How to reduce voltage from a car battery ?

Hi,

I would like to make a digital clock using a PIC 16F877 mcu, I need two supplies, a 5 V (max current 100 mA) and a 12 V (max current 150 mA). The device is to be powered from a car battery (12 V Lead acid re-chargeable).

I would like to know how I should reduce battery supply and current to a more tolerable level as the car battery supplies quite a high current ?

I plan to use a LM 7805 voltage regulator ICs to get the 5 V supply, and a 12 V zener diode for the 12 V supply.

Kindly advise a suitable method, the device must not overheat or consume excess current.

thanks
arunb

I wouldn't recommend a zener diode and resistor because that would mean a continious 150mA dran from the car battery.

Will your 12V circuit be able to handle 15V surges?

If so use just use the 12V directly from the car battery, you might want to use a fuse for over current protection but it's not really a problem.

You might also want to consider using a low dropout regulator which is probably a better solution.

The 12 V supply is basically meant for the seven segment displays (4 in all) . The displays would be driven through transistors.

15 V surges may not be problem, but how to limit the current to a more manageable level ??

thanks
arunb

Series resistor(s) to the 7 segment displays?

David.

arunb said:

The 12 V supply is basically meant for the seven segment displays (4 in all) . The displays would be driven through transistors.

15 V surges may not be problem, but how to limit the current to a more manageable level ??

thanks
arunb

Click to expand...

Batteries only supply the current that the load requires. You need to understand Ohm's law.

If you're worried about current then use a polyfuse to provide short circuit protection.

I'd agree with the low dropout regulator. Why deal with the hastle of blown fuses?

I think Ron H got the question.

A 12vc car battery (capable of hundreds of amps) does not supply any more current to a 12v 150mA load than a small 12v battery or transformer capable of only say 200mA. The load decides how much current to try to draw, not the battery.

Johnson777717 said:

Why deal with the hastle of blown fuses?

Click to expand...

You don't, that's the whole point of using a polyfuse rather than a normal single use fuse.