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Thread: Inverter meltdown.

  1. #1
    Join Date
    Aug 2008

    Unhappy Inverter meltdown.

    Please help me out.
    I recently purchased a Cobra Model CPI 875 inverter to use mostly with my laptop. It was working ok until I accidently connected the inverter to a car battery red to black and black to red and the sparks flew. I did not have the laptop or anything else plugged into the inverter, but the inverter was turned on.
    I called Cobra. The service rep told me for $37, they would repair it. I then took the inverter apart(even though the manual said not to) and discovered two fuses inside. They are located near the red and black input wires and appear to have blown.
    Then I called Cobra back and was told the fuses are each 50 amps and must be purchased from Cobra. I checked with NAPA and VIP stores, but they don't stock the type of fuse needed with a rating of 50 amps. Only go up to 40 amps.
    Have several questions about my problem. Can I get the right fuses anywhere else besides Cobra? If so, where? Do you think the inverter will work after replacing the fuses, or is there likely to be other damage? Do you think I could use 40 amp fuses? The inverter is rated for 800/1600 watts, which is much more than needed to power my laptop.

  2. #2
    And then I was mod. Tidder's Avatar
    Join Date
    Sep 2003
    New Mexico, USA
    It's been my experience that electronics manufacturers design their products so no damage will be done before the fuse blows, but there's always a possibility. For $37, why not just send it in for repair? At that cheap, no sense in trying to make a DIY project out of it. Maybe they'll even warranty the repair, and test the inverter to make sure it functions correctly.

    You could probably use 40 amp fuses with no issues, yes.

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    The best resurrected frontend I've ever used, period.

  3. #3
    Join Date
    Aug 2008
    Thanks for your quick response.
    I purchased the inverter on EBay for $37. It was second hand.
    Have purchased a new Cobra Model CPI 875 from, which should arrive in a week. In the meantime, thought I would try to learn more about inverters and possibly get the damaged one working by replacing the fuses.

  4. #4
    Variable Bitrate rEegLer's Avatar
    Join Date
    Jul 2008
    Mechanicsburg, PA
    He took it apart though so they might not touch it.

    Its really hard to say whether replacing the fuses will fix it. Reversing the polarity might have caused damage elsewhere on the board and only blew the fuses after causing damage somewhere else. I would call them back and see if they'll fix it even though you opened it.

    If the 40A fuses are cheap, buy 'em, just make sure they're the right dimensions and standard. Tidder is probably right, 40A should be fine. In any case your safe going lower than higher

  5. #5
    Join Date
    Aug 2008
    The fuses are most likely there to protect the rest of the system (auto, boat, etc.) and not the inverter itself. Generally, consumer products are made as absolutely cheaply as possible. It is tricky to design robust, fail-safe electronic circuitry AND make it affordable. It often seems that the power transistors blow to protect the fuse. . .

    Unless the electronic circuitry was robustly designed, the chances are that the inverter is toast. How much did this inverter cost when it was new? Is it made in China? It might even be that the $37 is the cost of a new circuit board (or even a new product).

  6. #6
    Low Bitrate Beehphy's Avatar
    Join Date
    Apr 2007
    Quote Originally Posted by Altazi View Post
    It often seems that the power transistors blow to protect the fuse. . .


    Well, as an electronics designer, and producer of car-pc related equiptment I can tell you, prepairing for your customers mistakes can be tough but it's definately part of the job.

    In my ACPPS power supply I put a diode accross the incomming power that intentionally blows the fuse if you hook the power backwards. The PSU doesn't event need to be turned on to protect it in that way.

    This topic reminds me of an pheonix gold amp I used to use. The M-25, standing for "moron". They designed it so an epileptic retard could hook up without killing it.

    Based on the price, I would say the fuses are all that need replaced in your inverter. $10 for parts, $25 labor for 15 minutes labor. Since you allready bought a replacement, I'd say try the repair.

  7. #7
    Join Date
    Aug 2008
    Quote Originally Posted by Beehphy View Post
    In my ACPPS power supply I put a diode accross the incomming power that intentionally blows the fuse if you hook the power backwards. The PSU doesn't event need to be turned on to protect it in that way.
    This approach is OK as long as the I^2t rating of the diode is greater than the I^2t rating of the fuse; this generally works for relatively low current devices. To blow a 40A fuse on a reverse power connection, you are talking about a pretty massive diode that could survive that kind of abuse.

    (also an electronic engineer)

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