Bridge Rectifier question

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fum

Well-known member
Joined
Jun 3, 2004
Messages
861
Location
Seattle
Hello,

I'm having some schematic collides with the real world problems, and since I'm dealing with some high V's, I want to make sure I got it right in my head first. Here's what I got in a schematic:

pwr.gif


Now the transfomer I have doesn't have a center tap, but has the overall voltage requirements ( they say 180-0-180, I've got 360V).

Question is what needs to be done about the missing center tap?

Regards

ju
 
Use a single cap with the appropriate voltage rating, or if none is available (they would have to be HV film or oils since 'lytics don't go much above 450V) make sure to put some fairly high value resistors across each of the two in your schematic to equalize the voltages. If you pull about 100uA it should be enough. Use resistors with an adequate voltage rating.

I'm assuming when you say you get 360V that it's the end-to-end a.c. voltage at the transformer secondary, right?
 
Yes, the unloaded secondary gives me about 390V. The spec'd caps are 100uF 450V
 
Cool. Then I would put a 2.7 Meg 1/2W or 1W (for the voltage rating, not the power dissipation) across each cap. Otherwise the lower leakage cap gets a larger amount of the voltage, and in the extreme case could be running over its voltage rating---you will have about 550V d.c. overall with that unloaded secondary voltage.
 
Well yes, there is that :grin: , but in general it could be said that I've got a ton of projects going on, and if it wasn't for the help of everyone here( it's not like you haven't helped me before), half of it would stall out, or would at least take me weeks to figure.

Brad's been on my :guinness: list for a few days now, although I often feel like I'm in the corner with my dunce cap on :green:

Regards

ju
 
> Then I would put a 2.7 Meg 1/2W or 1W (for the voltage rating, not the power dissipation) across each cap.

Hmmmm... I would think maybe 1/10th that size, hundreds-K not Megs.

I suspect we are both right, to a point. Indeed two new same-batch electros really should not need resistors. Until they get old. When they get old enough, no amount of swamping will be enough.

IIRC(???), electro leakage is often specified as 10uA per uFd after a 1,000 hour life test at high heat and voltage. So 100uFd caps will leak up-to 1mA at that moment (not new, far from old). We know that modern caps usually won't leak near as bad as the spec, but we might get a leaker. Say 0.1mA in a good cap, 0.9mA in a barely-passed cap, 0.8mA unbalance... heck, round-off to 1mA unbalance. We have 273V on 450V-rated caps, so the mid-point can be 177V off. 177V and 1mA means 177K impedance in the divider, or two 354K-max resistors. Allowing 10% resistor tolerance gets 319K. Oh, round-out to 330K standard value. This has to be 1W rating to ensure it won't smoke and quit just as the cap comes up to its 450V rating.

If indeed early-life leakage current is matched to 0.1mA (true for 99.9% of cap-pairs gotten in the same shipment) then we could go 3.3Meg, as you say. Even if leakage is a bit higher, unbalanced caps tend to equalize: the one taking higher voltage leaks more. And a little leakage is not a problem until it is enough to heat the cap.

I think 3Meg is fine for surviving warranty period and even for years. I think I'd consider 330K if it was meant to last for decades.
 
> I wonder if I will outlive my caps.

Look in the WE 121A thread. Stuff made in ~1940. I suspect leaky/dry caps cause the odd bass-lump. We can't know if old-age caught up with the designer before the caps. But odds are (knowing WE gear) that it was working perfectly as recently as the 1980s, 40+ years after it was made.

I have at least one box that I tossed-together for an immedate need, 25 years ago, still in service. If I'd known, I would not have put 36V on 35V caps; but they seem to be holding. (I also put 36V on a 28V-rated chip: it failed every decade and is no longer readily available.)
 
I put 18V on some 16V caps, which was fine until they started sourcing from a Far East dirt floor operation. PAFF! after about 15 minutes for a few, enough to let slip the dogs of political war. Just about the worst disaster in my career.
 
[quote author="PRR"]> I wonder if I will outlive my caps.

Look in the WE 121A thread. Stuff made in ~1940. I suspect leaky/dry caps cause the odd bass-lump. We can't know if old-age caught up with the designer before the caps. But odds are (knowing WE gear) that it was working perfectly as recently as the 1980s, 40+ years after it was made.

[/quote]

I don't know if I'd go so far as to say they're working perfectly today, but both amps came up w/o replacing anything. Amazingly quiet but poor frequency response. The voltage odometer that came with all this stuff is at 17,817 hours so it's safe to say they've been busy.
 

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