Date: Wed, 23 Apr 2014 11:07:20 +0200
From: peter chadwick <g8on@fsmail.net>
To: amps@contesting.com
Subject: Re: [Amps] Filter capacitors
Bigger capacitors also means bigger peak rectifier currents. Depending on the
duty cycle of the load, it could lead to more transformer heating. If it's for
an SSB application, there shouldn't be a problem as long as the rectifiers
aren't too wimpy. If you're talking continuous duty, it needs looking at.
73
Peter G3RZP
## IF the xfmr is a low Z type... IE: low sec dc resistance... higher C is
usually never an issue. Sure the peak current through the rectifiers is
higher, but the duration of the pulse is a lot shorter.
Model it in PSUD...and you will see that the average current stays the same.
The average current is typ the same as the load current. On these old style
plate xfmrs..with high sec dc resistance...
that were made for choke input, they will get a little warmer when run in a
high C config. The saving grace is the duty cycle on the diodes is only
50%..whether it’s a FWB..or a FWCT.
## stuff some 1N5408s or better yet 6A10s in there..and peak current every
8.3 msecs is a non issue. Typ what happens is the Z back on the mains line to
the main panel is high enough that it wont
allow the peak currents to flow. You can model that effect in psud as well....
by adding some more Z to the primary of the xfmr in question. For an
experiment, I have doubled, tripled and quadrupled
the OEM C..and ran the supply in CCS mode.... like a cxr into a dummy load,
etc... and noticed no additional temp rise on the diodes, nor the xfmr.
## with the low Z xfmrs, higher C is a non issue. Same deal no additional
heating. On those, I have increased the OEM C by as much as 12 times..with
no increase in heating. Ripple is
then proportional to C. Double the C..and ripple drops in half. Dynamic
regulation is much improved. Static regulation is also improved. But you
al;so have to be careful with glitch R values..
and wattage of them. Also... any C plus the glitch R forms a RC time
constant. Hence my use of a fast HV fuse inline with the glitch R.
## On all these old amps and old TXs from years gone by... lytics were only
available in small values...at least ones that would fit inside the cabinet.
Drives me nuts watching the 3% ripple when looking
at my RF monitor scope..with a cxr into a dummy load. You don’t get that
with just the xcvr alone by itself...and scope re-tweaked for the same full
height display. Triple the C..and ripple is now
down to an acceptable 1%.
Jim VE7RF
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