12v lineal regulator

kasar · Post by **kasar** » Tue Aug 19, 2008 5:49 am

I was looking for a regulator or regulators wich can handle arround 3 Ah.

But not for buy, I have low money those days and IÂ´m very lazy to wait them meanwhile shipping two or more weeks.

but I have a lot of stuff wich can have something usefulll inside, for example, I have a few originals gc power suply (ac-dc), more than 3, and I only need one, So I can open one of them and get some stuff from inside, I also have some broken computer ATX power suply. but it still output 12v from the yellow wires. So I think I can get them from those stuff.

anyway I dont have lot of idea of how them are exactly, I think they are something black with 3 legs and needs a small cooling, them usually are hooked by a screw to a heatsink, but IÂ´m not sure.and maybe I need more stuff.

help apreciated

timmeh87 · Post by **timmeh87** » Tue Aug 19, 2008 3:05 pm

Two 1.5A linear regulators in parallel is just as good as one 3A one.

A 7812 costs less than a dollar.

kasar · Post by **kasar** » Tue Aug 19, 2008 5:55 pm

timmeh87 wrote:Two 1.5A linear regulators in parallel is just as good as one 3A one.

A 7812 costs less than a dollar.

thanks

can I put them directly like this?

e= dc in

t= ground

s= regulated output

complete diagram

timmeh87 · Post by **timmeh87** » Wed Aug 20, 2008 11:02 am

The datasheet has the exact circuit for what you want. Well, at least the last one I looked at did.

They reccomend capacitors on either side, but otherwise thats about it. 100uF seems decent.

But look it up in the datasheet anyways.

Oh yeah you will also need some big heatsinks if you are cranking 3A through that.

Post by **Skyone** » Wed Aug 20, 2008 3:39 pm

I wouldn't recommend giving anything less than 14-15 volts as an input, the regulators won't support it.

As timmeh said, if you are actually pulling 3A (the maximum for those two 7812s in parallel), then it's still going to dissipate an assload of heat. I'd really suggest a completely different regulator, but I suppose you did say you're short on money/time.

kasar · Post by **kasar** » Wed Aug 20, 2008 6:15 pm

thanks for your comments guys.

what about 3 of them in parallel?

will help a little to get it less heating? it will give arround 4,5A So it will drain less power from individual regulators and cause less individual heating, right?

my system eat arround 2,65 A (tested with a multimeter)

when I plug the charger at the system it got arround of 20v,I suppose the regulators will also able to regulate the system meanwhile charging (the charger is 3,16A , I know it will charge slower if playing at sametime, but I think it will work fine)

what about the minimal voltage the regulator can handle?

skyone said about 14-15 volts

but , shoulndÂ´t it be able to work for example with 12v (its the same voltage it should output

)

timmeh87 · Post by **timmeh87** » Wed Aug 20, 2008 8:45 pm

Well with three regulators you will still have the same total amount of heat being generated, its just less concentrated as you said. If you cant move 10W of heat out of your portable then it will ultimately overheat no matter how many regulators you use. A fan would reduce the size of heatsink needed, and might be necessary for this application.

And the regulators need about 2.5v on top of what they output in order to function properly. You can get special ones that reduce that to 1.5 or 1v.

Another option is a switching regulator, off the top of my head, a PTN78020W would work. It only dissipates 2W, but it still requires 2v more than the output to operate (14v). There may be better options though. Samples are free

And if this runs on batteries, cutting 8W out is pretty significant.

kasar · Post by **kasar** » Thu Aug 21, 2008 4:41 am

thanks

http://focus.ti.com/docs/prod/folders/p ... 8020w.html

wow, it seems very efficient

and 6 A output is more than enough

and it ask about min 7v input, fantastic!

how about the price?

Post by **Skyone** » Thu Aug 21, 2008 4:49 am

I have a few PTN78020s on my desk right now actually, they're great with PS2s.

You can get them by free sample; otherwise it's probably something ridiculous on Digikey.

Rekarp · Post by **Rekarp** » Thu Aug 21, 2008 10:03 am

Ti cards are about $8-$14 retail. I get most of mine through my University Email.

timmeh87 · Post by **timmeh87** » Thu Aug 21, 2008 10:36 am

That is an adjustable regulator. It says min 7v input because you can set the output lower than 7v.

However, its a step-down regulator, and still needs 2v extra to operate. 14v minimum in your case. It says so in the datasheet. Which you should read thoroughly.

And even though its rated for 6A, it still starts to get pretty hot at around 3A. Generally its a good idea to not operate things at their "absolute maximum" rating.

kasar · Post by **kasar** » Thu Aug 21, 2008 5:29 pm

roger that!

thanks you guys

as usual you saved me again