Quote:
I want to change 4700uF caps (that has dimansions 30x30 mm) on bigger caps 30x40 mm. Is it fit in Lovely Cube body?
(Sory for my English. I have a little talking experience.)
You need to be careful about the size of capacitance of the smoothing capacitor you use; this is complicated, as it depends on a number of factors as to whether or not you blow up or stress your rectifier diodes at switch-on, and / or burn out the series resistors inline with the smoothing capacitors...
If you use, say 30VA 15-0-15, Toridial, (Lehmann used a 28VA with quite good regulation compared to what you can buy off the shelf), the 3.3 Ohm Carbon resistors will limit the inrush current to the 4700uF filter cap, as well as providing a basic first-order LPF. However, too bigger capacitor, you run the risk of stressing those resistors or even blowing them up.
To work this out, you need to know the internal resistance of the transformer's secondary, you can figure this out from the no load and full load output voltage, the difference between these two values times the full-load current will give you the internal resistance of the windings (Ohms law)
Then the next thing to factor is the ESR of the smoothing capacitors, this is the only thing that will limit the inrush current into the capacitor when it is fully discharged - this is given in the manufacturer's data sheet, use the lowest value for calculation purposes, as the production tolerances of Electrolytics are pretty big, typically as much as +80%!
Then through a series circuit, you can estimate the inrush current seen by the capacitors and the diodes, you can neglect the intrinsic resistance of the diodes, as this will be small.
From the current you calculate, you can then figure out whether the peak inrush current is within the repetitive ratings for the diodes (see the datasheet - there's lot's of useful information, but it's just in the form of numbers), within the I^2t characteristics of any fusing, especially on the secondary side, and within the peak power rating/ Imax of the 3.3 ohm resistors.
If you're hell bent on using a certain capacitor, like my friend had this 47.000uF 50V Electrolytics, that he mounted the rectifier and caps on the chassis, with Tag Strips. Being a NAIM Audio owner, and a believer in over-sized transformers, he went out an purchased 300VA 15V-0-15V toroid.
On power up he blew-up the T1A fuses, no surprises I guess, then we he wired out the fuses; switched on again he blew up the 3.3 Ohm resistors that he'd wired from the rectifier diodes, no surprises here I guess when you think about it. So after he replaced the resistors with wire links and switched on, all the rectifier diodes exploded into little bits - that was way cool!
He ended up getting much bigger Schottky diodes, which was fine, after wiring it all up, he blew the mains fuse on the transformer. It's not a good idea to go overrating the mains fuse on the primary otherwise you risk all sorts of trouble like blowing the internal thermal fuse of the transformer; it's only good for scrap then, as this fuse is usually buried in the middle of the windings.
So we built a soft-start circuit with a 20 Ohm 25W power resistor and a 250ms switch on delay circuit that shorts the 20 ohm fuse in series with the Active (Live) connection to the transformer - it all works fine now....it was a good learning experience if not a little expensive!
