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How to measure a fuse current

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Fahime

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I have some fuses in hand. There's no figure on them. I don't know what kind of fuses they are and how much the max current and other specifications are.
Could you tell me how I can found the the specifications out?
 
Get a constant voltage power supply that has a greater current than you think is necessary and then use a power resistor in series with the fuse to send current through it. The resistance is probably much larger than the fuse resistance so you can neglect the fuse resistance.

Use V=IR to predict how much current will flows through the fuse for a given voltage supply and resistor value. Start with a large resistor value that you know won't allow enough current to blow the fuse and let it run through the fuse for about 2 seconds. Then change to a slightly smaller resistance. Keep decreasing the resistance if the fuse survives for more than about 2 seconds or so.

When the fuse burns out in about 1 second or less, depending on the type of fuse it is (slow blow or fast blow) that current value is around double what the fuse should be carrying in normal operation.

If you have a current limiting power supply, it is much easier. Just turn the current limiting to zero and turn the voltage to 1V, or 2V, or 5V, or some low voltage. THen slowly turn the current limiting up so the current in the fuse increases. Ideally, turn it just slow enough so the current rises slowly enough so you don't overshoot but not so slow that the fuse will gradually build up heat to eventually blow (like a fuse at 100% current rating might take 4 hours to blow, but at 200% current rating it will take 1 second).
 
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you can do it the cheaper way...

for steady-state current

If=In/(RR*0.75)

If=Ideal fuse rating
In=Nominal operating current
RR=Temperature re-rating factor

for transient pulse
It has no steady state current. You can do calculation on how many pulses the fuse can withstand. This is done by comparing the I^2t of the pulse with the relative I^2t of the fuse. Ideally, I^2t is the heat energy of the pulse and fuse. You must consider also the melting time (the amount of time required to melt the fusing element).

Relative I^2t = (I^2t of Pulse)/(Melting I^2t of Fuse)

This is in term of percentage. Military std normally apply 70% passing. Some automotive company uses 90%.

for transient current (inrush)
It is actually a pulse followed by steady state current. So the calculation is done on how many pulses the fuse can withstand and if the fuse is strong enough for the steady state. This is using the cumulative I^2t fo the pulse and fuse. It is actually the cumulative energy over time.

BTW, the I^2t factor can be derived through the graph provide by the supplier. This is normally called TC (time-current) characteristics. Some suppliers provide graphs for Number of Pulse VS Relative I^2t.

Hope this information helps.
 
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