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My device runs from a USB supply, typical 5V, current typically 20mA, never beyond 100mA.

I want to add a fuse. The JFC0402-0500FS claims rated current 0.50A (perfect for this application) and a typical resistance of 500mΩ.

Since E=IR, I'm getting an expected voltage drop of 50 mV. Now, that resistance is at a full 100mA; perhaps the resistance is higher for less current, so let's double it to 100mV. Is that the correct voltage drop to expect?

Interestingly, the larger packages (e.g. 0603) had higher voltage drops, the opposite of what I'd expect from a resistor. I chose this package and this trip current to minimize the voltage drop.

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    \$\begingroup\$ Why do you think that the resistance would change with current? I'd expect change with temperature, so possibly the resistance would increase with higher current due to thermal effects but I don't know why it would decrease. \$\endgroup\$
    – John D
    Commented Sep 13, 2023 at 22:44
  • \$\begingroup\$ Note that a USB device needs to work with supply voltages down to 4.8 V anyways, so if these 0.05 V break the camel's back, the device probably has design issues anyway \$\endgroup\$
    – Marcus Müller
    Commented Sep 13, 2023 at 23:30

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Yes, 0.5Ω*0.1A is 50mV so at the max current of 100mA that would be 50mV max voltage drop for your design.

But one problem is the 500FS has a rated current of 500mA so if you are trying to current protect the device, the fuse won't start melting until 500mA which is around what the minimum of most usb devices source.

You may want to resize the fuse for a smaller current like 200mA, but this will come at the cost of higher resistance (~2Ω) which probably won't work for most designs. So this begs the question of why a fuse would be used at all.

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Source: https://www.electronics-notes.com/articles/analogue_circuits/power-supply-electronics/current-limiter-circuit.php

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  • \$\begingroup\$ My feeling is that: If there's a short, it will blow quickly, and if there's no short, I can't think of any failure mode that a fuse can protect against, period. \$\endgroup\$
    – SRobertJames
    Commented Sep 14, 2023 at 0:09
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    \$\begingroup\$ But here is the problem, some usb ports only source 500mA, so if you have a 500mA source with a 500mA fuse, the fuse might not blow or it will take a long time to blow. \$\endgroup\$
    – Voltage Spike ♦
    Commented Sep 14, 2023 at 2:37
  • \$\begingroup\$ Ahh! Now I see. So I need something that will trip at, say 200-400mA, if I want to catch a short in my circuit. Otherwise, I run the risk of blowing the fuse in the USB source, which may damage it. I want to ensure that my device's fuse blows first. What's the maximum trip current I can use that will blow before the device's fuse blows or the device gets damaged? \$\endgroup\$
    – SRobertJames
    Commented Sep 14, 2023 at 2:58

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