The main reason for using a PC PS is the need for a very well
regulated filament voltage for the directly heated cathode tubes.
We already know about the filament voltage topic.
At my country QTH, the line voltage is sometime oscillating
(and this is not only at 50 Hz, hi!) between 180 and 220 VAC.
Anyway, I gave up and used it for testing purposes only, because
that PC style PS had not support the 30 amps continuous needed.
I opened one power transformer from that PS type and found
the 25 A rated +5V secondary having 2x3 turns from 4 wires of
only 0.6 mm diameter. The 12V windings are in series with the 5V
ones and used two wires (this also explains why some PS are
labeled for not using the max. capability from all the outputs at
the same time).
The generated heat in these PS is the most important to care of.
Many of them, when loaded at 75% of the rated power will
blow if the fan will lock.
I found that older style computer PS are MORE rugged.
The PS from old computers (servers or not what we call now
"PC") are very good.
I found a such PS at the flea market, it is almost three times bigger
than the standard now PC PS. The +5V rectifier is a 300 amps
Motorola brand, 3600 uF total input filtering, 280W MOSFETs,
two solid state relays (25 and 10 amps), and is functional after
20 years, and all for $2!
>From the time when the computers became no more for
"professionals"only, but "for home or office use only", the
quality (and I'm not talking about the technical performances
here) of some components and construction was somehow
going down.
It is not uncommon for a big company (over 1500 PCs) to need
replacing a PS at every, or few days...
Most of these PS, even used in very well known brand name
computers are made in China or Taiwan, and I have found many
times undersized components inside them (compared with the
specifications).
I learned to not trust the labels, but to look inside for size and
quality of components and construction.
Look inside one 30A PS for CB use and you will understand why.
I found an interesting article in a RadCom magazine by an DL
ham who modified a PC PS for using it with a 100W HF
transceiver, with no problem. Just used the 5V rectifier in 12V
place, rewind the output choke with thick wire and modified the
feedback resistor. Also used some simple EMI/RF filtering.
If anyone interested, I can scan it and send by e-mail.
Regarding the RFI problem, good screening and filtering can
solve it. Only to not mount the switching PS in the pi tank
compartment for not transforming the amp in a microwave oven
(hi!).
Anyway, I think that the most important for us is experimenting
(anyone remembering about "experiment or die"?).
All the Best,
73s,
Traian
steven grant wrote:
> As Rich Measures, and others has pointed out: be
> careful of the filament voltage you are useing.
> The filament voltage on many of these 5 volt tubes
> should not exceed 4.9 vac. Any higher and you cut tube
> life, and you wont get any more power. If you have
> lots of money then go ahead, better yet take my advice
> and send the savings to me :-) Call is good on qrz.com
> 73
> STEVEN GRANT W4IIV
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