Why a no-no to use 40/50KHz high V electros for 50/60 Hz use?

for mains or hv rails?

If you want to use smpsu caps on mains there are 2 problems:

1. the lytic caps aren't bipolar
2. the caps also arent X rated (for L-N connection) nor Y rated (For L- E connection)

If you meant using them for hv dc rails, why not?

NT

absolutely NO PROBLEM. Usually the only time this kind of thing becomes an issue is if the electrolytic is expected to handle and extreem amount of ripple such as the rather physically big 4.7 uf @ 50V electrolytics used in some monitor deflection circuits.... if you try to substitute an equal value 4.7uf @ 50V normal size (much smaller) electrolytic it will OVERHEAT and burst because the "power factor" rating of the smaller cap is not adequate. For what you described, I have used those kinds of electrolytics in 60 Hz filtering applications many times. NO PROBLEMS. electricitym

with

high

For more clarity, I was meaning the 300V or 400V electrolytics used after rectifying the mains and before 40/50KHz chopping to drive a smps transformer. Then using those on the secondary side after rectification, of a totally

50/60Hz mains transformer, with high voltage secondary. I thought the SMPS ones had a different formulation or something to allow for high frequency current demand.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

The high voltage electrolytics in SMPSs are not specced for 40/50kHz. That would be the low voltage ones on the secondary, after the high voltage DC has been chopped at 50kHz, and transformed down. The high voltage one(s) are merely the smoothers for the primary DC rail, derived from the normal line frequency (50/60Hz) input. There is absolutely no reason at all that these primary smoothers cannot be used for exactly the same purpose on normal line frequency transformer-derived supplies, such as might be found in valve amplifiers, and indeed, I have used them for such purpose on many occasions.

Arfa

are

line

occasions.

I've never actually weighed or measured any but I always thought that V for V and uF for uF that the ones from SMPS were always lighter than the the more general application ones. Therefore implying something different about their construction.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Whether or not they are made lighter ( and I suspect that in a lot of cases it's that they are made smaller to better fit switchers and modern consumer equipment physically ), if you think about it, they are just working at 100 / 120Hz fundamentally, as their primary ( ha ! ) job is just to smooth the main rail after the bridge, and the ripple from that bridge is just line frequency x 2. Whether or not there is a transformer before the bridge, does not have any basic effect on this. I can't immediately think of any other differences that would cause the caps to *need* to be any different, but I would agree that they do tend to be small compared to those found as original in amps.

It might just be a cost thing. As these amps are not generally short of internal space, perhaps it is more economical just to use a cheap 'big' one

Arfa

Reusing 100Hz reservoirs as 100Hz reservoirs is absolutely fine.

NT

I'm not sure I go along with your ripple argument here.

The ripple in a SMPS input capacitor has two components. The AC current into it at 100/120Hz, typically rather peaky as the input bridge conducts for a fairly small angle when the mains exceeds the capacitor voltage. The current being taken out of it goes into the SMPS transformer, typically a sawtooth (with gaps) at the SMPS frequency. As current in equals current out of the capacitor (when averaged), the ripple thus has roughly equal components at both 100/120Hz and the SMPS 40/50kHz. Thus this capacitor does need to be rated for SMPS frequency ripple current.

However, I agree that a SMPS rated input capacitor should still be fine for use generally at 100/120Hz (so long as its ripple current rating is adequate for the application).

Regards, Mike.