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I have a 12V air pump (runs out of the car cigarette lighter). It had worked ok the first few times then stopped working, with smoke billowing out of the receptacle while it was plugged into the socket. I opened up the pump's cigarette plug and it was all melted inside. I replaced the entire plug with a new one. The result was that it worked for less than a minute, smoke started coming out, the inside of the pump's plug melted again. Reviews on this air pump from the store I bought it from showed this to be a common occurrence, along with repeated burnt fuses, etc. Except for one particular reviewer who seemed to have found a way around it with a mod as follows:

"...did not want to return the unit to get a replacement with the same cheap cigarette lighter plug, I simply cut off the burnt plug and replaced it with 2 strong alligator clips and an inline fuse receptacle and I now plug the unit directly onto my car battery's post. No more problems !!!!"

I guess I could just get a heavy-duty marine-grade receptacle, but I like the idea of just running off the car battery directly. This seems easy enough to do but since I'm no expert, I have the following questions:

  1. The fuse that was inside the original receptacle was 15A and quite
    tiny. I was told in another forum (that was of limited help) that I should find a 15A 1.25" fuse. The fuses at the electronic store were labelled 6x32mm. Since 32mm~1.25", this is the correct size fuse?

  2. The original fuse was glass. The store has glass or ceramic and slow-blow or fast-blow. Which should I be using?

  3. Is it ok for the in-line fuse holder to be rated at 20A (that's all the store has) or does it have to be rated at exactly 15A (the same as the fuse)?

  4. Is my diagram below basically how it all goes together?

BATTERY(+)--[Alligator Clip]---------------------------------------->PUMP(On/Off)

BATTERY(-)--[Alligator Clip]------->[inline holder+fuse]---------->PUMP(Ground)

Nick Alexeev
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user26831
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2 Answers2

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"is this the correct size fuse?"

The mechanically correct size fuse is the one that mechanically fits the fuse holder. The electrically correct size fuse is the one that protects against burnout (According to the Manufacturer - 15 Amps).

"...glass or ceramic and slow-blow or fast-blow. Which should I be using?"

Glass or Ceramic, it doesn't matter - even plastic would be okay. Fast-blow is for protecting sensitive things like diodes and transistors, not motors or light bulbs, etc.

"Is it ok for the in-line fuse holder to be rated at 20A?"

The fuse holder rating indicates the largest fuse you should use, smaller is okay.

"Is my diagram correct?"

While electrically your diagram will work, for safety reasons the fuse should be in series with the HOT side of the circuit as near to the power source "BATTERY(+)--[Alligator Clip]" as possible!!!

Dan Likins
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As long as the fuse fits in the fuse holder, the "size" (as in millimeters/inches) is fine. As long as the rating of the fuse holder is equal to or greater than the rating (in Amperes) of the fuse you plug into it, the holder is fine. As long as the rating of the fuse is the one recommended by the manufacturer of the device the fuse protects, the fuse is OK.

I would recommend looking for a resettable circuit breaker, rather than a fuse, btw. If this pump draws that much juice, having to replace glass fuses would be a nuisance. Automotive auto-reset or manual-reset circuit breakers aren't that much more expensive than fuse holder+fuse combinations.

Battery + -> (red)Clip -> circuit breaker -> (red)compressor

compressor(black) -> (black)Clip -> Battery -

Jon Watte
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