> -----Original Message----- > From: cctalk [mailto:[email protected]] On Behalf Of tony duell > Sent: 01 November 2015 18:47 > To: General Discussion: On-Topic and Off-Topic Posts > Subject: RE: Testing H7864 (MicroVAX II) PSU With No Load > > > > > 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. >
You mentioned DEC PSUs, this one is actually made by ASTEC. I will use a dummy load and put it on an isolating transformer when using the scope. Regards Rob
