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May be a little newbie question, but I since I am an electronics beginner and couldn’t find a direct answer I will ask it.

I blew the 600 mA fuse on my brand new multimeter, I think I didn’t change the leads when measuring voltage after measuring current, causing a short.

I ordered some new fuses (multiple just in case), the exact same rating, but can’t help but feel like I’ve already ‘tainted’ my new meter.

So my question really is: does the fuse protect the meter from damage, or is it more to prevent fires and thus protect the user?


It took no more than 5.5 volts DC, and the current was limited to about 2 amps, that is where the bench power supply was set.

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    \$\begingroup\$ Being cautious, I'd make some basic voltage/current/ohm/measurements, check that the DC, AC volt display zeros (with leads shorted) - just to ensure that damage wasn't done. Sometimes the internal fuses are somewhat exotic types: pay attention to more than just "600mA". \$\endgroup\$
    – glen_geek
    Sep 5, 2023 at 16:11
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    \$\begingroup\$ I do the same once in a while. This is why I have a bag of 4-5 spare fuses inside the 3 multimeters I use. \$\endgroup\$
    – fraxinus
    Sep 6, 2023 at 8:05
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    \$\begingroup\$ How energetically did the fuse blow? If you've got little bits of fuse splattered around the guts of your multimeter, then yes, you may have damaged it. \$\endgroup\$
    – Mark
    Sep 7, 2023 at 1:54
  • \$\begingroup\$ What kind of multimeter? Simpson / PSM6, or one of them newfangled digital meters that measures all of stuff you don't care about (for marketing purposes)? I remember 8th grade radio and electronics class in about 1953. Simpson multimeters were expen$ive, but in those times PHYSICAL PUNISHMENT WAS PERMITTED! By a strance coincidence, none of the meters were wrecked. (Same teacher for 8th grade science and 12th grade physics, coached me for my ham radio exam, he was one of the best I ever had and will never be forgotten.) \$\endgroup\$ Sep 9, 2023 at 20:05

5 Answers 5

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So my question really is: does the fuse protect the meter from damage, or is it more to prevent fires and thus protect the user?

Without making any assumption about how the fuse is blown, both scenarios are theoretically possible. It would require a case-by-case analysis to determine the consequence of a particular misuse. For serious lab work, it may be necessary to send the multimeter back to the calibration department for a recheck, since it's the policy of many labs to require re-calibration after an instrument is repaired - replacing a fuse may count as a "repair" as well.

That being said, I believe it's unlikely to damage a multimeter by a mild overcurrent, in particular, the chance of damaging expensive active components (such as its analog-to-digital converter) is extremely low.

To see why, let's take a look at the schematic of a representative multimeter, the Fluke 87. Other multimeters have similar protections in place.

Input Circuit of the Fluke 87

When the multimeter probe is plugged into the "milliamp" and "common" sockets and set to the "milliamp" range, the measured electrical current flows across a 1A fast-blow fuse, to the mA range switch, to a 0.99 Ω and a 0.01 Ω resistor, and finally leaves the meter via the "common" terminal. At the top of the 0.99 Ω resistor, an "amps sense" line connects it to the pin "AP3" of the multimeter ASIC (which has an integrated analog-to-digital converter).

How does the input circuit work? A multimeter measures current by sensing the voltage across a known low-value (e.g. 1 Ω in this case) shunt resistor, and uses Ohm's law V = IR to determine the current. What are the consequences if there's an overcurrent?

  1. Overvoltage. Due to Ohm's law V = IR, a slightly higher voltage drop appears across the shunt resistor. With an ideal low-voltage source, or a current-limited voltage source, this is unlikely to cause damage to the active components.

    • If it's connected to a "high" voltage source but it's current-limited (by series resistance, by active regulation, by capacitance of a charged capacitor). it's unlike to cause damage, due to the low shunt resistance value used, the voltage drop across the shunt would be small. It may also force the voltage source to discharge or drop quickly.

    • If it's connected to a low voltage source around a few volts without current limit, it's unlike to cause overvoltage damage to the active electronics in the meter. Suppose the multimeter is connected directly across a 5 V voltage source, the short-circuit current is 5 A, and the voltage across the two resistors is 5 V.

