car head unit power supply

Hi Friends,

Could you please clarify my below doubts on power supply to a car head unit in modern cars.

1) Is the power from alternator rectified and supplied to the head unit?

or

2) Is the battery directly connected to the head?

Or

3) Is the battery power boosted to 14.4 volts and supplied to the head unit.

Or

4) Is the accessory power regulated and provides constant 12 V.

5) In what circumstances does the head unit use accessory power and direct battery power.

Thanks,
Arun

Modern cars are no different to old cars, the radio connects direct to the battery (may be via the ignition switch) - no reason to do anything else.

On the car head unit and as per the ISO 10487 connections there are two power connections- direct battery and Accessory power (A4 and A7 respectively). No sure how they're used to power the head unit.

One is permanent power, the other switched power - it's been standard on car radios for decades (the un-switched one powers the clock and maintains memory).

You can connect both together, and to permanent power, if you wish - but the radio won't turn OFF with the ignition then.

The full-time power input usually draws just a few tens of uA, while the switched power could be up to a couple of A.