[Amps] Filter Capacitors
Carl
km1h at jeremy.mv.com
Mon Apr 16 08:45:15 PDT 2012
----- Original Message -----
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Monday, April 16, 2012 7:56 AM
Subject: [Amps] Filter Capacitors
> 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.
>
Thats because PSUD completely ignores the basics.
> ## By playing around for hrs on end with psud, you can minimize the
> effect some what.
More stupidity.
If you want to use a choke input on a real PS without ringing then you have
to bleed it heavy and use a sane value of filter, 8-12uF is sufficient for
acceptable ripple. Most of the old ham TX with a choke used too much L and
not enough R in order to save weight on the power xfmr for desktop rigs. On
a class C rig the modulator in class AB or B was a steady load in addition
to a hefty bleeder current. Key the Class C amp PS supply relay and there
was no spike. Look at an old Collins KW-1, 30K, etc, they were clean. A
Johnson Desk KW also.
> 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%.
Who cares, nobody can hear it.
> 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.
But not as replacements in PS with marginal transformers. It might be OK for
casual no coders but certainly for serious SSB, CW and digital....or AM.
>
> ## 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.
And the transformer is cooking along with it. You now are pulling the
difference between what the original did with the sag and now with no sag,
there is no free lunch as the transformer is now working harder.
Put an IR gun on it.
>
> ## BTW Roger, when you short out one of these large value lytics, it does
> NOT cause the cap to explode,
> why would it.
Thats more CB type talk. Shorting out a big cap CAN rupture it, its not a
given but when it does happen you may be in for a surprise. A cap should be
left to discharge thru its bleeder resistors until down to about 10% voltage
and the thrus a 10K o so resistor on the chicken stick. Hook the stick
directly from the HV to ground while working on the PS/amp to prevent a
partial recharge
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.
And all the necessary peripheal parts needed for safety add to complexity
and cost and the end result is zero added value to the amp unless the xfmr
is overated to start with. A kit step start wont do the job either and when
the relay shorts as they do...especially in Ameritrons.....then the xfmr
goes next.
For those with commercial ham amps its dangerous and stupid.
For HB the builder better have a clue what he is doing....many dont and just
get carried along with the hype.
Carl
KM1H
>
> later... Jim VE7RF
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