# Measuring current with a fuse

What could be the ways to measure a current based on the heat of the fuse (just by simulating on Matlab for example) ?

Thanks.

• I wouldn't bet that the characteristics of a fuse are reproducible enough that you could get accurate results. Temperature will also depend on the environment - if the air temperature is 40 degrees C, and your fuse is at the same temperature then that is different from air temperature at 0 degrees C and fuse at 40 degrees C. – JRE Oct 25 '16 at 15:46
• Use the fuse as a current shunt. Measure the actual DCR then use the voltage drop to calculate current. The DCR should be measured by forcing a current through the fuse and measuring voltage drop. The forced current should be around the same as the current you expect to measure. – mkeith Oct 25 '16 at 16:03
• This type of thing is probably better experimentally tested. You could always make a look up table with the current vs temperature from the datasheet of the fuse. After the fuse place a switch that triggers if the current gets to high to simulate an open condtion – Voltage Spike Oct 25 '16 at 16:53
• thank you all for answering. I was thinking about finding an equivalent model to help me simulating better.. But I can't find much information on internet, since it's not something very common .. I would be grateful if anyone could help me with that ! – user127838 Oct 25 '16 at 18:20
• It is not a good idea to use a fuse (or any other safety device) as something else if it is still being used as a primary means of safe failure. Interfering with it might make it fail unsafe! You don't see skydivers sewing pockets into their parachute silks :) – Wossname Nov 24 '16 at 20:17

Fuses are typically not uniform enough to use them as current shunts. You might get a very crude sense of how much current is flowing, but there are so much easier, cheaper and more accurate ways, using a fuse seems like a very poor and unreliable solution.

• thank you for answering. I agree, but I'm working on a project and this is what I must do. – user127838 Oct 25 '16 at 18:14
• Since we don't know what your project is, we probably can't offer many practical suggestions. If there is enough current flowing through your fuse to create a significant voltage differential (to measure and calculate current) then you are probably near the limit of the fuse and about to melt it. There are many disadvantages to this method and virtually no advantages. – Richard Crowley Oct 25 '16 at 18:20
• I'm just working on the theorical aspect for the moment.. Iam actually asked to do simulations on Matlab to see if this is possible. I'm aware that it won't be the best solution in terms of efficiency but the idea is still feasible – user127838 Oct 25 '16 at 18:24
• Before you waste too much more time with Matlab, get a few boxes of fuses and a milli-ohm meter and see for yourself how much variation there is and whether that fits within your (undisclosed) performance requirements. – Richard Crowley Oct 25 '16 at 18:31
• As mentioned above, a vendor once told me that the fuse DCR tolerance was 20%. This was an 0402 fuse. However, if you measure a specific fuse, then it can be a good current shunt. The tempco is not that bad. – mkeith Oct 25 '16 at 23:43

measure a current based on the heat of the fuse... just by simulating on Matlab

You can't measure current with a simulation. You can simulate measuring it, but nothing will actually be measured. So assuming you just want to simulate measuring current by the heating of a fuse...

Heating of the fuse is proportional to the electrical energy it absorbs. Electrically the fuse is a resistor, so we can apply Joule's First Law:- $$Heat = Current^2 \times Resistance \times time$$ Therefore:- $$Current = \sqrt{\frac {Heat / time}{ R_{fuse}}}$$

But how can you measure heat? You cant - directly. But with an appropriate sensor you could measure the fuse's temperature. As the fuse heats up its temperature rises, stabilizing as the heat flows through the fuse's thermal resistance to its surroundings. To do the job properly you would need two sensors, one to measure the fuse temperature, and another to measure the ambient temperature. The difference between them is the fuse's temperature rise, which is proportional to the rate of heating (power).

In reality this would be a difficult and unreliable way to measure current. Firstly, you would need to calibrate it to the thermal resistance of the fuse and its mounting (and the sensor needs to read the fuse's internal temperature, which might not be easy). Secondly the fuse's electrical resistance is not constant, but changes with temperature depending on the material the conducting element is made of (copper, silver etc.) and that temperature varies along its length. Thirdly, the fuse and sensors may have a relatively long thermal time constant, so the measured current could lag behind the actual current by several seconds.