DC-DC conversion question

Krepticor · Post by **Krepticor** » Fri Mar 31, 2006 8:35 am

i plan on using a 12v battery for many of my portables

Since this is the n64 (i'm working on it first)
where can i get a dc-dc converter that will tone down that voltage to about 7.2~8v ?

Do i also need a 7808?

timmeh87 · Post by **timmeh87** » Fri Mar 31, 2006 9:26 am

you need one or the other

why are you using a 12v battery?

http://focus.ti.com/docs/prod/folders/p ... 8000w.html

Krepticor · Post by **Krepticor** » Fri Mar 31, 2006 4:35 pm

12v battery because the dc-dc converter will increase battery life

v=IR
r= constant
v goes down I goes down total a mAhours go up

Also, the twelve volt plug i would use would run off of the laptop wall warts

so the 12v battery will work with the 7808?

timmeh87 · Post by **timmeh87** » Fri Mar 31, 2006 7:41 pm

you would accomplish the same this with 7.2v and a slightly higher mAh rating, and it would be roughly the same size.

dc-dc converters arent 100% efficent. 90% if youre lucky. the 7805 should work... i think.

Krepticor · Post by **Krepticor** » Sun Apr 02, 2006 4:43 pm

which one of these dc-dc converters would i use?

(another free sample heheheh)

timmeh87 · Post by **timmeh87** » Sun Apr 02, 2006 5:32 pm

first of all, let me explain again why i dont think you should be using a 12v battery:

one way to measure capcity of batteries is the so-called "energy density", which is the total amount of power (in Joules), divided by the weight. this gives you a numer in J/kg.

so total energy is what we are concerned about when powering something. people always talk about "volts" or "amps", but the energy (in watts, or J/s) is the volts multiplied by the amps.

so. if you have a 7.2v, 3Ah battery that weighs 1kg, and a 12v, 1.8Ah battery, with the same energy density (and therefore also weighing 1kg), they both weigh the same, take up the same amount of space, and have the same amount of power.

now with the 7.2v battery, you dont need a dc-dc for the screen, and the linear regs in the n64 are close to their dropout voltage.

With a 12v battery, you do. But dc-dc converters are never more than about 93% efficient (meaning they waste 7% of all energy put into them), and linear regs are incresingly less efficnet the further you take them from their dropout voltage. So some of the energy in the 12v battery is wasted, that wouldnt be wasted with a 7.2v battery. Assuming they have the same energy density, this means you will actually need a 12v battery that is physically bigger (in weight and/or dimensions) to power your system for the same amount of time.

now if you happen to have found a 12v battery with a signifigantly higer energy density than 7.2v batteries, then that would justify using it.

anyways, if youre set with your 12v battery because you already bought it, then go with the link to the reg i posted earlier. if you go into the datasheets, you will find tables listing the efficiency at certain input/output voltages. each model has a "sweet spot" where it is most efficient. Lots of models would probably work, but only one is the most efficient for your purposes. the one i found (above link) is pretty good for you. for the 3.3v line, get a pth08000

Krepticor · Post by **Krepticor** » Mon Apr 03, 2006 8:28 am

ok i'm sold I think the 7808 is a better choice after the research

The main reason i wanted a dc-dc convert was because i knew how to use them....voltage regulators not so much

Anywhere to get a crash course?
I'll check the wiki

BÃg Ã · Post by **BÃg Ã** » Mon Apr 03, 2006 10:49 am

yeah, try wiki, its got good info on it

Thank the Lord for Wiki

timmeh87 · Post by **timmeh87** » Mon Apr 03, 2006 11:08 am

the 7808 is less efficent than a dc-dc converter. by far. like 66% for a 12v battery.

and they are hard to find.

Krepticor · Post by **Krepticor** » Tue Apr 04, 2006 9:13 am

ok then i'll get a dc-dc converter
I can request free samples....

Just try digikey type in "7808 regulator" I found many

and Nothing will every be 100% efficient Batteries are not even 100% efficient

which dc-dc converter on the page you sent me do you recommend?

timmeh87 · Post by **timmeh87** » Tue Apr 04, 2006 10:51 am

all the converters on that page are the same. the ones that are in the EUS package have pins (that fall out easily when you solder to them

), and the ones in EUT dont. click the links that say "DIP MODULE (EUS)" and "DIP MODULE (EUT)" to look at a pdf with a picture of it.

Krepticor · Post by **Krepticor** » Tue Apr 04, 2006 6:55 pm

thanks

Krepticor · Post by **Krepticor** » Wed Apr 05, 2006 8:06 am

how would i wire upt this bad boy?

same as the 3.3v converter i sampled earlier?

Post by **Skyone** » Wed Apr 05, 2006 8:28 am

No, lemme draw up a diagram...

EDIT: It'll be up soon, I gotta goto school.

http://focus.ti.com/lit/ds/symlink/ptn78000w.pdf

Go there (PDF Warning) and check out the Standard application. All you have to do is replace that resistor on Rset (the one that says RSET# 0.05W, %1 (Required)) with a 5.99k Ohm Resistor. That's not a common value, so a 6k Ohm (6,000 Ohm) Resistor will do also.

Ok, so just follow that diagram with the input and output capacitors, and for that 1st ceramic capacitor, use 2.2uF or MORE. It must be ceramic:

Then, for the output capacitor, just use 100uF. MUST BE ELECTROLYTIC:

And for the input, put it on pin 2 (on that diagram). Voltage output (8 volts) is pin 5, which should also have the positive side of the 100uF electrolytic capcitor on it already.

Hope this helps!

Krepticor · Post by **Krepticor** » Wed Apr 05, 2006 9:04 am

ah yes when all else fails ASK THE COMPANY!