31 Jan
2009
31 Jan
'09
10:46 a.m.
On 30 Jan 2009 at 18:52, Tony Duell wrote:
Agreed, but the OP was interested in repairing them, I believe.
The design _is_ something of a black art (in
particular the design of the
transformer -- the exact characteristics of that are critical to the
design). But repairing an SMPSU that has worked (and in th case of a DEC
Q-bus box, has been made many time,s and has worked well for years in
general) is a lot easier. You know the basic deisgn ins sound, you know
that particular transformer can work in that circuit, and so on.
While repair is interesting in its own right, it's the design that
really shows the mind (or lack thereof) of the engineer. For example, it never occurred to me that putting an EMI
line filter in
front of a SMPSU, while it may not disable, may compromise its
operation.
Presumably because the SMPSU can draw 'spikes' of current when the
chopper decice turens on. Such spikes have a significant high-frequency
component..
However, I am pretty sure that any supply that had a significant
high-frequency compoent to its input current would not meet the
appropriate regulations in Europe, and could not carry a CE mark.
It would also be a poor design IMHO. You have no idea of the
high-frequency characteristics of the supply mains. While the
low-frewuency impendance is clearly pretty low (think of the short
circuit current :-)), the HF impecnace is going to be a lot higher.
How many engineers have simply spec-ed an
off-the-shelf PSU and stuck
it after one of those dandy little fuse-receptacle-power-switch-EMI
filter boxes without thinking to ask the PSU manufacturer if it was
advisable to do so? It certainly wouldn't have occurred to me.
Err, have you ever tried getting useful information out of manufacturers?
-tony