sometimes the reason for failure is a
design problem though, and that is a more difficult fix usually.
A copy of "Art of Electronics" would be a better investment, IMHO :)
Then again, the IOTA units are very reasonably priced, and you might
just prefer to replace it with the then current models.
"Fixing" an analog supply is probably easier, if we define "fixing"
as replacing the blown parts. Otherwise, it's the same as a switcher,
determine what failed, determine why it failed, and determine what
else probably took some stress, and repair all three. If the
fundamental cause is excessive heat though, you're pretty much stuck
since linear supplies are by nature, wasteful and that waste ends up
as heat, and heat kills.
A final note on electrolytic caps, their failure modes are not
limited to splattering their guts all over the inside of the box. Far
more often, they go leaky, or they go open. I've seen 1000uF caps
that read as 1uF on a cap meter, after a few years in service. Bad
caps may or may not cause problems in your system, but the problems
that they cause can be really difficult to find, if you don't
approach with a suspicious eye. Caps are one of the few electronic
components that really do have a finite lifetime, and a definite
wear-out. An open cap does not filter like it should, and a leaky
one can cause a circuit's bias to drift. Either one can cause
catastrophic failure, without it's own failure being all that obvious.
I've told this story before, but maybe not here, it's worth repeating:
Many years ago, I worked for Muzak, repairing amplifiers and such. At
the time, switcher designs were not popular yet, and we had enough
failures to keep several guys busy repairing them. These amplifiers
ranged from 10W to over 300W, and overall were not that different
from what you would use to power a similarly sized repeater. We
used replacement transistors by the bucket, some of these amps used
eight expensive transistors on the output with about a 10 x 20" heat
sink. After working there a while, I got curious why a given amp
would come back in pretty quickly after having it's output
transistors replaced, with another set of blown transistors. There
were issues at the time with counterfeit Russian transistors, but I
determined that this was not happening to us.
I finally traced it down to the electrolytics. I started replacing
the electrolytics and the failures plummeted, to the point where we
only had one guy doing repairs (me) and I was idle about half the day.
When was the last time you changed out the large electrolytics in
your repeater?
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