Fuse blowing in 12 multimeters rated for 200mA, only measuring 12.5mA. Why?

Question

I'm a high school electronics teacher and while doing a lab on Ohm's Law we just blew the fuses in 12 out of 18 brand new digital multimeters while measuring current, and no the meters were not hooked up directly to a voltage source!

Details:

• 18 brand new 30V DC power supplies
• 18 brand new Radio Shack 22-810 15 Range DMMs
• Smallest resistor 2KOhm

The students connected the resistor and ammeter in series to the power supply.

The multimeter is rated for up to 200mA and the fuse is a 315mA 250V ceramic.

Voltages ranged from 5V to 25V. Even with the smallest resistor of 2KOhm, the largest current that would have been measured is 12.5mA. Even if the students max'd out the power supply to 30V the current would still only be 15mA, which is way under 200mA and 250V.

Why did the fuse blow on 12 out of 18 multimeters? I was directly supervising the students and no one connected the multimeter to the power supply without the resistor. I looked for current surges by connecting the multimeter to the power supply in voltage mode and voltage didn't change at all. I also measured the voltage with an analog multimeter and it was also solid. And I doubt there would even be current surges because the power supply should hopefully filter those out, and they should also be visible on the power supply output meter.

I contacted Radio Shack to see if there was maybe a batch of bad fuses, and of course they said no and that the fuses are not covered under warranty.

After all the checking I've done would you recommend wrapping the fuses with aluminum foil to get the multimeters up and running? It's not like I want to wrap a fuse in the source panel of a house, car, or airplane. Only on the multimeters which are only connected to the power supply output.

Answer 1

Fuses blow when excessive current flows through them.

The likelihood of this being a batch-related problem is improbable in my opinion - most fuses are safety-critical components, usually with regulatory agency marking - especially parts rated 250V.

Can you post a photo of a failed fuse? The metallic end-caps usually have markings indicating ratings, manufacturer and regulatory information.

This issue presents a good opportunity to teach your class how to do methodical troubleshooting.

Try this:

• With a good meter, measure the resistor before installing it in series with the power supply.
• Wire up your circuit - power supply, the series-connected resistor and a good (one in which the fuse is not blown) series-connected DVM in milliamp range, as per your experimental criteria.
• Disconnect the positive power supply connection and measure the resistance between where the positive connection was, and the negative of the power supply. You should see the resistance of the resistor plus the resistance of the shunt inside the DVM (which should be extremely small; perhaps not even visible). If you see some resistance smaller than the resistor value (i.e. zero) something is wrong and you need to check your setup.
• Do your Ohm's Law calculation to establish the maximum expected current based on the expected power supply output and the resistance you measured. Also calculate the worst-case current based on the worst-case power supply output.
• Connect the positive power supply connection and do your experiment.

If you continue to pop fuses, there are two hypotheses that come to mind:

• The power supply output exceeded the worst-case prediction.
• The fuse could not handle the worst-case current.

Proving which was the cause is left as an exercise for the students ;)

Answer 2

Given the conditions that you have described there is no good explanation for what has happened except that the fuses are faulty.

This indicates either that the fuses are faulty :-) - or that there is something incorrect in your described conditions. When things like this happen it is a good idea to ask "what really happened compared to what I think happened?". Sometimes human error creeps in very subtly and Murphy loves to confuse us.

All things being equal I'd guess you have some mislabelled resistors, or wrong low value resistors in a batch of apparently high value ones. See below.

The 200 mA fuses are probably NOT a safety issue but a meter protection one. Chances are the circuit may be damaged if a much larger amperage fuse is used. You could sacrifice one meter to see. May not be a good choice.

One way to try to bypass your problems is to place a say 125 ohm resistor in series with the supplies at a point that the students cannot access. Then max possible current is 25V/125r = 200 mA. A 100r resistor would not blow a 200 mA fuse. Fuses are typically rated to blow at 2+ x rated current. If an external voltmeter is used downstream of the resistor then the drop in the resistor is unimportant. If you are using a meter in the psu it may matter. If these psus are used only in this experiment (unlikely) an internal resistor could be fitted upstream of the meter. Not advised.

