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Author Topic: DC-to-DC power supply design -- best resources?  (Read 786 times)
KC2ELS
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« on: June 01, 2011, 08:06:53 AM »

I have an old Asus Eee that I'm using for digital modes.  Its power brick generates 9.5V @ 2.5A -- where can I find schematics that will help me design a power supply that can use my shack's 13.8VDC to drive this thing?  Bonus points if RFI is kept to a minimum.

Jack.
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K0BG
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« Reply #1 on: June 01, 2011, 09:46:53 AM »

Seriously? Do a simple search on Google for voltage regulators. There are monolithic ones which can handle up to 10 amps, or you use a small one with a pass transistor. The former is probably a better solution for you, and I'd get an adjustable one. They require a few more parts, but not by much.
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K2DC
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« Reply #2 on: June 01, 2011, 10:06:01 AM »

Yup, LM-338 three terminal adjustable regulator $10 at Mouser plus a few simple parts and you're in business.  Data Sheet:

http://www.national.com/ds/LM/LM138.pdf

Mouser order page:

http://www.mouser.com/Search/Refine.aspx?Keyword=lm338

GL & 73,

Don, K2DC
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W6EM
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« Reply #3 on: June 01, 2011, 07:41:39 PM »

Let's see, 9.5V @ 2.5A from your shack's 13.6V regulated supply.  Hmmm.  A series-pass regulator's drop across it will be 13.6-9.5V or 4.1V drop.  Dissipation of the power loss, in heat, may be a tad of an issue.  Why?

Well, The TO-3 case of an LM-338K, will have to dissipate that 4.1X2.5 or 10.25W of loss as heat.  Make sure you include a good sized heat sink, capable of keeping that regulator from cooking...


73.

Lee

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N3QE
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« Reply #4 on: June 02, 2011, 06:32:49 AM »

I have an old Asus Eee that I'm using for digital modes.  Its power brick generates 9.5V @ 2.5A -- where can I find schematics that will help me design a power supply that can use my shack's 13.8VDC to drive this thing?  Bonus points if RFI is kept to a minimum.

Jack.
Go to any of the major semi manufacturer's websites (e.g. www.national.com www.ti.com etc.), select power management, select either linear or switching, you'll get a little screen where you put in Vin, Vout, and current, and then get a menu of parts.

Or... go to any online marketer and buy a car charger for your Eee, put a cigarette lighter jack on your 13.8V shack supply, and you're golden.

With most laptops the constraining factor isn't the electronics in the converter itself, it's the weird-ass coaxial DC power connectors.
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W6RMK
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« Reply #5 on: June 02, 2011, 08:03:26 AM »

If you're interested in designing from scratch..
Series pass regulator is one way, as mentioned above. Almost any three terminal regulator will work, make sure it has enough power handling capability, big heat sink, etc.  (you can use a 7805 type, with a resistor network.. works pretty much the same as the 338 approach, just different resistors).  Make sure you read the data sheet and ap note about low impedance capacitors on input and output: you don't want a power oscillator on your hands..

Switching buck regulator is the other.  Lots of off-the-shelf parts that can do it with one IC and some other parts.  The key is always the magnetics.  You need to pick a part and design where YOU don't have to do a lot of trial and error with the inductor.    Here's the big challenge: the load varies a lot, and switching regulators to handle a widely varying load are harder to design and build.  It may work fine at full load, but oscillate or lose regulation at low load.  It's doable, but most converter designers start with a basic design, and iterate it a bit with a box of parts next to them.  LTSpice (free) helps. BTW, I'd hate to have to debug a switcher without an oscilloscope, unless it's a cookbook design that exactly meets your needs.

If you don't care about efficiency, go with the linear regulator. Easy, 3 or 4 parts plus a heat sink.

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AA4PB
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« Reply #6 on: June 02, 2011, 08:32:32 AM »

The linear regulator will have the least amount of RFI issues to deal with.
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