what is the circuitry in a dual input-voltage car phone charger?

Question

I have several multi-voltage (12/24) power supplies, for phones, PDA etc, and I would like to know what type of circuitry is used to regulate the output voltage. Given that the 24 volt input can vary from 19 to 26volts how does it regulate the output to 5 volts or whatever? How does it determine the input volts in the first place?

To elaborate, how does the charger work on the 12V in my car, and 24V in my truck, without any external switching, also allowing for fluctuations in both voltage supplies? Dave

Answer 1

Most dual input chargers have the capability of running off of so called 12v dc or so called 120 v ac, so I'm not sure if I understand your question. For pure dc chargers, it makes no difference if it is running off of so called 12 v or 24 v dc as the charger will usually accept an input up to about 30 v dc with no problem and then it's out put is regulated to 5 v dc. The regulation circuitry can be quite complex with lots of filtering etc., or as simple as using a 5 v regulator IC such as an LA 7805.
The reason I say "so called 12 v dc" is the fact that it really never is. The normal power off vehicle battery only voltage is nominally about 12.5 volts and the power on can be as high as 14.7 or .8 volts depending on the vehicles regulator setting.