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Old 16th October 2010, 12:56 AM   #1
ew is offline ew  United States
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Default Wall wart alternative/AC power supply design & housing questions

Greetings, I have four pieces of rackmount equipment that require 200-600ma of 18VCT AC power, and each piece comes with a large wallwart that takes too much room and well as cause other neatness issues. Iím not sure Iím thrilled with the idea of daisy chaining them off one large(r) transformer, but am toying with the idea of housing four chassis mount transformers in my rackmount power conditioner (lots of dead space inside). Other than mounting the cores at right angles to one another, are there any other tips/recommendations/arguments to this idea?

The OEM powersupplies are nothing more than an EI transformer with fuses on each secondary, is there anything that could/should be changed or improved upon from the OEM methodology?

Thanks & take care!
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Old 16th October 2010, 01:10 AM   #2
sreten is offline sreten  United Kingdom
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Hi,

Converting to DC might be an idea, It keeps the charging pulses out of
the wiring to the equipment. Not many people know you can drive an AC
input with DC, all that happens is one half of the bridge is always on.

Even using the wall wart transformers you can improve matters
with adding bridge rectifiers and smoothing caps to them locally.

All that happens essentially is you lose ~ 0.7V per rail due to the extra
redundant diodes, it no big loss compared to the reduced ripple content.

rgds, sreten.

Replacing the equipments diodes with resistors for CRC filtering is an option.

Last edited by sreten; 16th October 2010 at 01:27 AM.
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Old 16th October 2010, 02:46 AM   #3
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i theory the bigger the transformer the more stable it would be. So you could get one big 18vct and go with a torrid like from antek.
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Old 16th October 2010, 05:18 AM   #4
benb is offline benb  United States
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Since these are just line-operated power transformers connected to the same source, I see no reason to mount the cores at right angles.

The single big toroid idea sounds good, but without knowing for-sure the insides of these four rackmount things, I hesitate to run them off one winding lest their almost-surely common ground cause a problem. I'd go for four 18VCT secondary windings, one for each device.
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Old 18th October 2010, 12:32 AM   #5
sreten is offline sreten  United Kingdom
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Hi, IMO one big toroid could be problematic, you'd lose fully floating supplies, rds, sreten.
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Old 18th October 2010, 04:26 PM   #6
ew is offline ew  United States
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Thanks for the input guys! I looked at toroidal transformers initally, but had a hard time finding ones that were either small enough to fit in a 1U power conditioner, or less than $40 per, or both!

Mouser has EI transformers <$7.00 per, so the fuses will probably end up costing more than the iron.
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Old 31st October 2010, 03:12 AM   #7
ew is offline ew  United States
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It turns out that the one of the two EI transformers I wanted to use wont physically fit, so I'm back to looking at the "single toroid" option.

The four rack units in question are all made by the same manufacturer & the manufacturer actually mentions daisy chaining power supply(s), and mentions that it is acceptable if balanced wiring is used exclusively. I assume this means balanced wiring between units sharing the same power supply (which I would be), and not exclusively as in throughout the whole system, (which I wouldnt be), but I've been wrong before....

Sreten, could you elaborate on losing the fully floating supply? I'm not versed in theory to quite that depth.

Thanks again to everyone for their input!
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Old 31st October 2010, 08:32 PM   #8
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by ew View Post
Sreten, could you elaborate on losing the fully floating supply? I'm not versed in theory to quite that depth.
Hi,

Any piece of equipment with in own AC supply can float its ground reference
level to agree with whatever its conected to. By using a common supply for
a number of pieces of equipment you force them to have the same level.

This either works or it does not, you'll get hum or you won't, it depends on
how they are wired up, more than likely you'd be OK with a single supply,
but I cannot guarantee it, which you can for floating independent supplies.

rgds, sreten.
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Last edited by sreten; 31st October 2010 at 08:35 PM.
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Old 31st October 2010, 09:55 PM   #9
ew is offline ew  United States
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Thanks for the quick reply, I've got an Anteck AN-0209 that I purchased for another piece of equipment needing 9VAC @ >2000ma so I'll give it a whirl setup as 18VCT & see what happens.

Thanks again!
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Old 31st October 2010, 10:21 PM   #10
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Quote:
Originally Posted by sreten View Post
Hi,

Converting to DC might be an idea, It keeps the charging pulses out of
the wiring to the equipment. Not many people know you can drive an AC
input with DC, all that happens is one half of the bridge is always on.
One thing to watch out for if trying to put DC into an AC socket is that half wave voltage doubling may be employed to obtain split rails, which won't work with DC.
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