[Amps] Filter Capacitors

[Jim Thomson]

Date: Fri, 13 Apr 2012 10:40:35 -0400
From: "Carl" <km1h at jeremy.mv.com>
Subject: Re: [Amps] Filter Capacitors
To: "Rob Atkinson" <ranchorobbo at gmail.com>, "Roger \(K8RI\)"
<k8ri at rogerhalstead.com>
Cc: amps at contesting.com
Message-ID: <06F3644C80174F748A720A59B65AA9CB at computer1>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original

A choke is not a good idea unless great care is taken to eliminate/minimize 
the switching spike which is present with tube and SS rectifiers. A scope is 
mandatory especially if the choke and caps are not well overated for 
voltage.

For decades this spike has been mostly ignored in literature and finally saw 
daylight again when more boatanchor amps were being built and then the audio 
types got involved and developed their own versions of reality.

I first noticed it about 10 years ago when checking ripple in a 2700V PS and 
had to dig into the old handbooks and QST's and saw the "ancients" knew all 
about it. I hadnt built a choke input supply since the late 50's and that 
was simply using WW2 surplus from a BC-610 so obviously the design work was 
already done.

Carl

## You folks are making a mountain out of a mole hill.   You can easily model
all of this on Duncans  PSUD.   Link to it on GM3SEK’s  site.  The only thing you
have to tweak is to add more diodes into each leg. Change it to 10 x 1N5408 
or 6A10 and it wont crash on you.     PSUD assumes the steady state load is
ALREADY  connected to the output of the power supply.   It also has the option 
of soft start or no soft start.

##  when playing with conventional choke input supplies, not the resonant types that
henry radio used,  you are in for an eye opener.  When you 1st turn it on,  you get this
huge  yo-yo oscillation on the P-P  v waveform, that finally settles down.   what psud wont
simulate is a varying load, like ssb-cw.  Once the supply is up and running, and no more
soft start involved, is when the trbl starts up.     Every time you hit the key,  you are slamming this
big load on it...and you end up toggling between full and no load.   Every time you hit the key, 
that oscillation starts up, and P-P V is sky high, till it settles down.   On cw, that’s next to impossible. 

##  By playing around for hrs on end with psud, you can minimize the effect some what. 
Psud will handle anything from doublers to FWB  to anything else.  Then you can use stuff like
L-C-L filters or  C-R-C filters, or just C.    Then its easy to see what the P-P  ripple voltage will
be for any given load, no rocket science here...and the software is dead on for the most part. 
You can also factor in the Z of the plate xfmr, and wiring back to main panel. 

##  In these 1960’s and 1970’s  supplies, with  the typ 8 x 200 uf caps in series, like what was
used in a SB-220  or  Drake L4B, ripple is typ 3%.   Use a string of 600 uf caps..and ripple is
down to 1%.   Ripple is just inverse to C used.   Rich measures  still sells his 560uf @ 450 vdc
caps.  A few friends have used em in several projects.  They work good too. 

##  back 40 yrs ago,  200 uf caps were used, cuz that’s  all you could get, that would fit inside
the typ box.   Larger value lytics  either did not exist, or were cost prohibitive. 

##  These days, large value caps are everywhere.  And no, they  wont cook your plate xfmr either. 
I wired  8 x 2500uf caps in series  for one of my L4B supplies, and it works great.  That’s
312 uf in total.   Ripple dropped from 3%  down to just .24%   Dynamic regulation is superb, it doesn’t
budge. 

##  BTW Roger, when you short out one of these large value lytics, it does NOT cause the cap to explode,
why would it.   Try this experiment some time.   Run a pair of HV wires  out to your back yard, with  the 
contacts of a vac contactor wired across the leads.  vac contactor also 100 feet out in your  back yard. 
  Charge the cap up to 400 –450 vdc.   Energize the coil of the vac  contactor remotely.   
You will get a bang alright...and  the bang will  be  100 feet away..out in the yard.  Your cap
in the basement wont even break a sweat ! 

##  put a 50 ohm 50-225 watt glitch R in series with a FAST HV fuse....and you will never have a problem 
with any high C supply.  3 kv divided by 50 ohms = 60A of fault current.     Its 60A of fault current
regardless  of whether you have 25 uf  or  250uf.   All that happens is the glitch R will LIMIT the fault
current to a safe value.  Meanwhile the fast HV fuse will INTERRUPT the fault current within 2 msec.  

##  You naysayers  who keep saying  High C  filters  cant or wont work, or are a bomb, have yet to try it. 
There are now several HV supplies built, using high C filters.  And several  of the 7-10 kv no load variety
using 3900-5400 uf caps, 450 vdc type..and enough in series such that they are only operating at
70-80%  of their 450 vdc rating.   

later...  Jim  VE7RF