Bench Power Suppy

Discussion in 'The Projects Forum' started by awwende, Jan 29, 2010.

  1. awwende

    Thread Starter Active Member

    Feb 17, 2009
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    I'm building a power suppy out of 2 regulated 24v @6.5A single output power supplies, both are over-voltage and short-circuit protected.

    I'm going to have 2 adj outputs one positive, the other negative. another adj regulator is going to be used as a +5,9,12v output, using a rotary switch to change different resistance amouts to control the output. finally I'll have a 5v output which will charge my ipod and play music out of it (i threw that in there, just because i could and i like listening to music when i work on my circuits :) )





    so my question is if my 24v is regulated, do i need to use filter caps in my regulator circuits?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You dont need a filter, but you do need a (close to the regulator) bypass to control potential oscillation of the regulators themselves. Look at the data sheet for the regulator. It will show the minimum input capacitor to be located "close".
     
  3. awwende

    Thread Starter Active Member

    Feb 17, 2009
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    ok thanks. should i attach it to the input or output of the regulator?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Like MikeML said, you will need a small "bypass" capacitor from the input of the regulator to ground; somewhere in the range of 0.1uF to 0.33uF - consult the datasheet of the regulators you are using to determine the manufacturer's recommendation.

    You may also wish to use a 10uF and 0.1uF capacitor from the output to ground to improve the regulator's transient response.

    Note that using linear regulators to regulate from 24v to 5v will make them dissipate a lot of power. If you have a 1A load on a 5v output, this means that the load will dissipate 1A x 5V = 5Watts of power, and the regulator will dissipate (24v - 5v) x 1A = 19 Watts of power.

    You will need large heat sinks on your linear regulators.
     
  5. awwende

    Thread Starter Active Member

    Feb 17, 2009
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    ok thanks.

    since you mentioned it, i was planning on using a large heat sink. any clue as to how big of a heat sink i'll need? I'm using 2 BJTs for the adjustable regulators to allow for more output current. I've got a chunk of aluminum i was planning on using that's 9x2.5x0.125" which should be plenty.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Sounds like you need to get rid of ~30W. To keep the flanges of your transistors to ~75degC, starting from room temp of 25degC, that is a 50degC rise, so you would need a heatsink with a thermal resistance (theta) of 50degC/30W = ~1.7degC/W. Look at these extrusions. Most of them have a theta of ~5degC/W per linear inch of extrusion. Your flat plate is WAY too small.
     
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