Trying to run a 5v PIC chip from a 12v car

Nick · Guest

I am trying to get my PIC chip to run off a 12v car power source. I have a 7805 voltage regulator but I dont think it can handle the current load from the battery. When I hook it up to a 12v battery the PIC goes crazy. Im only able to take the voltage regulator to about 7 volts before everything starts getting speratic.

Thanks,

Nick

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

Look up the datasheet for the LTC1625. It has a chapter on dealing with automotive "so called 12V" power suplies (p.16 "Automotive considerations").

Good Luck,
Nick

Ttelmah · Guest

You refer to taking the 'voltage regulator to about 7v'. This presumably implies you are testing from a bench supply?. If so, you need to look at what current is being drawn, but my immediate suspicion, is that you have not got the required HF capacitor properly fitted close to the regulator chip, and it is breaking into oscillation. Most regulators are prone to this (they are very high gain amplifiers internally).
Provided you are not drawing too much current, and have an adequate heatsink, the 7805, should be able to deliver the voltage you require, _but_ (and it is a major 'but'), you need to look very carefully indeed at the design of the decoupling, and input protection of the circuit. Automotive power systems, are quite 'unfriendly' areas, and the 78 regulators on their own, can have trouble handling some of the high voltage spikes that can be present on the 12v supply. In general, you should also look very carefully at how you clamp and protect every signal that connects to the chip, if the system is to be reliable...

Best Wishes

Guest · Posted: Tue Jan 11, 2005 5:45 am

Hiya,

The LM7805 should handle upto 35v input, i've used 100's of them in the the same situation. Even the titchy 100mA versions shouldn't get warm.

If you want to have an elegant solution use a LM2575 SMPSU (National Semi.), uses a few more components, but draws almost no current at 5v (app. dependent) which could be useful if the car is stationary.

Just a point to note, automotive 12VDC isn't 12VDC, whilst the battery is charging (engine on), it should be a healthy ~14.5VDC.

Hope it work out for you.

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Nick · Guest

yea I just hooked it up, i didnt add the caps. I will fix that.

Thank you so much for the help.

Nick

menonash · Guest

If you plan to use in an automobile, then be warned about load-dumps that will fry your device. Use a TVS of some sort at the input before the regulator to suppress these spikes caused by collapsing fields during transients.

Charlie U · Posted: Sat Jan 15, 2005 11:23 am

Take a look at the National LM2940 series regulators. They are designed for automotive use and can tolerate reverse battery and momentary high voltage that is generated by the load dump. They do require larger input and output caps than other families to keep them stable, but it can be a small price to pay for robustness.