NAD C446 PSU Common Mode Choke replacement

[theozaurus]

Hi All,

I'm in the process of repairing a PSU on a NAD C446. The common mode chokes on the input have been fried from too much current passing through them after the fuse was shorted (a stupid mistake). I think I have found the original problem, and need to replace these chokes.

The schematic lists the common mode chokes as being 10mH each. However they are in the UU9.8 and UU16 packaging (L902, L903) respectively. These packages have current ratings that are much lower (700mA for a UU16, even lower for a UU9.8), than the 1.25A slow blow fuse (FUSE901).

Surely this can't be correct? Should I favour lower inductance for higher current handling? What would be an appropriate value minimum inductance? It looks like a standard choke arrangement with the inclusion of a resistor ladder between the two. Does anyone know what the purpose of this is?

Any help would be greatly appreciated.

Cheers,

Theo

[PChi]

I think that R911, R917, R918 are included to discharge CX901, C902 etc. (which I believe is a requirement these days). It didn't used to be so one kept ones fingers from the mains connector.
I would favour lower inductance for a higher current rating and accept the increased conducted and radiated noise from the mains cable.
It's difficult to calculate the common mode inductance required without more information like how much capacitance the switching node has to the case.

[Kleinstein]

AFAIK the discharge resistors are only needs if the caps are relative large (e.g. more than 220 nF at 230 V). Still it is a nice to have feature.

The current rating of the chokes is really long time, so slower than even a slow blow fuse. So I see no need to have the chokes for 1.2 A. It's mainly important to have them to withstand the normal operation current. From the filter cap and missing PFC, I would expect a much smaller nominal current.

The two chokes don't have to be the same inductance - so maybe the smaller form factor one could have less inductance to get a reasonable current rating.

[theozaurus]

Thanks for the replies, that's been really helpful.

The schematic show it as a 25W PSU. The back of the unit says it can run from 100V to 240V. I guess this means the chokes need to handle 250mA (worse case 25 / 100, assuming 100% efficiency).

Interestingly, on Digikey the most appropriate I can find for the UU9.8 form factor is here: https://www.digikey.co.uk/product-search/en/filters/common-mode-chokes/3408553?k=uu9

The 10mH ones can only handle 200mA, but 6.8mH one can handle 250mA. So it's right on the edge by those calculations. I guess safer to opt for 6.8mH at 250mA. Then again, I live in a 230V area so in reality only expect the draw to be 109mA well within.

It seems strange that the ratings with those crude calculations are so close to the best available!

[singapol]

How about this 2A.. though you can buy 1 piece but there is a minimum monetary order? Search for others if you think it's not suitable. No worries if part is under or over speced or reliability.

http://uk.farnell.com/murata/pla10an1522r0r2b/choke-common-mode-2x1-5mh-2-0a/dp/9528423?MER=bn_search_1TP_SearchNB_9

[theozaurus]

Hi Singol,

Thanks for finding that part. That looks great for the UU16.

I'm struggling to find anything with reasonable current capacity in the smaller UU9.8 package which has a pin spacing of 7mm by 8mm is much harder. On Farnell a 1A choke is only 600uH (as opposed to 10mH). The 10mH chokes are more like 250mA.

I might get a selection and compare the awg of the windings to the original part to see what current rating the original had.

Cheers,

Theo