car cigarette adapter - 12v to 5v

[thaiguetz]

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!

[SherpaDoug]

The basic circuit with the 7805 and two caps is OK if you don't draw much current. The circuit will generate heat as P=I * (12V-5V) so a 1 Amp load will cause the circuit to generate 7 Watts of heat, which will get toasty without heatsinking. However a 1/10 Amp load will only generate 0.7 Watts which is not bad. A simple PIC and an LED will only draw maybe 0.02 A so the 7805 will only generate 0.14 W which you will hardly notice. You have to know how much current your PIC project will draw.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[Guest]

mmm...
cool

And about the diode?

Can I use an 5v1 zener diode or a 4007/4001 ?


Do you know any schematic image to guide me? ;)

[sapy44]

I would suggest just buying a universal 12V car adapter. The have selectable output voltages in 1.5V steps to mimic battery volatges. Setting the output to 9V should do it. Heck even 12V should be fine. The prototype board form CCS feeds 12V into the 7805 and it works fine.

Make sure you get the polarity right thoug.I thinks the tip is postitive and the sleeve negative. Good luck.

[SherpaDoug]

[Guest]

mmm...

I think that i will build as follow

capacitor (1000uf) -> LM 7805 -> capacitor (1000uf) -> zener 5v1 or 4001

what do you think?

Do you think that i will be safe? ;)

[Ttelmah]

[thiaguetz]

Hi Ttelmah!!

thank u a lot for the sugestion!

But which fuse an diode i must use?

What about a 0.20mA fuse and a 4007 diode?

What do you think?

thanks again!

[Paolino]

The diode 1N4007 or the 1N4003 are both good, in this case.

Fuse: the value to be choosen depends on how much current your circuit needs... Generally you can choose a fuse able to drain a current of 10% greater than max current needed by the circuit. In this way the fuses does not burn if you have peak of current.

Anyway, I think tha the best benchmark is the experience of each one.

Hope this helps...

Best wishes.

Paolo

[thiaguetz]

mmm...

pretty cool!
I will build the schematic this week!

Thanks for all that helped me!

[Ttelmah]

[dvsoft]

Bonjour

I use a power supply 12V battery for my Motorcycle logger
5V 1A Maxi 2%

1 Circuit protection
Polyswitch http://www.circuitprotection.com

2 Schottky diode 1N5822
3 Cap 10µF
4 Cap 100nF
5 Reg LM340A or MC7805
6 Cap 220 nf
7 Cap 100µF

Extra diode 1n4148 on 5 volt Regulator
Katode on pin 1 Anode on pin 3

for the analogique power supply i use PIE filter
cap 100nf
self 100µH
cap 100nF


ALain

[burnsy]

Another spin on this problem is to use a small switcher like in my seimens mobile phone adapter. It uses a MC34063 switcher ic. I use them a lot in the carwash industry (a very noisy environment) and they work great, no heat probems. Get the circuit staright out of the motorola data book and you're on your way.
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This is the last code change until its ready....

[Bill_Smith]

Since you are going to be working in an automotive environment there are a few things to remember when connecting a low power microcontroller to your car.

1. Before starting your engine, your 12V car battery supplies power. It's voltage can vary between 10.8V and 13.6V, during cold weather cranking it might even drop to 8V!

2. Once your car is running, the alternator or generator supplies the 12V power. If your alternator looses a regulator diode, which sometimes happens, the alternator voltage might fly up to 60v! There are also "Load Dump" and EMI issues but with a good DC-DC regulator you can handle the ugly stuff.

My suggestion is to take a look at switching regulator made for automotive applications. Maxim has a nice little 8-pin part, the MAX5035, which has an input voltage range of 7.5V to 76V, and will deliver 1A of current . You can choose 4 different fixed output voltage versions, and by adding a couple of voltage divider resistors, you can set your own.

The cool thing about Maxim is that you can order free samples from their website. If you talk to their technical apps guys, you might be able to get the schottky diode and inductor for free as well. If making a switching regulator PCB scares you, they also have a complete evaluation kit you can buy for minimal cost. www.maxim-ic.com

Good Luck!

[neil]

Hello, I found a car cigarette lighter adapter for a gameboy in a cheap bargain shop this weekend. It cost (get this) 50p! I bought one and immediately took it apart (when I got home). It has a SP34063 (same as the MC34063, but made by "yingsemi"). This adapter puts out 3v at (I guess), about 0.5A. You can change its voltage by adjusting the values in the potential divider which sets o/p voltage.

So, for 50p, I got an adjustable power supply, packaged ready to use in a car!

You can also use the 34063 as a controller/driver IC for a much bigger power transistor, inductor and schottky and get several amps from it. I am considering adapting one to power my 19V laptop from the car.

Keep a lookout for game or mobile phone car adaptors. They are very simple, can often be adapted to give different voltage outputs and are nearly always dirt cheap!!

Neil.