Help understanding SMD PPTC fuse specs.

[HellasTechn]

Hello everyone. I have this question now that i need help with.

I have a GPS with an rs422 output. I want to protect its output from burning if short circuited to ground.
According to the RS422 standard the RS422 lines have a max 150ma short circuit current.
I want to use fuses in serire with the output to prevent damage in case of short circuit.
I found the Littlefuse model MICROSMD010F-2 . I have the datasheet for it but i dont understand its specs. I am posting a picture with the specs.
It says Hold current : 100ma. I supose it means that it will not allow more than that to pass through.
Then it says trip current 250ma. I what does that mean ? maybe that the fuse goes completely open above 250ma ?
Last, it says Current rating max : 10 A. What does that mean ?

THANK YOU SO MUCH !

 

[Harald Kapp]

This kind of "fuse" is not a fuse in the classic sense which completely opens the circuit in case of overcurrent. It works like a PTC which has a low resistance in the operating temperature range. In case of overcurrent, the "fuse" heats up and the resistance increases considerably, thus limiting the current, not interrupting it. When the short circuit is removed and the component cools down, the "fuse" is reactivated. This type of "fuse" is also called resettable fuse.
Trip current is the current required for the "fuse" to be activated.
Hold current is the minimum current required to keep the "fuse" in the off state.
Max. current rating is the max. short circuit current the component will be able to tolerate. At higher currents the component can bfreak down and will not be resettable.

Have a look at the RS422 driver chips used in your GPS device. Some RS422 drivers are short-circuit tolerant and you may not even need additional protection.

 

[HellasTechn]

Trip current is the current required for the "fuse" to be activated.
Hold current is the minimum current required to keep the "fuse" in the off state.
.

So in other words the above fuse will restrain current to 100ma only after the load will draw above 250ma ?
hold current is required to keep the "fuse" conducting ? mening that it will block below 100ma currents ?
I dont understand...

 

[Harald Kapp]

So in other words the above fuse will restrain current to 100ma only after the load will draw above 250ma ?

Not exactly. The current drawn depends on the type of fault, too.

hold current is required to keep the "fuse" conducting ?

No. Hold current will keep the fuse in the off state, which is only partly conducting.

mening that it will block below 100ma currents ?

As long as the fuse has not been triggered, any current less than 250 mA can pass comparatively unhindered. Once the 250 mA limit is exceeded, the fuse will trigger and become highly resistive, thus reducing the current. As long as the short circuit current is 100 mA or more (hold current), the fuse will stay in the off state. When the current falls below 100 mA, the fuse will cool down and operate normally (allow some time for cooling).
The reason for the different trigger and hold currents is that in the normal on state the fuse has a low resistance. Thus a higher current is required to generate the power necessary to heat up the fuse until it becomes high impedance.
Once the fuse is in the high impedance (off) state, a much lower current is sufficient to generate the same amount of power to keep the fuse off.

Maybe this article is clearer than the Wikipedia entry in my first answer.

 

[HellasTechn]

Thank you !

So this in fact could be compared with a 250ma fuse. Well in a way... So it can not be used to protect devices that max allowed current is 100ma.

 

[Harald Kapp]

No, you'll need a 100 mA fuse or resettable fuse.

 

[HellasTechn]

No, you'll need a 100 mA fuse or resettable fuse.

I must be missing something.

As long as the fuse has not been triggered, any current less than 250 mA can pass comparatively unhindered. Once the 250 mA limit is exceeded, the fuse will trigger and become highly resistive,

if the above is correct then how will that specific pptc fuse be able to protect a circuit that can not run above 100ma ?

That specific fuse was titled as 100ma pptc fuse. maybe it was mistitled ?

 

[Harald Kapp]

I must be missing something.

a 250 mA fuse will trigger if the current exceeds 250 mA. Thus it wll pass currents between 100 mA and 250 mA and offer no protection in that range.

f the above is correct then how will that specific pptc fuse be able to protect a circuit that can not run above 100ma ?

It can't. Rember: hold current is the current required to hold the fuse in the "off" state once it has been triggered by the much higher trigger current. But current in the range between hold current and trigger current will not suffice to trigger the fuse.

That specific fuse was titled as 100ma pptc fuse. maybe it was mistitled ?

The datasheet you linked states trip current = 250 mA. Thus the fuse will trip (or trigger to use my previous phrasing) if the current exceeds 250 mA. Not 100 mA.

 

[HellasTechn]

Okay so Trip current if exceeded the "fuse" will introduce high resistance and will not allow more current to pass.
Okay then the circuit must force current more than 100ma to "enable" the "fuse" to conduct but not more than 250ma.
I think ive got it now

P.S.
like this one is wrongly labled as 100ma
https://www.ebay.com/itm/20Pcs-Rayc...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

 

[bertus]

Hello,

Perhaps the following document might give you more insight on the use and characteristics of a polyfuse.

Bertus