Date: Thu, 19 Apr 2012 22:47:55 +0100
From: Dan Mills <dmills@exponent.myzen.co.uk>
Subject: Re: [Amps] Capacitors vs No Capacitors
On Wed, 2012-04-18 at 18:15 -0500, donroden@hiwaay.net wrote:
> Thinking out loud.....
>
> A 15KV @ 10 mf in a Broadcast FM transmitter many years ago shorted...........
> No noticable hum on carrier ( yes, I know .... FM )
> AM noise above FCC specs ?? Probably, but they were back on the air.
If the rig was big enough to operate on three phase power then there
would only have been about 30% ripple even without a cap in there, so I
am not at all surprised that an FM TX would work without a cap.
73s, Dan.
## This is why you don’t toss out old Orr books. In the 19th edition, he
shows
3 x different configs of hv supplies, all using 3 phase. The best one is
called a
6 phase bridge, and uses 6 x legs of diodes. With just a tiny choke in the
output,and
NO hv cap at all, the ripple current is 4.2 %. Ripple freq is 6F or 360
hz.
## dahl made a lot of these 6 phase bridges. They look just like a 4 x
leg bridge,except
were wider, and had 6 x legs, 3 on top, and 3 below. And both the single
phase and 3 phase
versions all used the same small boards, which consisted of 8 x 6A10 diodes
in series. Each
diode had a mov across it. These 6A10 s were 1250 v piv rated. Each
small board was rated at
10 kv piv. Your typ 6A10 will measure 1200-1400 v piv. Since the didoes
were 6A CCS rated,
the entire 3 phase bridge assy was rated at 18A CCS. In actual usage, that
would run blazing hot,
unless air cooled. Without the air cooling, his 3 phase bridge assys are
good for 6A CCS.
## The higher voltage rated assys simply used more small boards wired in
series, like 3+ boards PER
leg, and 6 x legs in total. The biggest one I saw that he made for 3 phase
used 12 boards per leg, 72 x boards
in total. rated for 40 kv at 18 A CCS.....and all done with just 6A10
diodes. 720 kw CCS...with air cooling.
That assy was 18 inchs wide x 3 ft tall..and not very deep, perhaps 10
inchs. All the various frames and divider
walls were made from that red micarta sheeting. He used the same sheeting,
for the smaller single phase FWB,s
## I see in another tech note that another config has been used, called a 12
phase bridge. The ripple is so low,
no cap at all is required. I think Harris employed it for air craft use.
## for those lucky folks who have 3 phase power in their shack, like our
european friends, ripple freq is
still 6F, which is 300 hz in their case, since they have 50 hz power. Being
able to Top up the caps 300
times per second, instead of the usual 100 times per second is a huge
advantage. You can then get away with
a small filter cap. In NA, that would be topping the caps up at 360 times
per second, instead of 120.
And only 60 times per sec if a doubler is used. A 3 Phase plate xfmr + a 6 x
phase bridge, and one small filter cap
would be the ideal ticket. Plan B is just a single phase xfmr, the usual FWB,
and a bigger cap.
later... Jim VE7RF
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