Date: Wed, 28 Mar 2012 22:17:07 -0600
From: "Jim Garland" <4cx250b@muohio.edu>
Subject: Re: [Amps] How to know when filter caps begin to fail
To: <amps@contesting.com>
Message-ID: <D5A8680215A44DC98F1462A3896EA0AC@Garland>
Content-Type: text/plain; charset="us-ascii"
My experience is that the only time electrolytic caps explode is either when
the polarity is backwards or when a rectifier diode fails and AC appears
across them. I've found other age-related failures to generally be more
benign. Generally they just lose their capacitance but don't start
conducting. Now tantalum caps are another story. They almost always fail by
short circuiting, sometimes catching fire.
73,
Jim W8ZR
## I agree. With really old caps, the C just slowly drops off. On all new
supplies, I measure the
ESR and also the D factor on em..b4 they get installed. Then u have notes way
down the road.
## if u look at both united chemi con and also cornel dublier, EG eng notes,
and formulae, etc
They will have specific examples of cap usage for lytics. Like caps for
application XXX must be able
to handle 15 A CCS of ripple current 20 hrs per day...and caps must last 200
K hrs. (or 300k etc).
## If u look at their leakage current vs max stamped voltage ratings, you
will see that by operating the cap at less than
90% of its ratings has benefits....like leakage current drops. At 75-80% of
its ratings...leakage current is through the floor,
non issue. You are just asking for trbl by operating caps at 95-100% of
their V ratings. You don’t want to be operating
them at their max ripple current ratings either. Ripple current is typ 2.56
x dc plate current. The air temp, surrounding the caps
should be the same as the room temp, or maybe slightly more. A 100 k @ 3 watt
MOF is plenty for EQ. The bleeder current
drawn will easily swamp out the leakage current by at least a ratio of 15-20
or more.
## operate the caps at 75% of their V ratings,, and low temps inside the box
where the caps reside, 100k eq resistors..of the 1% tol type ,
and it will last a LONG time. The 1% tol eq resistors I get from mouser are
all exactly .2% on the low side. They are all identical in value.
No need to match em up anymore. The resulting vdc across each cap is
identical, within a few volts of each other.
High value caps like 1000-10,000 uf caps in 450 vdc are the ones that have
the real high ripple current ratings.
My 2300uf and also 2500 uf caps both have 10A CCS ripple current ratings.
The 3900 uf caps are even more. My 10,000 uf ones are 30A CCS.
## If one cap in a string goes wide open, who cares. It still has the eq
resistor across it. Its not gonna blow up. Now if any one cap shorted,
again,
nothing happens. Since I run em at only 75% of their rating... it’s a non
issue. Loads of headroom left. The energy stored up in just one cap,
operating
at 75% of its stamped 450 vdc rating is not gonna be a grenade. They blow up
cuz of AC through em, or excess VDC across em..or RVS polarity.
## here’s how u get around the problem if one leg of a rectifier fails
shorted....say one entire leg of a FWB. Just install a 1N5408 or better
yet,
the bigger 6A10 ..and wire one across each lytic in the string. The single
safety diode across each lytic is wired RVS connected !
In normal operation, they sit there..and do nothing. Now if one leg of a
FWB decides to short out, the normal deal is... you will be pumping raw AC
into the caps on half cycles. When and if that ever happened, the RVS
connected safety diode across each lytic will conduct.. (all of em)..and the
diodes simply short out the HV raw AC. Fuse blows..end of event. Nothing
is damaged, and you will never lose an electrolytic. Diodes like
the 6A10 are dirt cheap. Rated for 1 kv piv @ 6A CCS..and a 400A surge. The
actual PIV rating on em is 1200-1400 V.
## for added safety, I install a fast HV fuse between on leg of sec (either
leg) and input of FWB...like a Buss HVU-3 Use the smallest size HV fuse
that will just handle the normal average current. The peak current through
the FWB is sky high..and every 8.3 msecs.. but the average is way down. The
legs of the FWB alternate, so you end up with a 50% duty cycle on the
rectifier diodes anyway..even if using rtty-fm.
## I use a high power 50 ohm glitch R , typ a pair of 100-225 watt wire
wounds, 100 ohm each, in parallel, to make the 50 ohm glitch R assy. In the
smaller amps, its usually just a single 50 ohm-50watt WW. Just b4 the
glitch R, another HV fuse is inserted. ( 2nd hv fuse is the B+ fuse).
## any shorts, trbl in the RF deck, etc... and the 50 ohm glitch R will
LIMIT the fault current to a safe value. The HV B+ fuse will INTERRUPT
the fault current..and do so in 2 msecs.
## OK, now if u had a short in the lytics..say B+ to chassis from dangling
wires etc..... the B+ hv fuse will do nothing.... its on the wrong side of
the caps.
in this case, the 1st hv fuse, located between plate xfmr sec and FWB..will
blow open asap, protecting your FWB. I also install a fast, magnetic
hydraulic breaker
in the 240 vac primary, sized correctly of course. I use the instant trip
variety. These types have no oil in the reservoir..and open off really fast.
They are NOT heat activated either.
## lastly, I install one B+ meter directly between the B+ and the B-
of the string of caps. The 2nd B+ meter gets wired in the RF deck, again
directly between
the B+ and B-. Now if the HV B+ fuse ever blows open.... the HV meter in
the RF deck will read ZERO. Meanwhile, the HV meter in the B+ supply will
STILL read 3kv ( or whatever ur normal B+ is)
## Now if the AC HV fuse ever opens up, (located between xfmr and FWB), then
BOTH HV meters will read ZERO.
## rauch claims Ameritron uses.... “special lytics” . Formed at 600
vdc... then tested at 525 vdc... and rated at 450 vdc. Look up the
procedure used by CD
etc, and others..and it’s the EXACTLY the same. BTW, that 525 vdc “surge
rating” is only for 2 secs or less.... like for transients, spikes etc.
## The fault current is the same with any B+ to chassis short..regardless
of the amount of C used in the supply. Its just B+ divided by glitch
value.
It does form a RC time constant though, but who cares. The HV fuse that
precedes the glitch R will blow open extremely fast. 3 kv divided by say
25-50 ohm
glitch value will result in 60-120A of fault current. And 60-120 A of
fault current is gonna blow open a 1 A rated HV fuse lickety split....like in
less than 2 msecs.
Glitch stays intact every time. Forgot one thing. I parallel a bunch of
6A10’s to make one big diode. then make a 2nd similar assy. Then wire the
2 x assy’s
between B- of caps..and chassis. Then the B- can never wander more than
+ or - .7V Stick some more across the plate and grid meters and its now
bomb proof
later...... Jim VE7RF.
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