12vdc to 15vdc car charger for laptop

[kinarfi]

Thanks so much, if you want to use the LT1270 in a constant current mode, driving a bunch of LEDs in series, this works great.

 

[bigal_scorpio]

Hi mate,

Is there a formula to determine the resistor or is it trial and error?

The cable was posted on monday by air mail. I don't know how long it takes to the states but I'm guessing under a week so it should be with you soon.

Al

 

[kinarfi]

Hi mate,

Is there a formula to determine the resistor or is it trial and error?

The cable was posted on monday by air mail. I don't know how long it takes to the states but I'm guessing under a week so it should be with you soon.

Al

yes,

The feed back voltage is 1.244, so if you're string of LEDs needs 300ma, you divide 1.244 by .3 for a resistor of 4.16 ohms.
1.244 times .3 = 0.3732W so you will need a 1/2 watt resistor.
Formula would be R = 1.244 / current, wattage for the resistor 1.244 squared or 1.547536/R
This set up is basically the same as using a LM317 in current mode.
Kinarfi

 

[circuitspecialists]

12 vdc to 15 vdc circuit

HI
You need a DC/DC converter to convert the approx 12 V from your car battery to the 15 V DC that is required by your laptop. You will need to know how much current is required by your laptop in order to determine the correct DC/DC converter circuit. The laptop most likely has the battery charging circuitry built in to it so you don't have to be concerned with correctly charging the batteries in the laptop. Many companies have excellent application notes to describe how to build these types of circuits. Linear Technology, TI, Microchip, National Semiconductor, etc. are some of the many companies that have these circuits.