Another alternative is to set the supplies to current limit at 200 mA if that feature is available.

You could add series constant current limiter but that takes time and money and may not be desirable. An LM317 plus a 5.6 ohm resistor will make a 223 mA current limiter. It requires soldering one resistor to a TO220 part and 2 external wires. Wrap the whole unit in tape or heatshrink and put in series with psu +ve lead. IFt will drop 2 to 3 volts in normal operation.

Fuses: Inspect the blown fuses and unblown fuses CLOSELY. Are they all identical? Or at least, is there a mix of fuses that is essentially the same in blown and unblown batches. A clear physical variation points to a fuse rating issue.

Examine the fuse rating labelling with a magnifying glass. Are they 315 mA fuses or 31 mA fuses? 30 mA range would be very rare but do exist. Fuses will typically blow at about double rating but wide variation is possible so would be marginal in your application. Seems unlikely but ...

If you decide to bridge your fuses use the thinnest wire you can find and solder it across the blown fuses. You are unlikely to find wire that will fuse under 100's of mA. If you blow that then 'summat aglae'.

You could buy and fit "somewhat larger" fuses. Knowing what happens to your meters at larger currents may be good.

Some meters have very nasty internal resistances and so drop significant voltage. I measure and label cheap multimeters current range resistances to avoid being caught out. IF your meter has more than 3 ohms resistance on the 200 mA range it will drop > V = I.R = 0.2 x 3 =- > 0.6V. So a silicon diode across the meter will protect it from over current. A Schottky diode will protect against lower voltages. 2 x super cheap 1N4148 diodes back to back across the meter terminals may suffice.

Take an unblown fuse, place a small current (20 mA?) through it and measure the voltage drop across it. Note drop and slowly increase current. What is the voltage drop and current when it blows? Try this with a 300 mA fuse from other sources.

Measure the resistances of ALL the resistors that students used. Are the 2k resistors ALL REALLY 2k. Are there some 200 ohm ones in there. Or 20 ohms. Or 2 ohms ? :-). Note that it may not be the lowest intended value that is faulty. A 100k that's a 100R will fool you if you are lookng for a 2k/20r. Measure them all. Quickish way is to set psu high, put a meter on a eg 10 amp range then short psu through resistor and meter. If a 2k is 2k meter will hardly budge. If you get 200 mA+ you should notice it. Meter will not be hurt at 10 amps.

So - overall a mystery. Fuses are bad or something is not as it seems. Hopefully the above will provide some leads.

Answer 3

You may try running the experiment using the 10 Amps multimeter range. If the power supply has current limit (it should if it is used in education) you should not be able to damage anything but you may get more insight into the situation.

Answer 4

Update:

Thanks so much for all the replies. I got a lot of troubleshooting ideas and did some investigative work.

Today, I hooked up the same arrangement as before- power supply, resistor, and a multimeter (with a good fuse) and measured the output of the power supply with an oscilloscope for over an hour. Everything was solid and the voltage didn't change at all. Also, all the resistors were measured again and they are all within tolerance. The fuse did not blow in the multimeter.

Possibilities for what could have gone wrong:

• Resistors too low (Eliminated)
• Multimeters connected wrong (Eliminated)
• Bad Fuses (Highly unlikely, safety item and manufactured to strict tolerances, labels show correct rating)
• Bad power supplies (Somewhat unlikely, checked with oscilloscope for over an hour)
• Bad mains power from the school (Very likely as electrical work was being done all week, plus a power supply also blew on the same day)

Conclusion:

The most likely cause for blowing the multimeter fuses is bad main power when electrical work was being done at the school. I know work is being done for sure because the technician turned off all the power as I was working at lunchtime the next day. I think it's very important for the school to tell us when they are doing electrical work because we are using sensitive electrical equipment in the lab. Passed onto department head and upper management.