Looking for transformers to build +5V, +12V power supply for CD-ROM drive - diyAudio
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Old 18th September 2003, 09:43 PM   #1
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Join Date: Sep 2002
Location: Oregon, USA
Default Looking for transformers to build +5V, +12V power supply for CD-ROM drive

I'd like to build a linear power supply for my CD-ROM drive. I looked at Avel-Lindberg, but didn't find any transformers with different output voltage secondaries. One option is to buy separate transformers for the two supplies (say 15V and 8V secondaries or something). I guess another option would be to build a 12V supply and use a voltage divider to get the 5V? But then a sag in one supply would make the other one sag too... but a voltage regulator will probably take care of that?

Has anyone done this before? Are there any transformers out there that I can use for this? Anything else I need to know or watch out for?

Thanks,
Saurav
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Old 18th September 2003, 09:58 PM   #2
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Join Date: Mar 2003
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I done this before, used an 50VA 12Vac transformer.

LM7812 for the +12V dc / 1.5A
MAX724 (Switching regulator) for the +5V dc / 5A

Works very nice.
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Old 18th September 2003, 10:01 PM   #3
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I know what the LM7812 does (I have LT1085 so I'd use those), I'm not familiar with the MAX724. So it's a 'switching regulator'... takes in the 12V, and produces 5V using PWM? Doesn't this defeat the purpose of building a linear power supply though, if I use a switching regulator?
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Old 19th September 2003, 09:43 AM   #4
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I used the MAX724 to limit the dissipation loss in de regulator.

12Vac => 15V dc under load. The CDRom player I have uses about 1.5A on the 5V line, with a linear regulator this would be a loss of 1.5A x 10V = 15W. in the regulator. With the MAX724 I reduced the loss to 4W. You can add an output filter to make the dc more smooth, see the datasheet for examples.

You can find the MAX724 datasheet here:

http://www.maxim-ic.com/quick_view2.cfm?qv_pk=1269
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Old 19th September 2003, 02:21 PM   #5
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Join Date: Sep 2002
Location: Oregon, USA
Thanks, that makes more sense. I was thinking about the heatsink requirements for the 5V regulator, and wondering if I should drop some of the voltage in a power resistor and the rest in the regulator.

Thanks for the help.
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