    • If it's connected across a "high" voltage, high current source, there can be a serious overvoltage.

  2. Overheat. Due to Joule's law P = I²R, increased power is dissipated into the shunt resistors. In case the short-circuit current is 5 A, the instantaneous power dissipation into both resistors would be 25 W. If the overcurrent is persistent, heat can cause permanent damage to this (inexpensive) shunt resistor, changing its resistance or turning it into an open circuit.

  3. Blown Fuse. The fast-blow fuse disconnects the input, the speed depends on how severe the overcurrent is, from milliseconds to a few seconds - hopefully before damage has occurred. If the overcurrent is far above the tripping current, which is usually the case for a short circuit, it only takes milliseconds before fuse disconnection. But if the overcurrent is very close a fuse's tripping point, disconnection may take a few seconds (some multimeters have a 10 sec. max label on the current input terminal, which means it's safe to work near the the current range's upper limit, as long as you stop quickly before the fuse is blown).

For extra protection in case of overvoltage, the Fluke 87 has these designs: a diode bridge (I labeled it "OVP") would short-circuit the input to ground if the voltage across the shunt resistor is greater than the voltage drop of three silicon diodes (~2.1 V) for both positive and negative overvoltage. Furthermore, in case of overcurrent, the sensing line is isolated from the input current via a 100 kΩ series resistor (I labeled it "OCP"), ensuring that almost no current is fed directly into the data converter under a fault. In the milliamp range, the 110.1 kΩ resistor is also in series.

Conclusion: For just a mild overvoltage, it's extremely unlikely to damage the multimeter ASIC. It's possible to damage the shunt resistor, but it doesn't happen that often, since the fuse usually disconnects the input quickly enough. Thus, just replace the fuse and it's probably fine. If you're really concerned, compare the current measurement results of another known-good meter.

Even if you're very unlucky, the only thing that needs replacement is an inexpensive shunt resistor (and possibly a diode bridge in the Fluke 87). However, for a serious overvoltage, overheating and dielectric breakdown may cause additional component and circuit boards damage.

When measuring the same voltage source, the higher the range, the more difficult it is to damage it. The 10 A range is the most difficult range to damage, as it only has a 0.01 Ω shunt resistance and dissipates minimum heat. Many resistors can easily take an impulse of several hundred amps.

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  • \$\begingroup\$ Blown Fuse. The fast-blow fuse disconnects the input within a few seconds, hopefully before damage has occurred. I'm sure you mean milliseconds \$\endgroup\$
    – MiNiMe
    Sep 6, 2023 at 22:08
  • \$\begingroup\$ Btw, it's nice to know about the Flukes protection. \$\endgroup\$
    – MiNiMe
    Sep 6, 2023 at 22:19
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    \$\begingroup\$ @MiNiMe Well, it depends on how severe the overcurrent is... In case of an overcurrent above the tripping point, it only takes milliseconds (I've even seen microseconds on SMD fuses in a surge experiment with 100x overcurrent, unbelievable!). Meanwhile, if the overcurrent is very close to a fuse's tripping point, disconnection may take a few seconds (it's what happens when you're taking a measurement near its current range limit - the 10 sec. max label on the multimeter tells you to stop quickly before it's too late). Anyway, thanks for pointing that out, I just edited it into the answer. \$\endgroup\$ Sep 6, 2023 at 22:41
  • \$\begingroup\$ Thanks for that expo of the Fluke 87! I have had one for a long time, and it is still serving me well. It still has some functions that I have never used. \$\endgroup\$ Sep 9, 2023 at 20:14
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That's the whole idea of a fuse, to protect the device that it is deployed in. So no, tainted is all in your mind. Change the fuse and learn from the mistake, and welcome to the world of electronics!

Edit: Sorry, I missed the other part of your question: Yes, to protect you if you measure something high powered and the multimeter instead of the fuse blew up in your hands. Also to prevent fires in buildings etc.

