Capacitor balancing resistors.

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jack
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#16 Re: Capacitor balancing resistors.

Post by jack »

Also note that the leakage current is for caps in good condition that are well-formed.

Old caps that have been on the shelf for a while and not correctly re-formed or ones that have been mistreated in a previous life may behave differently (e.g. if bought off eBay).

When doing HV stuff (I know that this is LV), I always reformed any caps I used in critical places to bring them to a known spec, then did an internal leakage check by plotting their self-discharge characteristics.

This leads to curves like those in the enclosed plot for an PEH 169 2200uF@500V RIFA:
Cap 1.pdf
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Note that one and a half days later there is still 400V on the cap, amply demonstrating that with good caps, you definitely need to bleed them down :)
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#17 Re: Capacitor balancing resistors.

Post by Mike H »

I'll go sit on the naughty step. Image


Just couldn't believe the leakage could be that high.
 
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#18 Re: Capacitor balancing resistors.

Post by Mike H »

What threw me is Phil is using such large values.


Why are you using such large values Phil? :D
 
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#19 Re: Capacitor balancing resistors.

Post by pre65 »

Mike H wrote: Sun Feb 19, 2017 9:14 pm What threw me is Phil is using such large values.
Why are you using such large values Phil? :D
I didn't realise that 22000uf was excessively large on a solid state amp.

The PSU is 900va transformer, bridge, 1uf cap, 5mH @10A, 27200uf, 10mH @ 5A, 22000uf.

I need 60v @ 3.2A and PSU designer has been my help. :)
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#20 Re: Capacitor balancing resistors.

Post by Mike H »

Probably not then. Just haven't used values that large lately but on second thoughts it's only 4 x 4,700 -ish.

I'll stop talking now. :D
 
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#21 Re: Capacitor balancing resistors.

Post by jack »

That's a HUGE amount of smoothing :)

It's worth noting that most modern amp designs have pretty high PSRR numbers, i.e. they are mostly immune to reasonable PSU ripple.

Unless you have massive transients (I don't know you that well...), you really don't need that much...
Mike H wrote: Sun Feb 19, 2017 9:32 pm Probably not then. Just haven't used values that large lately but on second thoughts it's only 4 x 4,700 -ish.
Nope - its 27.200uF + 22,000uF = 49,200uF, i.e. 10 x 4,700uF or thereabouts.
Last edited by jack on Mon Feb 20, 2017 12:26 pm, edited 1 time in total.
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#22 Re: Capacitor balancing resistors.

Post by Mike H »

OK I'm going!

Image


:D
 
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#23 Re: Capacitor balancing resistors.

Post by Nick »

It's worth noting that most modern amp designs have pretty high PSSR numbers, i.e. they are mostly immune to reasonable PSU ripple.
Thats true, but the supply has to do more than just remove ripple, it also has to be a source of current on demand.

Though in the case of this amp, I am not so sure as the output stage has a CCS as a load. I mentioned that some months ago, but its Phils build.
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#24 Re: Capacitor balancing resistors.

Post by jack »

Oooooh....

Just noticed that t'other Nick has gone over 10,000 posts! Blimey, does he think he owns the forum or something?

Well aware of the tank requirements, but they should be pretty low with this amp, certainly a LOT less that that provided by 49,200uF....

However, I'm reminded of the Max Bialystock quote: "That's it, baby, when you've got it, flaunt it, flaunt it!"
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#25 Re: Capacitor balancing resistors.

Post by pre65 »

[quote=Nick
Thats true, but the supply has to do more than just remove ripple, it also has to be a source of current on demand.

Though in the case of this amp, I am not so sure as the output stage has a CCS as a load. I mentioned that some months ago, but its Phils build.
[/quote]

Nick, does having a CCS as the load negate the need for a low ripple PSU ?

And would your snubber PCBs be necessary, or an advantage ?
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#26 Re: Capacitor balancing resistors.

Post by Nick »

Nick, does having a CCS as the load negate the need for a low ripple PSU ?
I would th8nk so, which is why I said it a couple of months ago.
And would your snubber PCBs be necessary, or an advantage ?
That makes me unsure if you understand what the board is for.
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#27 Re: Capacitor balancing resistors.

Post by pre65 »

I'm confused. :?

I performed test 1 on the mosfet amp PSU. Got 60v at the load resistor so that is fine.

BUT for some reason I have a problem with the 22000uf cap bank. I'm using a 1K 5W resistor across each of the 4 caps, and each pair is in series and both pairs in parallel.

I noticed that on each pair, the 1K nearest the positive rail is hot (as I would expect for a 5W resistor) but on the other cap it's cold, and no voltage is measured at the mid point between, which I would have expected to be 30v.

Have I cocked up ?
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#28 Re: Capacitor balancing resistors.

Post by Mike H »

Must have.

Lower one is shorted out somehow.

Or you connected your ground to the wrong place. (?)
 
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#29 Re: Capacitor balancing resistors.

Post by Nick »

Or the cap is the wrong way around.
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#30 Re: Capacitor balancing resistors.

Post by pre65 »

Still thinking about this problem.

If the leakage current on my caps is 8ma (nearest whole number) then the resistor needs to pass 80ma, but as each cap is dropping 30v then surely each resistor should be 375R ? (ohms law)

Could this be my problem, as the hot resistor is dropping all the voltage.
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