On 3/29/2012 7:52 AM, Jim Thomson wrote:
> 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.
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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