Voltage regulator or resistor?

[sopwith99]

I want to use an old laptop power supply to drive my battery charger. The power supply puts out 20v at 3.5A, I need to drop that to 12v. What is the best way for me to handle this?

Thanks,
Sop

[rich smith]

I want to use an old laptop power supply to drive my battery charger. The power supply puts out 20v at 3.5A, I need to drop that to 12v. What is the best way for me to handle this?

Thanks,
Sop

First class would be UBEC modded for 12v. El cheapo route: parallel 7812 linears bolted to piece of aluminium. Diodes and resistors not a consideration.

[gigelus2k3]

another option is to open up the adapter and look for the resistors around the TL431 chip (a small SOT23 package). Chances are you can adjust the resistors to drop the output voltage from 20V down to 15V or even lower.

Two weeks ago I did that trick on an old compaq notebook adapter that was rated at 19V/3.16A. My Turnigy charger has an OVP protection that triggers at 17.5V, so all I had to do was to adjust one resistor such that the oputput voltage dropped to 17V.

Şerban