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Old 22nd November 2006, 10:05 PM   #1
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Default PC smps Question

HI

Is possible +12 +5 +3.3 in series for total 20.3 V ???
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Old 22nd November 2006, 10:18 PM   #2
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Hi nicoch46,

No!
If you are talking about a PC supply.
3.3, 5 and 12V all share the same grund.
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Old 22nd November 2006, 10:25 PM   #3
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humm...

ok, -12 can mod with same diode at +12 ,

if use -12 and + 12 the diff poteanzial is 24v ?

can I use for classe-d ??

thanks for help
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Old 22nd November 2006, 10:35 PM   #4
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Hi,

-12V and and +12V could be used for +/- 12V with ground as common or +24V with -12V as commmon. -12V is usually very weak, about 0.5A or so. With 0.5A as your weakest link there should be very little current to deal with.
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Old 22nd November 2006, 10:40 PM   #5
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thanks 4fun

I reads on forum that -12v rail can mod ,only diode is weak ,the rail have the same current of +12
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Old 22nd November 2006, 10:51 PM   #6
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I have not checked that.
In a supply that may supply in excess of maybe 20A on +12V, why should they waste a transformer winding capable of 20A to an output that not need to supply more than 0.5A?
I could be wrong but I don't get it.
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Old 22nd November 2006, 10:58 PM   #7
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I search the thread.....

If I remenber well... because the trafo is the same ! at 50kHZ or so is only some coil turn, very cheap HIMO
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Old 22nd November 2006, 11:03 PM   #8
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found

link
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Old 22nd November 2006, 11:08 PM   #9
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Ok,

I shall have a look and I'll stand by.
I must tear apart one of my wasted power supplies tomorrow and have a look myself, interesting........
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Old 22nd November 2006, 11:20 PM   #10
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IIRC, there is a lm7912 on the -12V rail, limiting current to 1A maximum, provided the transformer allows for it.

possibly not all designs. I only know this because I took an lm7912 from an ATX power supply for use in a project.
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