What does a multimeter read when a fuse is blown?

Changing the Fuse – One of the most common mistakes with a new multimeter is to measure current on a bread board by probing from VCC to GND (bad!). This will immediately short power to ground through the multimeter causing the bread board power supply to brown out.

As the current rushes through the multimeter, the internal fuse will heat up and then burn out as 200mA flows through it. It will happen in a split second and without any real audible or physical indication that something is wrong. Wow, that was neat. Now what? Well first, remember that measuring current is done in series (interrupt the VCC line to the breadboard or microcontroller to measure current).

If you try to measure the current with a blown fuse, you’ll probably notice that the meter reads ‘0.00’ and that the system doesn’t turn on like it should when you attach the multimeter. This is because the internal fuse is broken and acts as a broken wire or open. Next, remove the two screws hiding behind the battery plate. Lift the face of the multimeter slightly. Now notice the hooks on the bottom edge of the face. You will need to slide the face sideways with a little force to disengage these hooks. Once the face is unhooked, it should come out easily. Now you can see inside the multimeter! Gently lift up on the fuse, and it will pop out. Make sure to replace the correct fuse with the correct type, In other words, replace the 200mA fuse with a 200mA fuse, Warning! DO NOT put a 10A fuse where a 200mA fuse should go. The placement of the fuses may not match the placement of the probe ports.

  1. Read the metal cap on either end of the fuse to double check which is which.
  2. The components and PCB traces inside the multimeter are designed to take different amounts of current.
  3. You will damage and possibly ruin your multimeter if you accidentally push 5A through the 200mA port.
  4. There are times where you need to measure high current devices like a motor or heating element.

Do you see the two places to put the red probe on the front of the multimeter? 10A on the left and mAVΩ on the right? If you try to measure more than 200mA on the mAVΩ port you run the risk of blowing the fuse. But if you use the 10A port to measure current, you run a much lower risk of blowing the fuse. Remember: If your system has the potential to use more than 100mA you should start with the red probe plugged into the 10A port and 10A knob setting. With sub $50 digital multimeters, the measurements you are likely to take are just trouble shooting readings, not scientific experimental results.

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If you really need to see how the IC uses current or voltage over time, use an Agilent or other high quality bench unit. These units have higher precision and offer a wide range of fancy functions (some include Tetris !). Bunnie Huang, hardware designer behind Chumby, uses high-precision current readings to trouble shoot boards during the final testing procedures of a Chumby.

By looking at the current consumption of different boards that have failed (for example a given failed board uses 210mA over the normal), he could identify what was wrong with the board (when the RAM fails, it generally uses 210mA over normal). By pinpointing what may be potentially wrong, the rework and repair of boards is made much easier.

Can a fuse be bad but not blown?

Car fuses are sort of safety devices in a car’s electrical system that protects the electrical components from blowing up. Technically, it saves everything in your car with a wire from getting toast by the excess current or extremely low current in some cases.

When an electrical surge or overload passes through a car fuse, it melts or gets broken to stop the current from reaching the car’s electrical circuit. So in a way, it’s like a sacrificial device. Can a car fuse go bad without breaking? Yes. Although it rarely happens, a car fuse can go bad without it breaking.

In very rare cases, there might be a break in the metal strip from the terminals inside the fuse, due to vibrations or a manufacturing defect. Also, in rare cases, corrosion due to moisture can cause a fuse to go bad without breaking, Cars, usually feature about two fuse boxes that contain different fuses, with other amps, used to protect specific electrical components of the vehicle from harmful electric currents.

When a fuse gets broken or goes bad in any device or machine due to a harmful current, it literally just served its purpose by protecting the whole thing from getting blown up. However, when this happens in a car, asides from having to change the broken fuse, there might be a need to have the electrical system of the vehicle checked.

A broken car fuse might be a sign of trouble or bad wiring somewhere that needs to be fixed. This means changing the bad fuse might just fix the problem temporarily. NEW: Is It Safe To Drive With A Broken Brake Line? Image: Cqdx / Wikimedia Commons

What is the voltage across a blown fuse?

There will be the full supply voltage across the fuse if it is blown, and zero voltage if it is good.

How do you know if a fuse has power?

How To Tell If a Car Fuse is Blown – To diagnose a car fuse, you simply take it out of the box and look through the fuse’s plastic cover. If the link inside the plastic looks broken or there are dark marks or metal residue in it, then the fuse is blown. You may also use a multimeter to check for continuity between the blade terminals. The type used in cars is also called an automotive, blade, or knife fuse. These fuses have a peculiar look that features two short blade terminals at both ends plugged into a box. The automotive types are designed in a way that they are easy to remove from the car.

