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 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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