16 Apr
2012
16 Apr
'12
1:59 p.m.
Tony Duell wrote:
No Tony, for sure this is not such an circuit.
I've meausred the pieces of the wirewound resistor, it must had a
resistance with approx 4-5 Kohms before. The Place for the FET is marked
GDS on the PCB and lat but not least the circuit isn't connected to the
Capacitors on the mains side.
Regards,
Holm
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I don't
know the function of the Part in question until now, I think
this makes some standby Power for the PSU itself. I've tried to trace wha=
t's
going on there, but it is getting to complicated.. 2 boards, 3
Transformers, one is connected over a diode to the wirewound resistor and
than to the drain of the FET ...???
...and not even the Pinout of the connectors on the PSU.
I think the FET was blown sometime in the past and the Resistor was
overloaded then. That dosn't prevented the PSU from working..it was only
the burning smell that alarmed me..
I don;t know this supply at all, so I am totally guessing here...
Is it possible that this is an inrush limiter circuit to prevent the PSU
taking too high a surge current when it's first conencted to the maisn
(and htus the capacitors are discharged)?
At power-on, the wirewound resistor is in series with charging circuti to
the capactors (either in series with the mains supply, or the output of
the rectifier), thuis limitign the current. Once the supply has got
going, the resistor is shorted out by that semiconductor device (are we
sure it's an FET, I ahve seen triacs and SCRs used in such circuits too).
If the cirucit fails, the PSU tries to run with the resistor in series.
The resisotr gets hot and bothered, and the chopper is passing more
current than it should be, due to the lower input voltage to that stage
(some of the mains votlage is dropepd across the resistor).
I seem to rememebr one of the DEC PSUs (H777?) has a relay used to short
out the inrush limiter resistor, and there's a fuse in series with said
resistor. If the relay or its drive fails, the fuse will blow, hopefully
before too much damage is done.
-tony