Car Battery Charger from Spare Parts in Mint Tin

Step 4 Theory of Operation

The LM317 is normally used to create a constant voltage. In this application its behavior is exploited to create a contant current source. The LM317 regualtes int he following manner:

The device does whatever it can to keep the OUT pin exactly 1.2V higher than the ADJ pin. For example, if ADJ is shorted to ground, then OUT is 1.2V. In this application, we have connected a 1 ohm resistor from OUT to ADJ, and connected ADJ to the car battery. If the battery is 12V, then OUT pin of the LM317 is 1.2V greater, or 14.2V. Because there is a 1 ohm resistor from out to ADJ (and car battery), and there is 1.2V across this resistor, therefore 1.2A flows per ohms law

V = I * R ===> 1.2V / 1 ohm = 1.2A ( a bit of algebra here)

The LM317 will perform exactly as described as long as the following conditions are met:
- the input voltage of the LM317 must be about 1.5V greater than the OUT pin
- The LM317 does not overheat and go into self protection mode (hence the heatsink)

Comments

[Sethro]

" If the battery is 12V, then OUT pin of the LM317 is 1.2V greater, or 14.2V. " 14.2V? shouldn't it be 13.2? if the input for the LM317 needs to be 1.5V greater than the out (which is 13.2 or 14.2), The computer charger needs to be around 16 - 17 V. Mine puts out 19V. I have an extra 18V cordless drill charger that puts out 19V 2.2A, would this work in place of the comp cord?