# Did I choose the correct fuse for my USB powered circuit?

I need to drive a circuit (operational current 150mA) with a USB port. I would like to protect the port with a fuse and from this question I concluded, that what I want is a PTC fuse. Is that the correct conclusion?

I would choose the MF-PSMF035X, but I am somewhat confused by all the different currents provided in the data sheet and can not really tell, if this fuse would "blow" (how do you say that for a resettable fuse?) earlier than the fuse on my motherboard, which I would want to not blow, of course.

So here is my interpretation of the different currents:

• $I_{MAX}$ = current at which the fuse gets destroyed
• $I_{HOLD}$ = normal operational current
• $I_{TRIP}$ = max current through "blown" fuse
• Max. time to trip [Amperes @ 23 C] = max. current it takes for the fuse to "blow"

Now the USB fuse of my motherboard should blow at 1.5A according to the USB specs. If my PTC "blows" at 8A, as it says in the sheet, then it would effectively NOT protect my MB, would it? And also $I_{TRIP}$ of 750mA is larger than the 500mA one can continuously draw from a USB port. Is that a problem?

Oh and additionally: what is the best thing to do with the data pins of a USB port, if I only need the power pins? I read somewhere, that you are supposed to short them, but that does not seem like a good idea to me.

My interpretation is slightly different to yours:

• $I_{MAX}$ = The maximum current the fuse can handle without exploding in a shower of dust
• $I_{TRIP}$ = The current at which the fuse will "blow" at
• $I_{HOLD}$ = The current that is "safe" to pass through the fuse
• Feed it 8 amps and it will blow within 0.1 seconds.

So yours will definitely "blow" at 750mA (give or take), but could also degrade and eventually "blow" at anything over 350mA.

• Initial resistance: The resistance of the device as received from the factory of manufacturing.
• Operating voltage: The maximum voltage a device can withstand without damage at the rated current.
• Holding current: Safe current through the device.
• Trip current: Where the device interrupts the current.
• Time to trip: The time it takes for the device to trip at a given temperature.
• Tripped state: Transition from the low resistance state to the high resistance state due to an overload.
• Leakage current: A small value of stray current flowing through the device after it has switched to high resistance mode.
• Trip cycle: The number of trip cycles (at rated voltage and current) the device sustains without failure.
• Trip endurance: The duration of time the device sustains its maximum rated voltage in the tripped state without failure.
• Power dissipation: Power dissipated by the device in its tripped state.
• Thermal duration: Influence of ambient temperature.
• Hysteresis: The range between where the device trips and where the device returns to a conductive state.

PTC Fuses are basically heat based. When they get too hot they go very high resistance. That heat could be rapidly produced by an overcurrent situation ($I_{TRIP}$) or a prolonged higher current ($> I_{HOLD}$) over time.

• Oh well, I guess searching the English Wikipedia would have helped :-). Thanks! – user35915 Dec 3 '14 at 20:14

Ptc are self resetting devices, that rapidly rise in resistance as temperature rises (hence positive temperature coefficient). They are not like traditional fuses, as they will start working again when the temperature of the ptc goes down, typically by removing the over current situation.

When you say blows at 8 amps, it means that the PTC is permanently fused closed/high resistance and will no longer reset.

When you say tripped, it means the resistance has gone up due to an overcurrent situation but can return to normal once fixed. If you allow too many trips, or allow an over current situation for too long, the PTC will be damaged just like blowing.

The I Hold is the normally safe current through the ptc. This should be your devices current plus in rush so 120% or so. The I Trip should be the current you want the fuse to trigger at, say 200% of the regular current. Keep in mind I trip is time sensitive, e.x. 400mA for 2 seconds will trip it.

I Max is the catastrophic worst case scenario and not relevant to your situation.

As for the usb lines, if you are using a charger, ignore them. If you are using a pc usb port, do not tie then together. Just leave them alone. Most usb ports are not current limited.

• Thanks! What does it mean, that they are not current limited? Can I just short the power pins of a USB port and nothing will happen to my MB? – user35915 Dec 3 '14 at 20:17
• Well, macs are built with high standards. It does have current limiting and some protection. I need a usb Y cable to power a 600mA external harddrive. Most pc towers tend to connect the usb power directly to +5V or 5VSB rails, and you can pull more than 500mA from a single port. But shorting any power rail is ALWAYS A BAD IDEA Current Limiting in USB means that it prevents a usb device from pulling more than it requested. – Passerby Dec 3 '14 at 22:17