input a 9V Bat. and get 3.3V @ 66mA to 3.3V @ 75mA How do I do it?

Macdonnie

Ok so I don't know where else to go, so I will ask here.· I need to know how to take a 9V Bat. and get 3.3V @ 66mA to 3.3V @ 75mA tops·regulated current?· What would be the easiest/cheapest way to do this.· I really need your help ty for looking.·

Oh yes and I have a Basic Stamp HomeWork Board, Adjustable voltage regulator, 100 Ohm 1/2 watt resistor, and some other parts.· But I am willing to go to·Radio Shack and pick some more parts up.·
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I need a regulated voltage for a GPS system.
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Phil Pilgrim (PhiPi)

Is your voltage regulator an LM317 by any chance? If not, what is it?

-Phil

Macdonnie

Yes it is.· LM317T

allanlane5

Well, I hope your 9-volt is more than a 9-volt "transistor" battery. Those little guys max out at like 50 mA.

Macdonnie

I was going to use a normal 9V·Duracell Alkline bat.· But if I have to I can use an old vex robotic system bat..· It's rated at 9.6V and 1000mAh Ni-Cd

Macdonnie

Here is a link to the GPS: http://www8.garmin.com/manuals/237_TechnicalSpecifications.pdf
Pg. 12-13 tells what i need

Mike Green

Download the datasheet for the LM317 from National Semiconductor's website.· They give a formula there for choosing the resistors for any voltage from 1.25V up.· For 3.3V, one example uses a 240 Ohm resistor from the output lead to the adjustment lead and a 390 Ohm resistor from the adjustment lead to ground.· Other similar values that give the same ratio should work.· I would get a 10 turn 1K trim-pot and connect it between output and ground with the wiper connected to the adjustment lead, set the pot around midrange and use a voltmeter to adjust the pot for the voltage you want (before you connect the load to the regulator).· You may have to adjust the pot a little once you connect the GPS unit to it.


Post Edited (Mike Green) : 6/4/2008 1:16:53 AM GMT

Larry

to save you a little math, there are a couple of online calculators available, so you can calculate your exact needs. R1 should stay in the 220-330Ω range

[url=http:// http://www.cpemma.co.uk/317calc.html?voltage1=3.3&res1=220&res2=394] http://www.cpemma.co.uk/317calc.html?voltage1=3.3&res1=220&res2=394[/url]
and
www.electronics-lab.com/articles/LM317/?voltage2=200&voltage1=350&dBgain=3.07

commonly available resistors are here:
www.mikroe.com/en/books/keu/01.htm (fig. 1.2:b)

Post Edited (Larry) : 6/4/2008 2:04:00 AM GMT

Macdonnie

Ty all for the replies! I think I figured out what I need to do.