My Variac (made by Zenith of London some decades ago by the look of it) has two fuses: one in the plug as usual and another in the body of the variac. A curious looking thing with a tight, curly filament wound round a tiny former and a rating of 3.15A.
You probably want the fastest rated fuse you can get away with for a variac.
In UK equipment (with no doubling) the electrolytic is usually at least
385V - frequently 400V.In doubling types, the capacitors are usually 200V each.
The ones with PFC front end don't have a reservoir after the bridge - that comes after the PFC transistor, the most common voltage rating I've seen is
450V.
Well, 230VAC RMS comes to 325V P-P., so Philips are pushing their luck with the 350V ones they use in this scope. Especially since there will be surges above this nominal level and general power station fluctuations around 230V RMS anyway. The standard caps' voltage rating leaves precious little allowance for these expected variations.
When PC PSUs were part of my bread & butter work, I often found they'd used
150V MOVs to protect each of the series 200V electrolytics. Once or twice I encountered PSUs that had suffered a cascade failure, started by one of the under-rated MOVs breaking down.Maybe I saw a dozen or so that blew up - there must be thousands out there that didn't.
Argh! Thanks for that, Arfa. You're right of course. I still have linear PSU topology burned into my consiousness (large electrolyitcs in parallel). I need to get it into my thick head that switchers use a different approach altogether (I'm a bit slow to adapt given my advanced age).
I was going to use the existing transformers for the HT stuff and just rig up my PSUs for the sub 60V stuff, tbh. But I admit it would be messy and fiddly.
Whilst it was out of the scope I did test it and it was dead; no chopping and no supply volts even to the PWM chip. HOWEVER, ISTR from somewhere that these things shut down if they don't see a load, so I'll leave my "outthinking" behind for a mo (you're not the first to say that) and reconnect everything back in circuit and re-try. I'll report back in due course.
Indeed! I have actually ordered one (a 500W model) as I wasn't comfortable with a mere RCD in combo with a variac between the scope and the wall socket. In addition to the safety aspect conferred by a IT, it's always annoying and frightening when you blow a bloody great hole in a probe's ground shield - as I'm sure we've all done at some point - when completing an earth loop you didn't know was there. :-)
Secondly, it's not easy to safely measure
Quite, good point. I'm in the especially dangerous category inasmuch as I only know enough to be dangerous. I have gaps in my knowledge. I need to spend more time refreshing my knowledge of testing safety procedures. I do recall the old 'hand behind your back' rule from a long way back, though. And I don't fancy working two-handed with some of these DVMs you see nowadays that claim to be CAT III or IV but are anything but when you take them apart.
Many thanks for your observations here.
I managed to get a DS for the chip pretty quickly, it was the scope schematics that were the big problem. I'll have to check out that site you mentioned in your email and see what's there. I've got another 2 or 3 Philips scopes to sort out when this one's done so I'm sure it would pay off.
[...]
Cheers, Arfa. You're a diamond.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.