What are the biggest challenges of an automotive electrical system?

Discussion in 'General Electronics Chat' started by ke5nnt, May 7, 2011.

  1. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
    When designing electronics intended for use with an automotive system, I understand that while a vehicle is off, Voltage can be as low as 9V (lower in cases where something is wrong with your battery), and typically, while running, the system is at 13.8V. However, I'm curious to understand what makes the electrical system of a running vehicle so harsh? Will you see spikes in voltage when accelerating quickly? Drops in voltage when you turn on the headlights?

    I imagine that current ripple could be a problem as well, but just how much does the current in this type of environment ripple? Let's say you connect a device to the poles of the battery, and you want to regulate as best as possible, but also using the FEWEST components as you can to get the output voltage down to 9V while supplying about 500mA. Thoughts on this might be a larger electrolytic capacitor across the poles, then series or parallel resistors to drop the voltage and limit the current? How about a voltage divider?

    I've seen devices after opening them up that run on an automotive system, that seem to only have a couple components between the 13.8V and the operating components of the device, none of which are any kind of component like a 7809.

    This is more of an open discussion than any specific question. Just want to get feedback from people that have more experience... how would you do this?

    Also, let's assume that the device being designed is only used while the vehicle is running.

    One more note, this discussion does not imply, and is not intended to be used to modify any existing automotive circuit or device, nor does it discuss puncturing the vehicle's firewall. Therefore, this thread is not intended to, nor should be construed as a violation of the ToS of AAC.
    Last edited: May 7, 2011
  2. Adjuster

    Late Member

    Dec 26, 2010
    It's worse than you think. Probably the worst thing is the "load dump" phenomenon, where the disconnection of a heavy load such as the starter leads to an inductive kick-back. See this link: http://www.industrologic.com/autotransients.pdf
  3. someonesdad

    Senior Member

    Jul 7, 2009
    Besides voltage drops, current spikes, HF noise, etc., also remember that a design needs to address the rather wide temperature and humidity ranges encountered in vehicles, not to mention vibration and perhaps exposure to fumes. Oh, and don't forget nimrods like me who spill their coffee on things... :p
  4. #12


    Nov 30, 2010
    I think those 2 about covered it. Anything that can go wrong, will. It seems that designing something that is immune to the voltage fluctuations is only half the battle. The other half is designing something that will resume being in a working condition after the transients stop. If you think that's easy, Toyota is looking for consultants.
  5. Jaguarjoe

    Active Member

    Apr 7, 2010
    The SAE has a specification for vehicle powered electronics. It explains load dumping, applying reverse voltage when bubba puts his battery in backwards, and double voltage occurances when jumping a car with another.
    I think the worst you'll see is disconnecting the battery while the vehicle is running and the alternator is working hard.
    Putting a capacitor across the battery poles is like peeing in the ocean- you'd need many farads to accomplish anything.
    On newer cars at least, alternators have "bear claw" rotors that allow the alternator's AC output to be real close to a square wave which jacks up their efficiency quite a bit.
    One challenge electrical designers have is weight. They've went so far as to develop lighter wire insulation to save a tiny bit of weight. Older Cadillacs had as many as 53 wires going into a door, a few years ago its down to nine, maybe less now.
    Another challenge is modularity. The old Ford Escort had about 15 different wiring harnesses, none alike but all so close to each other it was hard to tell them apart. Now, the Ford Focus which replaced the Escort might have 5 harnesses. A great cost savings.
    Alternator reliability is so good, there was talk of eliminating the conventional belt driven unit and also eliminating the starter motor. The flywheel/flexplate would be electrified to become a combination alternator/starter.
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    Dont drink and drive! ;)
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    But his CD player has such a great automatic cup-holder! :D
  8. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
    Ok, thank you for all the responses, a lot of good information in here.

    The .pdf file attached to a post above talks about suppressing transient voltages using a central suppressor. My initial thought would be that all vehicles, at least newer ones, already have central suppression systems in them somewhere?

    Some electronics that go in cars seem extremely simple. So simple in fact, that I have a very hard time believing that each one of them is designed to handle the kinds of transient voltages and wattages listed in that article, particularly the examples given for load dumps.

    I took apart a cell phone charger that plugs into the cigarette lighter I had lying around as an exploratory experiment. What I found inside was nothing more than a switching regulator, a 34063 to be exact, which operates in a step-down state. The schematic for the MC34063 on www.onsemi.com shown for the step-down converter is exactly what is inside this charger, with the exception that a small LED and a current limiting resistor for the LED were added to the Voltage Output pin of the IC to indicate when the device is on.

    The point I'd like to make in bringing this up, is that this simple design, while remaining plugged into the cigarette lighter of my truck, has been plugged in for 2 years and has gone though vehicle off, start cycles, jump-start cycles, etc... and still works. A 9-component device that takes up about 1 square inch survives in an automotive environment. Is it not possible then, to design other in-vehicle devices without too much thought to transients?
  9. retched

    AAC Fanatic!

    Dec 5, 2009
    This is an AUTOMOTIVE switching regulator, designed to handle the spikes of an automotive environment.

    If folks who want to build for automotive applications took the time to learn and use the right parts, then its not a problem.

    But you cannot take a 555 led flasher that works on your breadboard and expect it to survive.

    It is all about using the right tool for the job.