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Brownout on start explanation?

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  • Brownout on start explanation?

    Hi all,

    I have been reading the forums for a while now, keen to make a system in the future when I have time.

    I have always wanted to use a laptop for the benefit of having its own UPS built in. My question is as follows. Every car I have known, on start the radio normally drops out, due to the starter motor sucking too much juice when you start the car. I have not seen any people who have a car computer that isn't a laptop complain about this? How does a DC-DC converter fix this issue? Without another battery somewhere (ie in the laptop) how does the computer avoid running out of juice and restarting as the car turns over?

    Thanks Andrew

  • #2
    The DC-DC intelligent transformers are able to compensate for the voltage drop and keep the computer online when cranking the starter. I don't know the specific technical details but I think the general technology is referred to as a switching transformer. The voltage is actually switched to AC, stepped up to a sufficient AC voltage, then reconverted to DC at the proper voltage. If the DC voltage drops, the transformer is able to compensate unless it drops quite low.

    However, their ability to do so is dependent upon the quality of your installation. You need to ensure that your power cables are well connected, your ground wires are well connected, the wire gauges are sufficient and so forth.

    You may not have seen people complain about it, but there are a fair number of posts about computers not surviving crank from forum members.
    Originally posted by ghettocruzer
    I was gung ho on building a PC [until] just recently. However, between my new phone having internet and GPS and all...and this kit...Im starting to have trouble justfiying it haha.
    Want to:
    -Find out about the new iBug iPad install?
    -Find out about carPC's in just 5 minutes? View the Car PC 101 video


    • #3
      That's about it.
      dc-dc converters are designed for a certain input voltage variation to supply whatever output voltage.
      EG - 9V batteries are often replaced by "bigger" 1.5V AAA or AA cells. A dc-dc conv can convert the (say) 1V = 1.5V from 1 AAA to 9V etc.

      Car PC supplies are similar. They may convert 8V-16V to 12V or whatever the PC requires (maybe 19V etc for a charging laptop).

      Technically is a SMPS - Switched Mode Power Supply - the same as used in modern power supplies for DC equipment.
      Instead of using a bulky 50Hz or 60Hz transformer to convert 230VAC or 110VAC to whatever voltage, the convert the AC to DC (via diodes/rectifiers; eg to ~320V or 150V DC), then chop/switch it at high frequencies (say 50-100kHz) which requires a much smaller transformer.
      In practice, a coil/choke is used instead of a "multi-winding/coil transformer".
      The high frequency output is rectified (from its chopped/transformed AC) to DC.
      Being high frequency, only small capacitors are required for filtering (though chokes can also be used).

      For $1 you can buy chips that have the smarts to do the above - it monitors the output voltage and changes the chopping "duty cycle" to regulate the voltage.

      A dc-dc converter is the same except it already has the dc input (no rectification and filtering of the AC input is required).
      And whilst AC-input SMPS usually convert down (eg, 110VAC to lower DC voltages), dc-dc converters often convert up and down.

      dc-dc converters can or are used in dc battery charging systems and power supplies (like phone chargers etc), HID lamps, hi-power car audio amplifiers, etc...

      FYI - some audio systems do not "fail" during cranking.
      It could be that they are connected to the ACC circuit (off during cranking).
      I have an Alpine powered direct from the battery, but it has an internal cut-out when the battery voltage dips. This can be to prevent brown-out issues (damage), or simply Alpine's design logic that if you are cranking, you are likely to get spikes through the system so let's disconnect the circuits...


      • #4

        Cheers guys,

        Well explained, it's not so complicated when you know how.
        Hopefully this will enlighten a few other people too.

        Think it might be beneficial for me to stick with a laptop.

        Thanks Andrew