[Amps] How to know when filter caps begin to fail

Thu Mar 29 09:05:57 PDT 2012

On 3/29/2012 7:52 AM, Jim Thomson wrote:
> Date: Wed, 28 Mar 2012 22:17:07 -0600
> From: "Jim Garland"<4cx250b at muohio.edu>
> Subject: Re: [Amps] How to know when filter caps begin to fail
> To:<amps at contesting.com>
> Message-ID:<D5A8680215A44DC98F1462A3896EA0AC at 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.

Wayyyy back I had a Dentron DTR2000 about a month old.  One of the 
filter caps blew the can right off it.  AT least it was a relatively 
small cap that was only about an inch tall and maybe 3/4" in diameter. 
That can hit the top of the amp so hard it flattened it. It looked like 
a 38 wad cutter that had hit something hard.  It was darn near as loud 
as a 38 too.

73

Roger (K8RI)

> 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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