Design considerations for automotive related project??

[arunb]

Hi,

I am planning on making a seven segment based clock for my car, I plan on using PIC 16F690 mcu.

My question is what kind of power supply should I use for the clock, will a 7805 be reliable for this kind of an application ??

Also what other problems can be expected, current/voltage surges etc ??

Please advise..

thanks
arunb

 

[picbits]

Expect lots of problems on the power side.

Use something like the LM2940 as its been designed for use on automotive power. Also read the datasheet carefully and choose the output capacitor accordingly as it has to be within certain tolerances to stop the LM2940 oscillating.

 

[RadioRon]

I agree, you need to use a voltage regulator IC that is specifically designed for the automotive environment. The hazards that such ICs help with include:

- reverse polarity connection
- overvoltage during jump starting (to at least 24V)
- load dump. This is an overvoltage spike in the + or - direction that can be as high as 200V and occurs when high current loads are switched off (like headlights) and the parasitic inductance of the vehicle wiring causes a back EMF spike. It is short duration but may be relatively high energy.
- impulse noise due to alternator and other rotating or intermittent electrical loads
- high level RF from VHF two-way radios. This is usually only a problem if you have a radio in your vehicle.

If you know someone with IEEE membership, try and get this paper:
Welcome to IEEE Xplore 2.0: Electrical environmental characteristics for automotive electric systems

Another example of an LDO specifically designed for automotive application is the National LM90xx series, like this one:
https://www.electro-tech-online.com/custompdfs/2009/02/LM9071.pdf