Dave's points are good, I'd like to point out a couple
of other things.
We have used Astron supplies for years, and overall
have been happy with them. There are some drawbacks,
mainly the analog design is very wasteful of power,
and the units get very warm (hot). I have done some
measurements, and a RM-50 will waste as heat about 480
watts when putting out 40 amps at 13.5 volts. An IOTA
DLS-45 will waste about 136 watts as heat under the
same conditions. Note that the RM-50 is only rated at
37 amps continuous, while the DLS-45 will put out 45
amps all day long.
If you do the calculations on wasted heat, this comes
to an additional 4147 KWh per year on the Astron, and
an additional 1175 KWh/ year on the IOTA. If you live
in an area with 10 cents/KHh power (typical) then your
power savings in one year will be almost $300.
Many of you do not run this kind of power
continuously, but we do. Even at lower duty cycles you
will probably realize these savings within 5 years, if
you are paying for power out of your pocket.
Even if I could not fix these power supplies, and even
if I had to change it once a year, I would still be
money ahead with the IOTA. Also, we do not use ours on
commercial power, we use it at a generator site where
our cost to generate power is about 30 cents/KWh.
Did I mention that the Astron weighs 50 lbs and the
Iota weighs 5.5?
Joe
--- Dave VanHorn <[EMAIL PROTECTED]> wrote:
> 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.
>
=== message truncated ===
____________________________________________________
Start your day with Yahoo! - make it your home page
http://www.yahoo.com/r/hs
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Repeater-Builder/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
