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Resistor to ground - suspected inrush current problem

Dan K said:
I've just tried a spare 550VA 240V traffo from an NCC200 build in my Bedini 25/25. This one is a torroid and the original 115V is a 210VA EI.

When I flick the switch the transformer hums quietly as expected and then the ground lift resistor, that I installed from the centre of the PSU caps to the safety earth, glows orange and melts its insulation.

I quickly switched off the amp and haven't attempted a restart. The resistor is a 1W 10R and was added by me (with a 0.22uF cap in parallel) to get rid of an earth loop, I don't need it at the moment, so I could just remove it.

Why is it doing this? Is it normal to have a current flow to ground on startup (from the transformer centre tap)? Should it be a 5W resistor?

All helpful advice greatly appreciated :)

Dan
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Did you have any other pieces of equipment connected during this switch-on?
If something else is connected to earth and for some reason is pushing current through the 10R resistor in the power amp you could get this situation.

I suggest refitting the 10R and winding the supply voltage up using a variac. Monitor the voltage across the 10R as you wind the supply voltage up. Keep the 10R voltage below about 3V.
You need to find out where the earth current is coming from!
 
Is take any bypass caps, rc filters in the psu that aren't in the original spec off for starters.
 
Thanks chaps, all good suggestions. Caps are/ were Epcos 40V 68,000uF easily available from RS/ Mouser etc. £31 each, awesome things. I've now swapped back to a pair of 15,000uF LCRs that I had.

All is back to normal and music is playing through my 'stunt speakers'. Will update if anything changes :)
 
snowman_al said:
Best guess is it bursts into oscillation at times or there is a faulty replacement cap.
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How would one of the caps be faulty? High DC leakage? I can't very well test a 68,000uF cap, certainly not for uF anyway

I'm starting to believe myself a bit more regards the idea that this is all to do with the recovery time of a bog std rectifier Vs the tiny ESR of these monster caps. Faulty cap would be an easier explanation.
 
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I think what might be happening, with such large caps, is that an unbalanced effective impedances the rectifier (a weak diode) under the massive current draw on the bridge is pulling the psu output-side DC side off-centre from the transformer centre-tap for long-enough to cook the resistor to a parallel return path to 'dc 0v'. This could also help explain why one bridge gave up spectacularly.

(while the bridge might be rated '25A' or whatever, you've no idea of the matching of each of the four diodes involved esp under heavy current. And 68000uF in place of 15000uF is one heckuver a change in inrush, esp at 30VAC or more)

If reverting to more reasonable values fixes it, well, that's effectively that. And use a 35A bridge...
 
I think you likely have it there Martin :)

It's playing happily in my system at the moment as I type, although I have to say that the dynamics with the LCR caps are not quite what they were. Maybe that's my imagination.

I'll give these LCR caps a few day to break in whilst I look into an active bridge or something to allow use of the EPCOS's.

Perhaps just a 'balanced' 35A Bridge would be enough
 
Dan K said:
Thanks chaps, all good suggestions. Caps are/ were Epcos 40V 68,000uF easily available from RS/ Mouser etc. £31 each, awesome things. I've now swapped back to a pair of 15,000uF LCRs that I had.

All is back to normal and music is playing through my 'stunt speakers'. Will update if anything changes :)
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Ummm, I was pointing out that, original spec for the amp is 2 x 6,800uF caps and you had installed 2 x 68,000uF caps. Ten time larger...
then do you know what the NCC200 transformer voltage output was? Perhaps it over volted the 40 volt rating of them? Just thinking out loud.
 
snowman_al said:
Ummm, I was pointing out that, original spec for the amp is 2 x 6,800uF caps and you had installed 2 x 68,000uF caps. Ten time larger...
then do you know what the NCC200 transformer voltage output was? Perhaps it over volted the 40 volt rating of them? Just thinking out loud.
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The big UK traffo (that was previously used in an NCC200 amp) puts out 35V
 
Pete MB&D said:
So nearly 50v across the caps.

Pete
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Not sure where 50v comes from.

Are you saying that if a single PSU cap is able to pull the traffo balance to one side of the centre tap then what you have is a 0/70v single secondary? Perhaps theres an efficiency factor to be accounted for that drops that to 50v - either is not good. Std bridge rectifiers are not at all impedance balanced, mine are all over the place
 
martin clark said:
I think what might be happening, with such large caps, is that an unbalanced effective impedances the rectifier (a weak diode) under the massive current draw on the bridge is pulling the psu output-side DC side off-centre from the transformer centre-tap for long-enough to cook the resistor to a parallel return path to 'dc 0v'. This could also help explain why one bridge gave up spectacularly.

(while the bridge might be rated '25A' or whatever, you've no idea of the matching of each of the four diodes involved esp under heavy current. And 68000uF in place of 15000uF is one heckuver a change in inrush, esp at 30VAC or more)

If reverting to more reasonable values fixes it, well, that's effectively that. And use a 35A bridge...
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I was thinking along these lines earlier today.
I wondered if one of the caps was at an extreme of the spec, or perhaps the huge cap has popped one or two of the 4 diodes in the bridge - causing an imbalance.

Dan - I would recommend checking any surving bridge rect, if you are still using it.
 
I 'checked' the rectifier. ie I measured its impedance in either direction and all was as it should be. Each diode was slightly differt to the next.

I'm not sure what the max capacitance is that I can measure, maybe 500uF. Certainly not 68,000uF.
I can imagine that if one cap took longer to charge than the other then that would certainly make things worse
 
It;s not really that - but the fact that the peak current is so large, that even small differences in the dynamic impedance of each diode can lead to really quite large voltage offsets, quite quickly - esp when charging such monstrous caps
(It'd be quite a thing to measure the effect in operation...)

NB I found a schematic for a Bedini 25. There are far simpler/lower-hanging fruit than more PSU capacitance to get more out of its basic design...
 
martin clark said:
NB I found a schematic for a Bedini 25. There are far simpler/lower-hanging fruit than more PSU capacitance to get more out of its basic design...
Click to expand...

Go on...

There are a couple of diagrams on the net and I'm not sure that either are totally correct. Some have the 470uF front end decoupling caps and 100R resistors missing, some don't have the 1R resistors that are between the front end and traffo centre tap zero volt ref.

Some have a seperate rectifier board with individual diodes and some (like mine) don't

What are your thoughts Martin? These are properly fantastic amps, certainly my favorite. I'd rather listen to this thing than a KSA 50, with most speakers I prefer it to it's bigger brother the 802.
 
I'd like to see what the schematic of your amp actually is, before developing any ideas!
Certainly there are gross differences between nominally similar schematics I found...
 
So Martin, what are you thinking? Seperate regulated PSU for the front end? Seperate PSUs for each channel? Or simply push the rail voltage up to +/- 30V with a new 240V traffo?

It did sound stunning with the front end directly linked to the back (apart from the annoying 100Hz buzz) - SLB!

On balance I reckon an active bridge and back to the mega Epcos ultra low ESR caps, but before I go down that route I'll try adding a couple of 0.1uF K73-16 across the LCR PSU caps. Although normally I prefer a single low ESR cap
 
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