Hi Guys,
I am making a quick car battery charger from a wall wart to get the car started (have a habit of leaving the park lights on). I am using a wall wart from an old portable TV rated 15v@ 800mA and my target is 13.7 volts @ 700mA
I quickly realized that the 15 volt wall wart puts out ~ 21 volts unloaded and then I remembered that I couldn't quite remember how to apply Ohms law in this case.
I need a bit of help.
I want to calculate the correct "dropping" resistor to place inline with the positive of the wall wart to limit the current (so as not to burn out the wall wart) and to bring the voltage down to the desired 13.7v.
When I calculate for the correct resistance using Ohm's law, how should I consider the voltage part of the Ohms law equation?
Do I use the target (13.7v) or the Voltage that my dropping resistor will "see" ; the difference between the 21V and the ~ 12.5 volts of the car battery? Ie a potential of 8.5v ?
OR
Should I calculate based on the desired voltage 13.7 ?
Help calculate that resistor and its required rating in watts. Please!
I am making a quick car battery charger from a wall wart to get the car started (have a habit of leaving the park lights on). I am using a wall wart from an old portable TV rated 15v@ 800mA and my target is 13.7 volts @ 700mA
I quickly realized that the 15 volt wall wart puts out ~ 21 volts unloaded and then I remembered that I couldn't quite remember how to apply Ohms law in this case.
I need a bit of help.
I want to calculate the correct "dropping" resistor to place inline with the positive of the wall wart to limit the current (so as not to burn out the wall wart) and to bring the voltage down to the desired 13.7v.
When I calculate for the correct resistance using Ohm's law, how should I consider the voltage part of the Ohms law equation?
Do I use the target (13.7v) or the Voltage that my dropping resistor will "see" ; the difference between the 21V and the ~ 12.5 volts of the car battery? Ie a potential of 8.5v ?
OR
Should I calculate based on the desired voltage 13.7 ?
Help calculate that resistor and its required rating in watts. Please!