I was just robbing an old dead PC Power supply of some bits, and I saw this inside, and wondered what it was for?
It looks like a transformer, but it only has two wires connected to it, one of them looks to be connected to one leg of a bridge rectifier, the other, not so sure at the moment...
I haven't seen one inside a PSU before - it was screwed to the inside of the PSU case, separate from the main board.
Ah, okay, but this one is pretty big
280grams 5x4.5x3.5cmNormally they just have these in them from what I have seen before
I suspect this was an SMPS, and it was in series with the input (some rough smoothing on the rectified DC before it went to be chopped up again).
Cannot see it of course but there are significant attempts in at least some to suppress the crap switch mode supplies give out. This might be part of that. Brian
Most likely that is what the choke is for, more modern designs tend to have active PFC allowing the choke to be smaller and thus cheaper.
Of course some "ultra budget" PSUs still don't have any chokes or PFC correction but hopefully there are not too many of these around these days...
In which case, it's a pretty BIG choke.
Possibly it would be better to describe it by the more general term 'inductor'. Its function might have nothing to do with suppressing interference.
;)
Ok, it's primary job is to improve the PFC thereby increasing efficiency and reducing harmonic noise. Since the reduced noise is a regulatory requirement it can still be argued that it's part of its primary function....
Lee
However, I can't help thinking that it's unlikely that an inductor would be used in a PFC circuit. It's more likely to be an essential part of the guts of the actual switchmode circuit (although, to me, these beasts tend to fall in the 'Act of God' category).
No THESE are pretty big chokes
most probably its output smoothing..
well actually that is exactly wrong in an SMPS which is oversupplied with capacitors as part of its design.
The PFC section of switchers that employ that type of front end, does indeed usually use an inductor, which normally appears to be roughly between the bridge and the main filter cap. However, I don't believe that this is such an inductor for two reasons. First, PFC stages run at high frequency, much like the main supply that follows them, and this requires that the PFC inductor has a ferrite core. The inductor shown in the photo clearly has some kind of laminated iron core, which puts it firmly in the low frequency category. Also, the inductor usually has an ancillary winding on it, which connects back to the zero crossing detector pin on the controller IC, and again, it is clear from the photo that no such additional winding is present. I have also never seen a PFC stage inductor mounted off the board. Indeed, considering the frequency that they are running at, flying leads to an off-board inductor, would be asking for EMC problems.
I am also loathe to believe that this is any kind of interference suppression inductor, as the hash that switchers generate, ranges from several tens of kHz to the low megs, so again, a ferrite cored inductor is what is required to block this noise.
Arfa
If it's connected between the bridge and the storage cap, it will spread and flatten the current peaks which replenish the storage capacitors at the mains cycle peaks, and this will improve the power factor. Off board could be because it's a retrofit on top of an earlier design without PFC.
It would also provide inrush current limiting, and could correct the tiny leading phase shift, although I doubt either of these are its primary purpose.
It's not.
I think you are right. It probably is an early crude PFC corrector. It's only in the last few years that designs using chip-based PFC correction, have become the norm amongst better quality switchers. I guess since the eco-bollox hysteria took hold ... :-)
Arfa
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