How to know if multimeter fuse is blown

How to know if multimeter fuse is blown

One of the most common mistakes when using a new multimeter is to measure the current on the board by measuring from VCC to GND (a mistake!) . This immediately shorts the multimeter to GND, cutting power to the test board. Due to the rapid current flow through the multimeter, the internal fuse heats up and blows when 200 milliamps pass. This happens in a fraction of a second and there are no real audible or physical signals that something is wrong.

Wow, that's great, now what? Well, first remember that the current measurement is done in series (interrupt the VCC line to the test board or microcontroller to measure current). If you try to measure current with a blown fuse, the multimeter may read "0.00" and the circuit may not turn on as before when the multimeter is connected. This is because the internal fuse is faulty and behaves like a broken or open wire. Don't worry, this happens all the time and costs about $1 to fix.

To replace the fuse, grab your handy mini screwdriver and start unscrewing. the SparkFun DMM is relatively easy to remove. First, remove the battery plate and batteries.

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Next, unscrew the two screws on the back of the battery plate.

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Lift the front of the multimeter slightly.

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Now look at the catches under the face. You will need to use some force to push the face to the side to disengage these catches.

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Once the face is separated, it should come off easily: now you can see inside the multimeter!

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Lift the fuse carefully and it will fall out on its own.

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Be sure to replace the fuse with the correct type. In other words, replace the 200 mA fuse with a 200 mA fuse.

Warning. Do not put a 10 amp fuse in place of a 200 amp fuse. The position of the fuse may not match the position of the probe connection. Read the metal caps on both ends of the fuse to determine which fuse is correct.

The components and leads inside the multimeter are designed to withstand different amperages. If you accidentally send 5 amps across a 200 mA terminal, you can damage or possibly destroy your multimeter.

There are times when you need to measure high currents, such as in a motor or a resistor. See the two places on the front of the multimeter where you can insert the red probe? On the left is 10 A and on the right is mAVΩ? If you try to measure more than 200 mA on the mAVΩ connector, you run the risk of blowing a fuse. However, if you use a 10 A connector to measure current, the risk of blowing a fuse is much lower. The trade-off is sensitivity. As mentioned above, if you use a 10 A connection and a dial setting, you will only read 0.01 A or 10 mA. Most of my systems use currents greater than 10 mA, so a 10 A setting and connection works well enough. If you want to measure very small amounts of power (microamps or nanoamps), a 200mA to 2mA, 200uA or 20uA connection may be just what you need.

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Remember: If your system potentially draws more than 100 milliamps, you must first connect the red probe to the 10-amp connector and set the dial to 10 amps.

With digital multimeters under $50, the measurements you can make are only troubleshooting measurements, not scientific test results. If you really need to see how an integrated circuit is doing in terms of current or voltage over a period of time, use an Agilent or other high-quality tester. These devices are more accurate and offer a wide range of advanced features (some include Tetris!) Bunnie Huang, the hardware developer behind Chumby, uses high-precision current measurements to troubleshoot board problems in Chumby's final test procedure. By observing the current draw of several faulty boards (for example, one faulty board drew 210 mA more than normal), he was able to determine what was wrong with that board (when RAM fails, it typically draws 210 mA more than normal). By identifying what might be wrong, the PCB is much easier to rework and repair.