Best way to run a 328 from a car battery

[FFMan]

My little shift light project proved solid and reliable during the weekend race test. So now the prototype needs to get small and most importantly powered from the car (the prototype was powered by a lipo).

The race car doesn't have an alternator but does have an unsuppressed igntion system. I've been using a standard linear reg with the lipo but will this be ok on the batt, and should i place a few capacitors here and there to filter any spikes ?

What circuit should I use to run from the battery ?

The prototype was also in a grounded metal case, is this a sensible carry over to the final verison given the RF emanting from the HT side of the ignition ?

thanks

[dkinzer]

[FFMan]

that is sobering reading, i must have been lucky so far but i need to take some steps clearly.

thanks don

[spamiam]

[twesthoff]

Many of the major semiconductor manufacturers now make special IC's for use in automotive applications. TI makes voltage regulators that are designed to withstand the normal automotive environment. One example is: TLE4275 5V regulator. The voltage regulator is only one part of the solution and some of the other ideas suggested may have to be done.

See:
http://focus.ti.com/lit/sg/sgzb002e/sgzb002e.pdf

http://focus.ti.com/apps/docs/appcategory.tsp?appId=209

I'm sure other manufacturers have similar information available.

If you let us know some details about the inputs and outputs you circuit is using, we may be able to suggest what to try.
Tom W.

[stevech]

Toyota's people that mistakenly believe that the engine control unit is flawless need to that document. And review the how-to on life-critical firmware design and test.

[FFMan]

this app is very simple. There is an rpm interface module from Stack (http://www.stackltd.com/index.html) that has a single wire to the switched side of the coil. the interface cleans this up to a nice square wave and this is fed into a pin on a zx24 (to be changed to a 328l or 328n in final model) where i use caputreinput to provide the timing info to work out the rpm. The interface is currently powered from my onboard 11v lipo.

the outputs simply consist of 4 LEDS in a pod each with associated transistor driver.

on the enclosure itself (which is metal at the moment and grounded) there is a 3 position (centre off switch) that starts the code with one of two settings (wet or dry) simply by optionally grounding a pin input.

There is also an activity LED that flashes at various rates but this may be ommitted in the final version.

Any help on how to run this from the 12v batt direct much appreciated.

[FFMan]

i should add i used this circuit to good effect before http://www.seetron.com/an_vpwr1.htm to run a picture-in-picture box in the car that used to be 240v powered and used for CCTV. It works a treat but the regulator alone is £20 for the 1 amp version, but i'll be ok with the 400ma verison in this app which uses a cheap linear reg.

[twesthoff]

That power supply circuit should do well. Use the analog version, that regulator is very cheap. Don't use R1 (as it mentions). Put the regulator circuit and all of the rest of the parts in the metal box and you should be good to go.

Mount the tab of the 7805 regulator to the metal box to act as a heat sink.

The RPM interface module you are using should protect the input lead.

Tom W

[spamiam]

I have had a LM7805 fail in my car. It did not have enough protection, but it still is not rated for automotive conditions. It needs to tolerate jump starting, and load dump, at the very least. As I said, I have had good results with a LM2940, and it is reasonably inexpensive!

The zeners are OK in the circuit from Nuts & Volts, but you are probably better served by TVS diodes, which are designed for this purpose. Unidirectional will be fine, rather than bidirectional. They can handle the power of the spikes. A plain old zener may not. Something like a 1.5KExxA, where xx is the breakdown voltage. A MOV (metal oxide varistor) is not a bad idea too.

It is a good idea to have the 6V diode on the output of the regulator too!

When I was designing something recently, I ran my design past a real EE, who does power supply design for Linear. He was not too happy with my use of devices rated for 30-something volts for automotive application. He much preferred 100V ratings if at all available.

A load dump from an alternator is a horrible thing. There is a document somewhere on the web about the subject, and it is quite harsh.

-Tony

[FFMan]

thanks for the feedback

fortunately no alternator here but the engine starting process may give rise to some nasty spikes too.

[twesthoff]

The 5W zener on the input will protect the regulator just fine. I have used similar circuits for many years with no problems. The only thing I might add would be a small series resistor between the fuse and diode, but it would depend on the maximum current the circuit would use and I don't know that.

[FFMan]

max current draw is 200ma, mostly down to the ultra bright leds i need to use for outdoor use.

[sturgessb]

Being the bodger that I am, I just stuck a zx24n, 7805 and xbee under my hood in a solid abs enclosure secured to the bulkhead, connected directly to the 12v battery.

Been using the setup for a few days now, seems fine.

Time will tell i guess

[twesthoff]

It is common to have voltage spikes up to 100V on automotive 12V rails.  You may have problems later.  Some type of high voltage spike suppression should be used.  As a minimum, put a zener or other device (such as Tony suggested) on the input to the regulator.

On 5/18/2010 9:23 AM, General wrote: