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Old 1st March 2012, 11:00 PM   #1
diyAudio Member
Join Date: Feb 2010
Location: Austin, TX
Question How do I calculate power supply current after rectification?

So I have a power supply I'm using to power a chip amp. The transformer is rated at 250VA with 24V secondaries. I calculated the rails voltage after rectification with this equation: Er = √PR(1.4) + 4 (or, more simply, 24V * 1.4), which comes out to around 33.6V.

I would like to knock the voltage down to 15V to use with a preamp, probably with a switching regulator since I understand they're more efficient than a linear regulator. I need to understand a few things before I can do this:
  1. How do I calculate the power supply's current after rectification?
  2. How do I calculate the current going to the preamp after regulation?
  3. How do I figure out the current requirements for the preamp?

Here's an image if that helps:
Click the image to open in full size.

Thanks, thanks, thanks!
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Old 1st March 2012, 11:43 PM   #2
pdul is offline pdul  Denmark
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Join Date: Jan 2009
Location: Copenhagen
You forgot -1,2v drop in your rectifier bridge
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Old 1st March 2012, 11:48 PM   #3
pdul is offline pdul  Denmark
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Location: Copenhagen
The most simple would be to use LM317/337 as regulators for your preamp. you can draw up to 1,5 amp from those, more than enough for most preamps.
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Old 1st March 2012, 11:55 PM   #4
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Join Date: Mar 2007
Location: Auckland, NZ
PSUDII will help with the basics.

The current requirements of your regulator are specific to its circuit, as are the current requirements of your pre-amp - gotta know those first...
"It may not be easy for some to not hear differences, even if they are not there." - Vacuphile,
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Old 2nd March 2012, 12:58 AM   #5
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Join Date: Apr 2006
Location: Minnesota
You need to work backwards. Determine the output power of the amp and preamp. Then determine the power required at their inputs. This is not a simple task. Now keep going until you finally determine the total input power.
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