30 Jan
2009
30 Jan
'09
10:52 a.m.
On 29 Jan 2009 at 19:49, Tony Duell wrote:
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.
Beware of the last fault. Some SMPSUs don't
have bleeder resistors on the
mains smoothing capacitor, relying on the chopper circuit to discharge
them. If this isn't running for any reason, the capacitor stays charged
and will zap you. Now how do you think UI found that out :-)
Although the basic principle is simple, SMPSU design to me is still a
black art and it seems that the practitioners of it are all older than 50. One of the better websites on the subject
is:
http://www.smpstech.com/index.htm
Other references are the sevice manauls for some early-ish devices that
used such supplies (the HP7245 printer/plotter manual springs to mind as
containing an explanation of a '2 stage' SMPSU). And the second editon of
The Art of Electronics contains a basic description of the Tandy 2000
PSU, complete with a schematic (and an insulting comment about IBM and
Apple who refused to provide scheamtics!).
-tony