Author Topic: Power Supply Voltage Conversion  (Read 1289 times)

0 Members and 1 Guest are viewing this topic.

Offline spoonTopic starter

  • Newbie
  • Posts: 7
Power Supply Voltage Conversion
« on: August 11, 2014, 09:15:54 pm »
Hi all,

EEV blog noob here. I have what is probably a rather noob question as well:

I'm interested in converting an old 16V 4.5A laptop power supply to 10V 4.5A (partly to actually use for a project and partly just to see if it can be done). I found the TI LM338T/NOPB, which I think would work for this, correct?

I know I also will need to dissipate the excess power, which I believe would be: (16V - 10V) * 4.5A = 27W. So I would need a heatsink capable of dissipating at least 27W.

I'm a little confused as to how I actually adjust the regulator to output 10V. I've gathered that it has to do with feedback into the adjust pin, but I'm not sure exactly how that works.

Anyway, I hope this isn't too noobish, and I appreciate any help you all might have!
 

Offline mariush

  • Super Contributor
  • ***
  • Posts: 4982
  • Country: ro
  • .
Re: Power Supply Voltage Conversion
« Reply #1 on: August 11, 2014, 09:43:07 pm »
See datasheet : http://www.ti.com/lit/ds/symlink/lm138.pdf

Page 2, footnotes.  Power dissipation is internally limited, for TO-220 packages it's 25w.   So even if you'd have a heatsink and fan capable of dissipating 27w of heat, the regulator will not do that, it will stop at 25w.

27w is A LOT. It's not the way to go.

(I suspect this is too much for you, as you really sound like a beginner) My advice would be to open up the laptop power supply and try to understand how the circuit works.  There's a chip on the secondary side which controls the output voltage - by changing one, or a couple of resistors inside the laptop adapter, you may be able to reduce the output voltage down to lower voltages. I wouldn't say 10v, but something like 12-14v should be within possibilities.

LM338 would need about 2v above the output voltage to regulate output voltage properly, so either way you'd need 12v+.

But my advice would be to forget about it and switch to something like LM1084 or LD1084 or *1084 : http://uk.farnell.com/jsp/search/browse.jsp?N=2025+225134&Ntk=gensearch&Ntt=1084&Ntx=mode+matchallpartial
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf