Date: Thu, 25 Jan 2018 07:15:40 +0800
From: Alek Petkovic <vk6apk@bigpond.com>
To: Jay Sturtevant <k2ztdx@gmail.com>
Cc: amps@contesting.com
Subject: Re: [Amps] Commander HF 2500
<No. Not necessarily but what is critically important, is to check the
<value of the resistors across each of the filter caps.
<My friend, K4HB, had a filter cap erupt and burn out the section of
<board it was mounted on. He ordered a new set of caps and I advised him
<to check the resistors.
<Not a single one was in spec. The one across the faulty cap was open
<circuit.
<With new caps and resistors and a repaired board, he is back on the air
<with the 2500.
<73, Alek.
<VK6APK
## This is good advice. The V across any given cap will be proportional to
the
equalizing resistance value. Caps that happen to have higher than normal
values of
resistance will also get the highest value of dc voltage across em. The
actual value of
eq resistor is not the issue..... what Is important is to make sure ALL the
eq resistors
are the SAME value. Worse case scenario is when a resistor goes open... or
connection
at either end of the resistor is lousy. Infinite resistance across a given
cap means that cap
gets the FULL B+ applied to it !! IE: a 2800 vdc supply will apply 2800
vdc across that
one cap, which of course blows into a million pieces, damaging everything
around it.
## I use Vishay brand 100 K at 3 watt MOF eq resistors across any 450
vdc electrolytics.
These are the 1 % Tolerance resistors.....available from Mouser. 100 per
box. I bought
several boxes of em.. and tested ALL 400 of em.. they ALL measured the same
99,800 ohms.
Tolerance was zero.
## Vdrop across each lytic was identical ! The eq resistor does not have
to be wired directly
across the cap terminals. They can be several inches away if you like. BUT
the connections to
the cap have to be 100% rock solid.
## Vishay PN CPF3100K00FHB14 . Customer PN 71-CPF3-F-100K-T2 Then no
more
need to play matchup. Just grab XXX amount of resistors to do the job at
hand. My lytics have
machine screw connections and use an internal lockwasher. I use sta-kon crimp
connectors, closed
round eye types. Crimped, then also soldered. Then crank em all down tight.
I also wire a 1N5408
or 6A10 in parallel with each cap.. diode is wired RVS connected.
If any leg in the FWB or FWD shorts, you will end up with raw AC, on half
cycles, across the entire series
string of lytics. Lytics dont like raw AC. Diodes are cheap insurance, esp on
expensive lytics like the types
I use. 2100 uf, 2300uf, 3600 uf, 10,000 uf. Diodes get their own sta-kon
crimped + soldered ring
connectors.
## 100K at 3 watt MOF is a good value. Even with as many as 24 x caps in
series, the resulting
eq bleeder current swamps the internal leakage current of each cap by well
over 20 X.
Internal leakage current in any lytic drops like a rock as soon as the applied
vdc across that cap
is below its stamped Vdc rating. IE: dont run a 450 vdc rated cap at any
more than 400 vdc.
I usually run mine at no more than 70-75 % of their 450 vdc rating. There
is NO requirement
to be using lower values of eq resistance.
## max plate current you can safely run is typ way less than the cap max
ripple current rating.
Take the ripple current rating...and divide by 2.56 to get the max plate
current you can safely use.
If the amp runs at say 1.5A.... MIN ripple current rating should be 1.5 X
2.56 = 3.84 A.
Typ the bigger uf caps also have higher ripple current ratings. Like 10 A,
13A etc. You can also
parallel lytics to increase the ripple current rating.
later.... Jim VE7RF
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