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    \$\begingroup\$ PS, if you nevertheless wanna get a new multimeter, can I have this one? \$\endgroup\$
    – MiNiMe
    Sep 5, 2023 at 15:31
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    \$\begingroup\$ Nonetheless, I'd recommend a) you open the meter up and look for signs of any damage, and b) do a couple of comparison measurements against the your bench supply or another meter. It's important you can trust your meter. \$\endgroup\$
    – jonathanjo
    Sep 5, 2023 at 16:08
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    \$\begingroup\$ Did the same with mine! In fact I blew this 10A fuse as well on the high current range. I just replaced the fuses and everything worked as it did before. If you only went to 2 Amps and the meter still reads current, I'm sure everything is fine with it. \$\endgroup\$ Sep 6, 2023 at 4:11
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    \$\begingroup\$ " tainted is all in your mind." Well, no. Very likely not, but it's a good idea to check after replacing fuses. There is an old instance of Murphy's Law which says that, if a fuse is intended to protect a solid-state circuit, under too-large a load the solid-state circuit will protect the fuse by blowing first. Modern meters are pretty close to bullet-proof, but you should never take that sort of thing as certain. \$\endgroup\$ Sep 7, 2023 at 0:55
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So my question really is does the fuse protect the meter from damage, or is it more to prevent fires and thus protect the user?

It's both. But it's also possible to damage the meter by going grossly outside the parameters it's rated for. If you connect a meter rated for 1kV to 10kV, you may very well expect damage before the fuse blows.

If you're worried test your meter against another meter after replacing the fuse.

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I did this on a 1980's Goldstar, by measuring the 240V mains AC voltage while leads were connected to the Current input.

With the leads corrected and the fuse replaced, the meter now underreads AC by about 50% and wobbles around. However DC voltages and all other settings function as-expected.

So I still have the meter, but I don't use it for AC readings any more. I use a much more modern DMM but can't throw the old one out for sentimental reasons.

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  • \$\begingroup\$ Big difference between 240V and 5V though. \$\endgroup\$ Sep 6, 2023 at 20:05
  • \$\begingroup\$ @user253751 good point - I missed that in the comments, so edited it into the original question. \$\endgroup\$
    – Criggie
    Sep 6, 2023 at 20:55
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    \$\begingroup\$ It's still good to know that permanent damage can happen. \$\endgroup\$ Sep 6, 2023 at 21:08
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    \$\begingroup\$ @user253751 If you use the multimeter connected to a lightning rod in order to determine either lightning voltage or lightning current, I am fairly confident that the fuse will not really be successful at preventing permanent damage elsewhere in the meter. \$\endgroup\$
    – user107063
    Sep 6, 2023 at 21:26
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    \$\begingroup\$ Great confession. You to be commended for your humility, young fellow! \$\endgroup\$ Sep 9, 2023 at 20:21
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It depends.

Given the applied voltage was small, the meter may have survived intact. Or it may not. Depends whether it was slow 'oh, it doesn't work' or audible pop. A loud pop has a high chances of something being damaged. It will show as meter giving weird readings in some modes, or not working at all. You will have to verify every function (voltage dc/ac, current, ohmmeter, diode tester, transistor tester) against a good known sources with known values.

However, if the meter was not expensive, it may be safer to replace it.
A basic meter from hardware store is 5-7$, and it has enough accuracy for anything you will ever use it for. You probably don't need specialized meters, and its easier to learn on cheap ones.
(If you never burned a digital multimeter, do you even measure, bro?)

How to verify your meter:

  • V DC - fresh AA battery should read 1,6Volt.
  • V AC - measure a wall socket, it should be 120/240 depending on where you live at.
  • Ohms - if you have some resistors for your project lying around, measure couple with different values. Remember to always measure resistors that are not connected to anything.
  • Current - this will be the hardest - most reliable would be to connect it in series with your other* multimeter and see if they give same reading.

(*) I recommend having two multimeters at house for times when you need to read voltage and current at same time. Or two voltages. Or as backup.
Also, it is very handy if your multimeter has a beeper for continuity testing. Don't underestimate audio cues when trying to find which wire is which.

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  • \$\begingroup\$ Also need to verify some functions against known BAD components. \$\endgroup\$ Sep 9, 2023 at 20:24

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