----- Original Message -----
From: "Jim Thomson" <Jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Friday, April 09, 2010 8:13 AM
Subject: [Amps] HV lytic max V ratings..some notes.
> Somebody was asking abt how high you can safely operate a typ 450 v lytic.
> Per Cornel dublier's eng notes, buried pretty good, they say..[ and I
> don't know
> if this pertains to new ...'wonder caps' that pack a lot of uf into a
> small can,
> or the older types].. for commercial / military applications, don't run
> em over
> 75% of their V ratings. And for aerospace applications, don't run em
> over 50%
> of their ratings.
>
> That's 75% of the working vdc.. NOT the ...'surge rating'. Typ surge
> V rating is
> 525 vdc. That surge rating is typ 3-8 seconds MAX.
>
> They also depicted a graph of leakage current in micro amps VS voltage,
> from 90-110%
> of a 450 vdc cap. The leakage current dropped like a rock, when the cap
> was run at 90%
> of it's rating VS 100% . They don't show leakage current < 90%...
> but it drops even more.
>
> I notice the stock 200 uf @ 450 vdc caps in my 4 x old 1977 era Drake
> L4B's seem too last
> forever. B+ on em is 2650 vdc NO load on RX. 2650/3600 = 73.6% A
> heath SB-220 and similar
> amps with the same 8 x 450 vdc caps, run at 3300-3600 vd.... don't fare
> as well after yrs of use.
>
** The Drake secondary is 780/1065 VAC. The Heath is 860/1171 VAC. Where do
you get your numbers from?
** Most of the SB-220 series I get thru here have good original caps. Get
rid of the 30K heaters and go with 100K MOX and verify that the voltage
deltas arent excessive. I use the standard Mouser 5% Xicon, no need for 1%.
** Amazingly Im still getting mid 60's NCL-2000's in with original caps.
National used a custom built 80uF 450V rated for high ripple current, about
an inch longer than a comparable Sprague Atom.
Carl
KM1H
> The L4PS uses 100 k 2 watt carbon comp resistors's and even those
> things didn't drift anywhere
> near as bad as I thought they would. [ I replaced both the caps and the
> 100 k resistors with bigger
> caps and 3 watt, 1% tol resistor's]
>
> Regardless of hv cap value, [from 200 uf up to 4800 uf], a 100K 3 watt
> MOF works very well.
> The total bleeder current more than swamps out the leakage current of the
> caps... by a ratio of
> 10-15 : 1
>
> The absolute ideal eq resistor is the VISHAY 100k 3 watt MOF 1% Tol
> units. I bought 300
> of em [100 per box] and all 300 of em are exactly 200 ohms on the low
> side of 100 K. [.02% low]
> The kicker is.. there is no more than a few ohms difference between all
> 300 of em... and this is
> 3 x different lot #'s. NO more playing..'matchup'. Just grab as
> many as you need for a
> given project.
>
> Mouser cat part # CFP-31003FB14
> Cust PN 71-CPF3-F-100K-T2
> Vishay PN CPF3100K00FHB14
>
> These are only [I think] 50 ppm temp coefficient too. W4ZE, Tony King
> [sk] put me onto
> these Vishay / Mouser resistors a few yrs ago. Tony was fed up with
> playing matchup all the time.
>
> On these HB HV supply's I have been building, using surplus HV lytic's,
> I don't run the caps at
> more than 68-75% of their working vdc ratings.
>
> The resulting leakage current is through the floor. Also, if any
> spikes, transients, surges,etc
> gets past the 250/277/120 vac MOV protection, the HV caps will just
> absorb the spikes + transients.
>
> These surplus caps I have been using have bucket loads of uf anyway.. so
> it's not like I'm short
> of total C. As far as surplus lytics.. I have seen everything from
> 1600 uf, 2000 uf , 2300 uf
> 2500 uf, 3900 uf, 4800 uf and now 10,000 uf. There are also some good
> deals on 350 vdc rated
> caps.. you just require more of them. A buddy used a string of 30 x 5200
> uf caps on a 7700 vdc
> supply, with great success. Every config tried always had the 100 K 3
> watt mof across it... and
> also a RVS connected 6A10 safety diode. The resulting V drop across
> each cap is virtually
> identical... within a few volts. I put the Vishay resistors in the 4 x
> drake amps as well.
>
> later... Jim VE7RF
>
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