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Resistor to ground - suspected inrush current problem

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Dan K

pfm Member
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
 
martin clark said:
NO.

There;s something seriously effing wrong there. Be careful, and double-check ALL the wiring; draw out the whole diagram of it, as you have things connected, and take a cold hard look.
Click to expand...
In addition to that, you should get an RCD breaker installed.
 
I can find nothing wrong :-s

The traffo is 550VA and is centre tapped to the middle of the bar between the 2 PSU caps (Epcos 40V 68,000uF), via a 1R 7W Welwyn W23 resistor. The resistor to ground from the middle of the Caps was a 1W 10R item.

Why does the grounding/ zero volt ref. resistor from the traffo centre tap need to be 7w? Surely this is just here to achieve the same result as if connectig the traffo centre tap using a 't-bar' bewteen the PSU caps?

My guessing at the moment is that there are some pretty massive currents swinging around during in-rush, and before the transformer settles down, and these are finding their way to ground via the 1W reststor before the Caps have had a chance to charge and stabalise the PSU.

Thoughts?

Thanks
 
The transformer secondary is floating. You should be able to connect any single point on the winding to ground without any current flowing. If you're getting a large current, there is probably a second point also connected to ground.
 
How about posting some pics? Particularly of the transformer connections and also the little wiring diagram that's usually on the side transformer itself to show you how to connect it.
 
Short from a transformer winding to Earth somehow? Check your transformer outer for damage to mylar wrap/enamel layer on copper. Have you use rubber pads to mount it and not just put the steel disc directly in contact with the transformer (assuming it's not an epoxy-potted mounting style)? Sounds like you're very lucky that the only casualty so far is a resistor TBH!
 
Very odd this. Still looking like No Fault Found. Actually S_J this is the very same spare transformer that you swapped out for my mate to power his NCC200 amps, some years ago now.

No visable arcing under the traffo. Very odd
 
Being realistic it's too big for the Bedini 25/25 casing anyway. It fit's but only just. If I was going to use it to power my 25/25 then a whole new case would be needed.

I need something with max dimentions of 100mm x 50mm. 30 to 35V secondaries ( I want +/-28 to 30V at the rails) and 250VA+ Goss band and shield would be great as the amp boards are not far away. This transformer is 550VA and 125mm x 65mm

I'm tempted to build this whole amp into another case and then put a Le Monstre & SLB in the Bedini case
 
Maybe a Neutral to Earth short somewhere?
It is very important with toroids not to crate a shorted turn, likely if the case is not tall enough
 
I have an Antrim 2x30v @5A 50x120mm

Mint, just been a box spare.

About 5kg

Yours for £20+ p&p
 
@Dan K - I hope you understand I was blunt for effect: there was clearly something seriously Not Right, and we'd all rather you - and everyone else - lives to DIY another day.

I know from previous chat you are nearby geographically - happy to help if I can. ATB.
 
davidsrsb said:
Maybe a Neutral to Earth short somewhere?
It is very important with toroids not to crate a shorted turn, likely if the case is not tall enough
Click to expand...
Also, something that wasn't obvious to me, but if you have a loop through the transformer via the mounting hardware (e.g. bolt touching case bottom and also contacting something metallic up top) then you can have a big surprise. Here's some diagrams:

49977992061_17778680a3_c.jpg


I'm not sure how this could cause what was observed above but I guess if PSU ground touched the toroid mounting hardware something like this could happen.
 
Thanks Martin.. All good.

I've actually screwed the lid down on this amp - for now. I've been wrestling with it for maybe 4 years. I finally got the PSU utterly silent on the same day as the parts for the SLB arrived. Not making it up. The 100R front end decoupling resistor on the negative rail was just a metal jumper! In hindsight I could have found the fault by simply counting the components!!

The main reason that I kept this amp over the other 25/25s that I had repaired was that it had more of everything, but with an additional loud 100Hz buzz. I know why now, and I also know quite a bit more about amplifier design.

The only thing now is that the transformer gets hotter than I'd like. It's a 60Hz 115V 210VA item. Gets hot to touch but very slowly throughout the day, maybe 40 deg or so.
 
That'll be because the transformer will very likely be effectively undersized on UK Mains: 50/60 (=0.8) of the core CSA it needs to be for 50Hz mains. Not ideal, but liveable-with if not left 'on'.

Now we understand the desire to find a replacement trafo...
 
Dan K said:
Thanks Martin.. All good.

I've actually screwed the lid down on this amp - for now. I've been wrestling with it for maybe 4 years. I finally got the PSU utterly silent on the same day as the parts for the SLB arrived. Not making it up. The 100R front end decoupling resistor on the negative rail was just a metal jumper! In hindsight I could have found the fault by simply counting the components!!

The main reason that I kept this amp over the other 25/25s that I had repaired was that it had more of everything, but with an additional loud 100Hz buzz. I know why now, and I also know quite a bit more about amplifier design.

The only thing now is that the transformer gets hotter than I'd like. It's a 60Hz 115V 210VA item. Gets hot to touch but very slowly throughout the day, maybe 40 deg or so.
Click to expand...

I presume you are using a step down transformer then....
 
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