Powering up a 20-year old MicroVAX II

Tony Duell ard at p850ug1.demon.co.uk
Tue Feb 24 13:05:14 CST 2009

> >> I haven't been into a BA23 PSU, so this is a question, not a
> >> suggestion - does that PSU have a separate "jumper" plug for
> >> 50Hz/60Hz switch?
> > SInce the first thing an SMPSU does with the mians is rectify and
> > smooth it, it's not going to care about the mains frequency.
> Um, not quite.  If it's designed barely-sufficient, a lower mains
> frequency (eg, 50Hz when it's designed for 60) may cause output voltage
> sags as the filtering dips below what the regulation can handle during
> the non-peak portion of the mains cycle.

That is, of course correct, and therefore an SMPSU designed for a 400Hz 
(or similar0 input is not necessarily going to work correctly on normal 
50 or 60Hz mains of the same voltage.

On the other hand an SMPSU designed for 60Hz is almost certainly going to 
work correctly on 50Hz especially if it's supplying a lighter load (the 
OP had pulled the oards from the Qbus backplane, so the load was a lot 
lighter than it would be when the MicroVAX is in use).

And _no_ manufactuer is going to make an SMPSU where there's a 50/60Hz 
swtich to add extra smoothing capacitance in the former position. Period.

I think it's reasonable to assume that any supply with an 115V/230V 
voltage selector switch is going to work correctly on both 50 and 60Hz 
mains unless there's clear statement to the contrary. The point being 
that worldwide _most_ 115V mains supplies are 60Hz, most 230V mains 
supplies (at least ones used for small equipment) are 50Hz, so the point 
of such a selector switch is to allow it to be used worldwide.


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