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Thread: Car Start Waveforms

  1. #1
    FLAC
    Join Date
    Apr 2001
    Location
    Portland OR
    Posts
    1,244

    Post Car Start Waveforms

    A few months ago I told a bunch of you that I would upload some waveforms of my car starting onto this board. Well I moved into my new house and have a garage now so I can actually do this, not living in a college appartment anymore. So here are the waveforms of my car starting. I have a battery that has been killed a few times so it is not in the best shape. It works just fine and has a year of rated life yet.

    So here they are: http://skylab.org/~chugga/mpegbox/ca...t/TEK00000.gif http://skylab.org/~chugga/mpegbox/ca...t/TEK00001.gif http://skylab.org/~chugga/mpegbox/ca...t/TEK00002.gif http://skylab.org/~chugga/mpegbox/ca...t/TEK00003.gif

    You can see that the car starts up in 744 ms (depends on how long you crank). It starts at 12 volts, goes down to 8 and then back up to 13.8 If I had an inverter that cut out at 10 volts, I would need >51 ms of filter capacitance to stop that drop down to 8 volts.

    Those of you who told me to go by a new battery, HA! I can start my car with my custom power supply and my computer doesn't restart and it does drop to 8 volts.

    So anyway, you can also see in these waveforms how much the voltage swings. These measurements were taken with a TDS3054 directly AT the battery.

    Hope this helps those of you who still want to use 7812s see what your up against...

    Even if you run heavy gauge wire to the battery, you still have internal resistance of the battery to fight with, and unless your battery is brand spanking new and super capacity, you will have to deal with this.

    Jeff
    MPEGBOX - Plexiglass Computer
    www.mpegbox.com

  2. #2
    Constant Bitrate
    Join Date
    Jul 2000
    Location
    Israel
    Posts
    134

    Post

    Jeff,there are some people here that were telling that car voltage has many spikes and very "dirty". It would be great if you could take pictures of voltage with small Volt/Div to see how much noise is on the line.
    Enjoint life!

  3. #3
    FLAC
    Join Date
    Apr 2001
    Location
    Portland OR
    Posts
    1,244

    Post

    Originally posted by Dima:
    <STRONG>Jeff,there are some people here that were telling that car voltage has many spikes and very "dirty".</STRONG>
    I don't know how much validity that really has to this application. The battery is like big buffer. The voltage across the battery isn't going to change much, and it is most likely that it will absorb dangerous spikes. If you power your box right from the battery just like you'd power your subs, I don't think you will see any of these spikes. In order to see a big spike at the battery it would have to be 10's of amps or more. I really don't think it is an issue if you have a decent battery hooked up and you power your devices directly from it. The internal resistance of the battery is typically very small.

    For Example with my 85 accord:
    If my car is drawing say 100 amps to start the car, (I don’t know how much it really is, but that sounds like a good number.) Then if it drops from 12 to 8 volts during cranking (hence my nice oscilloscope waveforms), that is 4 volts/ 100 amps = .04 ohms of internal resistance. If you had a spike in the system that went back to the battery, it would have to be able to deliver 25 amps to raise the voltage at the battery 1-volt. My 40-amp alternator is not likely to deliver a 40-amp spike higher than 13.8 volts. Plus, if it did, that would be less than 2 volts, so 15.8 volts? Well, you get the idea. I'd be hard pressed to find any spikes that go over 15 volts at the battery of my car.

    Jeff
    MPEGBOX - Plexiglass Computer
    www.mpegbox.com

  4. #4
    FLAC
    Join Date
    Apr 2001
    Location
    Portland OR
    Posts
    1,244

    Post

    Oh, and about small spikes, I'll try to see if I can find some noise, I'll turn on the windshild wipers and see if there is any AC noise riding on the DC voltage at the battery. But Again, it is going to be small. You only really run in to problems if you use the cig-adaptor and there is some small gauge wire that has a lot of current running through it.
    MPEGBOX - Plexiglass Computer
    www.mpegbox.com

  5. #5
    Variable Bitrate
    Join Date
    Mar 2001
    Posts
    292

    Post

    My alternator is 100amps peak..
    Not that I'm worried about spikes.
    Player: Celeron II 633MHz, 256MB RAM, 20GB IBM 9mm 2.5" Laptop HD (180G/2ms), onboard ethernet/sound/video/tvout, 10"11"x3" case, MPBS1 70W DC-DC PS w/auto-shutdown controller, in-dash lighted switches, 7" NTSC TFT widescreen in-dash LCD, touchscreen, rear-window brake light installed Garmin GPS35 GPS, credit card sized IR remote w/IRMan, mini-wireless keyboard/mouse (sits under seat), PowerMate black knob, MP3s and GPS Navigation (Winamp, CoPilot, SA8.0).
    Car: 1993 Nissan Maxima, Black Emerald

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