KR wrote:
> On Saturday, July 14, 2012 6:34:55 AM UTC+10, larrymo...@my-deja.com wrote:
>> On Wednesday, July 11, 2012 7:46:18 PM UTC-7, RayLopez99 wrote:
>> > On Tuesday, July 10, 2012 9:36:42 PM UTC-4, larry moe 'n curly wrote:
>> >
>> > > DSL modems are similar to
>> > > TV converters in that they both contain switching mode voltage
>> > > regulators, which can be rough on capacitors, and my DSL modems were
>> > > made with cheapo brand capacitors. So maybe it would be a good idea
>> > > to disconnect the AC power if it's not used much, and I mean turn off
>> > > the power strip/surge protector so the modem's standby power circuitry
>> > > doesn't stay on. OTOH a lot of times the standby power fails when
>> > > it's turned on, including in computer power supplies.
>> >
>> > So if I read correctly, the most likely time for a component to fail is when > the power strip is turned on? Is that right? Assume the power strip does
>> > not have APS (battery) but is just a cheap Joule surge capacitor whatever
>> > they use in cheap $12 power strips.
>> >
>> > If that's what you are saying, since there is a small temporary power spike
>> > whenever you turn on a switch (I think), please let me know as that would
>> > argue for not turning on and off the modem power supply, even though it does > have a cheap SWITCHING AC TO DC POWER SUPPLY which I think for power spike / > power surge purposes is inferior to an old, heavy, oil filled traditional
>> > wired up old-fashioned transformer.
>>
>> That old-fashioned transformer of yours must be huge to be oil filled. 
>>
>> It doesn't matter whether the power supply is connected to a plain power strip or a UPS because normal voltage surges from the AC lines will be absorbed by the line filter built into the power supply. OTOH it does seem that lots of power supplies fail at turn-on, probably because (I'm speculating here -- not an expert) of the higher currents that flow momentarily to charge up its capacitors, but PSUs are supposed to be designed to to withstand that. My main concern is with the awful capacitors used in lots of power supplies, whether the power supplies are separate (wall cube, PSU for a computer) or built-in (all those voltage regulators inside modems, routers, TVs, and monitors), and for those, I'm guessing it's best to turning off the AC when they're not used.
>
>
> There would be relatively a large "surge" current at start up, when the capacitors
> charge up, but doubt this is much of a problem in typical PC and consumer supplies.
This is true. And some supplies, the specs actually state what the surge current will
be. It can range from 40 to 80 amps for a short period of time, like a single cycle.
My ATX supply here, causes the lights to flicker when first switched on at the back,
and that's the surge. The transient is too short, for my UPS to declare the event
to be an "overload". So the duration is short.
That is the purpose of NTCR1 on the upper left of this schematic. It is the
inrush limiter, and has a negative temperature coefficient. To work properly,
when you switch off the supply, you should wait 30 seconds for NTCR1 to cool
off. Then, the next time the back switch is energized, the inrush (surge) will
be limited to the stated value in the spec for the supply. Rapidly toggling of
the rear switch, defeats the protective action of NTCR1, and could lead
to a primary side failure. It needs time to cool off.
http://www.pavouk.org/hw/en_atxps.html
>
>
> The reason caps fail in switchmode supplies (excluding the mains filter caps) is the high frequency appearing across them. There are large ripple currents involved, over very short periods, and unless the cap is designed for this (Low ESR) it will overheat, bulge and fail. Same with motherboards. Even caps supposedly designed for this can fail over time.
There are two reasons for caps to fail.
The first, is engineering. You have to take the ripple current rating of the
application, the expected temperatures into account, then select the right
capacitor or number of capacitors in parallel for the job. The Arrhenius
equation, with curve fitted exponent, helps predict how long the capacitors
will last, as part of the engineering exercise. One capacitor company claims
you can get up to 15 years from an electrolytic capacitor, before the rubber
bung on the bottom of the cap dries out, and with it, the electrolyte.
So the manufacturer thinks they last for 15 years. Less, if they're constantly
being overheated. Life is very short, if they run at 105C all the time.
The second form of capacitor failure, is purely chemical. Billions of bad
capacitors were made, with an electrolyte formula lacking a stabilizer.
Such capacitors will fail after two years, even if the power supply is
sitting on a shelf, cold. The metal corrodes, and juice leaks out the top.
The pH of the electrolyte is wrong. I had an Antec fail that way, in storage,
and there were four caps leaking inside. The capacitors do not need to be
under bias, or in stress, for a "pure chemistry failure". No engineering
equation would predict it. Because it wasn't intended to work that way.
If the chemistry is right, the caps can last a long time. My 440BX motherboard
still works for example, and must be close to 12 years old.
If you abuse a capacitor in an application (remove an OSCON and replace with
a regular electrolytic capacitor), then the capacitor would be out of its
league, and shouldn't last very long. It wouldn't be rated for the ripple.
Paul