Need 5V 4Amps from 9V

T

tom61

[H]ard|Gawd
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Feb 3, 2001
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I just measured the current draw from my uPC, and it's around 3.9Amps at 5 volts. I'll need to get 5V from the 9V battery I have. Since the 7805 has a limit of 1 Amp output, I was thinking of just putting four 7805s in parallel.

Any problems you see with using four 7805s in parallel, or precautions I should do (such as using diodes to isolate them from each other). Also, any more elegant solutions that don't cost very much that you're aware of?
 
putting regulators in parallel will work to get more amps, but, it can cause oscillations and some other weird stuff to happen.

look at the LM1084 regulators, they can handle up to 5 amps.
 
Looks to be what I'm looking for, thanks!

What's the difference between the IS and IT versions of the chip?
 
the difference is the package type i think. you probably want IT (TO-220 package)
 
Since you plan on running it at 80% operation full time I would recommend a LM338. Its a 5A adjustable regulator, but uses a full size TO-3 can that you can affix to a heatsink and even a small fan.

Spec page at National for LM338

Datasheet for LM338, scroll down to Circuit Hints (starts page 6). Note they also have a 15A circuit incase you want to lessen the stress on the individual compnents.

Also note that 4A drawing off a 9V battery will require a substantial amp-hour rating on the batteries part for any prolonged amount of time. Rereading your thread, I guess this is a carputer or something with a good source of power??

Parts are available from Digikey or Mouser Electronics. I would also recommed against the regulators in parrellel for the issue of stability. Amplifiers really frown at oscilation :(
 
Since you plan on running it at 80% operation full time I would recommend a LM338. Its a 5A adjustable regulator, but uses a full size TO-3 can that you can affix to a heatsink and even a small fan.
Click to expand...

Yeah, a place to mount a heatsink will help.

Also note that 4A drawing off a 9V battery will require a substantial amp-hour rating on the batteries part for any prolonged amount of time. Rereading your thread, I guess this is a carputer or something with a good source of power??
Click to expand...

It's for my Ultra-portable PC project. I'll likely have two 5.6 Amp hour batteries in parallel, for an 11.2Amp hour 9V source.
 
A passive voltage regulator is going to be hella inefficient. You're going to waste about 16 W of power and cut your run-time nearly in half. You'll also need a big, heavy heatsink, good airflow, and willingness to get burned, or maybe all three. Look for a DC-DC converter, it'll run cooler and you'll get more battery life out of it.
 
TO-220 packages have a place for heatsinks...
 
Fenris_Ulf said:
A passive voltage regulator is going to be hella inefficient. You're going to waste about 16 W of power and cut your run-time nearly in half. You'll also need a big, heavy heatsink, good airflow, and willingness to get burned, or maybe all three. Look for a DC-DC converter, it'll run cooler and you'll get more battery life out of it.
Click to expand...

I wholeheartedly agree, but I don't know of any vendors who have DC-DC converters that'll work in these parameters (9V in, 5v out, 20 watts nominal; something like 40 watts peak to be safe)

Do you have a link to such a product?
 
mikeblas said:
I wholeheartedly agree, but I don't know of any vendors who have DC-DC converters that'll work in these parameters (9V in, 5v out, 20 watts nominal; something like 40 watts peak to be safe)

Do you have a link to such a product?
Click to expand...
20W will mean buying an eighth or quarter brick, and will cost ~$50. Which could be reasonable compared to the price of a big used heatsink...

Something like a homemade, MAX787-based switcher would be good for this - there's plenty of "run a PC in your car" circuits based on this chip floating around the internet. And there's probably newer/better parts from linear.com, texas instruments, micrel, etc and probably maxim themselves...
 
Will the circuit at the bottom of page here: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1271 be sufficient for my needs? (since I only need +5V)

What size of heatsink would I need for this? I was really hoping to use nothing bigger than a chipset heatsink. Will I need more than one 787?

Also, where to source these? Digi-Key doesn't sell them.
 
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