1 Nov
2015
1 Nov
'15
10:47 a.m.
If it has a transistor at the primary side, and a high frequency transformer, it's a
switching supply.
Those want a load, usually. Especially the older ones.
If there are large-ish electrolytics (100's of uF) with a voltage rating around 200V
and no reason to
believe that there is a 200V output rail then it is very likely to be a switcher with
those being the
rectified mains smoothing capacitors.
Some SMPSUs (Switchers -- it stands for Switch Mode Power Supply Unit) need a load,
other's
are happy without one. Occasionally DEC put the specification of the supply in the
printset -- they
did for the 11/730, and IIRC that supply is happy with no load on all outputs apart from
something
odd like the +12V rail. I've yet to find a DEC PSU that fails destructively if not
loaded, it is more likely
to just trip the overvoltaeg protection crowbar, shut down, then try to restart and repeat
the cycle,
tweeting as it goes
If it has a laminated iron transformer, big
electrolytics for filtering, and transistors at the
secondary side, it's probably a linear regulator supply. Those don't need a
load. You
may want to do full testing under load to observe correct regulation, but for initial
testing
you can test them without.
Be careful!. There is a third type of supply, and it's one that DEC used a lot, going
back at
least to 1970 (that is, it turns up in most older PDP11s, etc). This has a big laminated
core
mains transformer, rectifiers and smoothing and then _switching_ regualtors (not linear)
to produce the right output voltages. As with all switchers, that type might need a load
(but my experience is that the DEC ones don't).
Linear supplies are older; I think by the time of VAX
you're likely to see switchers (with the
possible exception of first generation stuff like the 780, I don't remember if those
big
supplies are linear or switchers).
Certainly the 11/730 is a mains switcher (it rectifies the mains to get about 350V DC,
then
chops it). In fact it's 4 SMPSUs in that little box.
The 11/780 draws a lot of power. I don't think they would have used linear regulators
(which
by definition are inefficient) in that machine.
I would bet quite a lot that all MicroVAX PSUs are mains switchers
Also watch out that DEC somethimes had a little linear PSU to power the SMPSU control
circuitry, at least before the SMPSU had rattled into life. The control circuitry
(oscillator,
regualtor, etc) is on the _output side_ of the isolation barrier in such supplies, the
drive
signal is transformer-coupled to the base of the chopper transistor. Those turn up in
all sorts of things, the VT100 being the most common example.
I hate to say this, but DEC seemed to like complicated and unusual power supply designs,
and I would not want to try to repair one without a good schematic.
-tony