DC 14.4v-12v down to 3.88v help

[jer67stang]

I'm trying to take a variable 14.4v-12v from a automotive source to make a constant voltage at 3.88 or close to that, im not sure how many amps it will draw but i believe it less than 1amp I am taking out a battery in a device and hard wiring it to the car(i.e. no need to charge the device) I am planing on using a LM317T voltage regulator. and i believe i have it figure out at R1=240ohms and R2=490ohms, but not really 100% in any of this, so ive been looking for a car charger with a output around 3.8v just to make it easier. any help would great!! thanks

 

[Militoy]

Your numbers are OK - though to dial in the voltage, you'll want to use 1% resistors - and the 240 ohm resistor listed in the datasheet isn't available in a 1% tolerance. You can go with a 243 and adjust your ground resistor for an accurate final voltage - or, get as close as you can from your parts stash, and trim R2 final value by paralleling. You will need to consider the 10W you are going to dissipate at full load. A TO-220 will rise 4 degrees C per watt, junction to case - so you need to be conscious of getting rid of all that heat. 50 degrees is enough to be uncomfortable for most people to touch - so 40 degrees added to 25 ambient will be downright hot! You'll need to make every effort to shed those watts. I've seen designs where the designer used unusually large heat sinks, or dropped the voltage in 2 stages - to spread out the heat. In any case - the LM317T is a real workhorse.