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car cigarette adapter - 12v to 5v

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thiaguetz

New Member
Hi doods,

I am searching for a 12v cigarette adapter power supply that can derive 5v to my PIC project.

But I saw some schematics that the person who made place one capacitor 1000uf on the 12v and another on the 5v side, regulating the voltage using the 7805.

And i heard that the battery can supply a voltage that we must have a diode on the 5v side to prevent some burned components.

And i heard too that the 7805 get hotter and hotter when the schematic is on.
I dont have the necessary space on my circuit box to place a heatshrink.

So my question is,
Anyone have a secure schematic that can derive 5v to my PIC project and prevent the "hothothot" problem?

I saw a lot of schematics, but no that convince me (secure).

thanks a lot!
 

Exo

Active Member
An 7805 will drive a pic without getting too hot. Question is, what is the rest of the circuit? i guess there must be other components besides the pic, what are there current requirements ?
 

thiaguetz

New Member
Hi!

The rest of the circuit is composed by a buzzer, and a RF hibrid circuit (5v too).

My preocupation is the security of all circuit.
I dont want any burned component... :)

If someone know a complete secured 5v power supply using a car battery, please.. post here, or try to explain :D

thanks!
 

Trini

New Member
Hello Thiaguez,

Consider these things:

The 7805 regulator has to drop the input voltage from 12 (or about 14 if you are using it in a vehicle, as you mentioned cigarette lighter) to 5 volts. The power that it has to dissipate is this voltage drop multiplied by the current going into it. Check it and see the watts.

These regulators have built-in protection. If the load draws excessive current the regulator heats up until it reaches a point where it will shutdown. Remove the load and it will come to life as soon as it cools off. You can place a dead short across the output and it will do the same thing without damage.

About the only thing that will damage one is the application of a reverse voltage across its output. Say, for example, you apply 14 volts across the output while there is 12 volts across the input. To safeguard against this it is wise to place a diode, like a 1N4001, across it with the cathode connected to the input pin and the anode to the output pin.

The 1000uF capacitor on the output is overkill. A 10uF is adequate. In fact if you use a very large value capacitor in this position the regulator will try to charge it with a high initial current which will cause it to shut down. When the load discharges the capacitor the regulator will start up and go through the cycle again ... and again.

Some times, depending on the length of the leads between the unregulated source and the regulator, it can go into self-oscillation and overheat and then shutdown. In this case place a .01uF from input and output to ground.

Good luck!

Trini
 
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