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Car Fuse Visual Inspection

Once you identify the exact unit you need to check, you pull it out of its slot. Although car fuses are covered in colored plastic, they are still fairly transparent. The link is typically a flat solid metal and when it blows out, the short gap created is also visible.

Testing Car Fuse With a Multimeter

Just like the cartridge types, however, the multimeter is also the most accurate tool to use to diagnose the blade types for faults. Run a continuity test between the two blade terminals to see if the link is broken or not. If the multimeter does not beep, it is bad and you need to replace it. Sometimes, testing other types of electrical fuses with a multimeter may not be so easy. Thankfully, these different types typically have specific visual pointers that help you know whether they are blown or not. For example, a drop-out type has a holder that disconnects from a contact and drops away from the body when the link is blown.

How many ohms will a blown fuse read?

When testing a fuse, touch the end of each ohmmeter cable to the opposite ends of the fuse. A reading between 0 and 2 indicates continuity—the circuit is good/complete. If you read infinity (∞) or OL (open line) the circuit is bad.

Should a fuse read ohms?

Testing fuses Page Testing procedure for fuses

It is extremely important to understand that if a fuse looks good, it might not in fact be good! Fuses can appear to be fine and have a break in them so small that it cannot be seen by the naked eye! Therefore it is important to check a fuse with an Ohmmeter or a continuity tester instead of assuming that it is good. A significant percentage of all boards returned to Dinosaur as “dead” have nothing wrong with them other than a blown fuse. Assumptions can waste a lot of time! The fuse shown below looks perfectly good, but is open under one of the end caps!


Use a multimeter and set the range switch to the lowest resistance range to check the fuse. A good fuse will read 0 ohms (depending on what the meter reads when the leads are held together) and a bad fuse will read the same as holding the leads in the air. The illustrations below show how to test fuses, showing each fuse as good. There is no gray area, the fuse is either good or it is not.

Testing a standard fuse Testing an automotive style fuse

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You can also use a ” battery operated continuity tester ” as shown in the illustration below (also indicating a good fuse) *Important note: A standard test light without the built-in battery will not work! It must have a battery built-in to make the lamp light up!

Testing fuses Page

Should a fuse read voltage?

Step 1: Identify the blown fuse. – If the fuse can be removed from the circuit, then the easiest way to determine if it has blown is a continuity test. Grab a multimeter and select the continuity or resistance setting. Perform a quick test of the meter to ensure it’s functioning properly by touching the leads together until you hear a beep or see 0 ohms.

  • Now, after the circuit has been powered off, place the leads on either side of the fuse and if you hear that same beep, and the meter has a very low resistance reading, the fuse is still good.
  • If you do not hear the beep, and/or the multimeter reads OL, then the fuse is blown.
  • Another way to test the fuse is by measuring the voltage across it with a digital multimeter.

This is helpful if the fuse is not able to be easily removed from the circuit. To do this, leave the circuit powered on and switch your meter to measure voltage. Make sure to select DC for DC circuits and AC for AC circuits. Take proper precautions to isolate yourself from dangerous voltages.

How do you know if a 13A fuse is blown?

Procedure – Many electrical devices used in eye care have an externally accessible fuse near the electrical cord ( Figure 1 ) that you can check and replace by following these steps.

  1. Disconnect the device from the electrical system.
  2. Remove the fuse from its holder. In some cases you may need a small screwdriver to unscrew the fuse holder cap.
  3. Look at the fuse wire. If there is a visible gap in the wire or a dark or metallic smear inside the glass then the fuse is blown and needs to be replaced. If you cannot see whether the fuse is blown, follow steps 4 and 5. If the fuse is definitely blown, go to step 6.
  4. Set a multimeter ( Figure 2 ) to the resistance or Ω (Ohms) setting.
  5. Place one of the multimeter leads on one end of the fuse. Place the other lead on the other end of the fuse. If the reading is between 0 and 5 Ω (Ohms), the fuse is good. A higher reading indicates a bad or degraded fuse. A reading of OL (Over Limit) definitely means a blown fuse.
  6. If the fuse is blown, replace the fuse with one that is exactly the same (see panel). Make sure to note the fuse amperage and voltage ratings, which should be marked on the fuse itself ( Figure 3 ) or on the panel label near the fuse holder. Additionally, note the size and whether it is a slow-blow or a fast-blow type fuse. If there are no markings on the fuse itself or on the equipment you must consult the device’s operating manual.