# How to select the right polyfuse? By hold or trip current? Why the difference?

#### volkerforster

Joined Oct 2, 2022
3
Hi,
can someone explain the large gap between hold and trip current? For example, I look at a datasheet and see a holding current of 2.5A and a tripping current of 5A. It means, the fuse always holds 2.5A at 25 degrees Celsius, and always trips at 5A.
My real-life circuit is USB powered, by a supply that delivers 5V and 3A max. (USB 2.0 through type-C USB), and the maximum current drawn by my circuit is 2.2A.
If I look at a polyfuse that holds 2.3A then the trip current is 4.6A and way too high to protect the power supply.
If I look at a polyfuse that trips at 3A, then it can only hold 1.5A, which is much too low.
What kind of polyfuse would I need to look for, and why?
Thanks a lot.

#### ronsimpson

Joined Oct 7, 2019
2,710
The hold current is for "24/7" time.
The trip current is for some amount of time. Do not remember now.
They trip by temperature. If you double the current the trip time is 1/2. (more or less)
Fuses are not very accurate.

#### Sensacell

Joined Jun 19, 2012
3,180
Polyfuses are crude, primitive thermal devices, don't expect any sort of precision from their behavior.
Since they are thermal devices, just the way you mount them can have a significant impact on their behavior, the heatsink effect of the leads, airflow, etc.

They are really only useful when the expected fault current is 4X or greater than the normal current- in cases where their crudeness falls within wide margins.

#### volkerforster

Joined Oct 2, 2022
3
They are really only useful when the expected fault current is 4X or greater than the normal current- in cases where their crudeness falls within wide margins.
Thanks, that makes sense.

#### Hymie

Joined Mar 30, 2018
1,133
To hold a polyfuse in the trip state requires a certain amount of dissipated heat (by the polyfuse), therefore the lower the supply circuit voltage, the higher this hold current is. If the hold current for a 2.5A trip polyfuse (based on a 5V supply) is too great a value to protect your circuit from damage, then you should consider some other fault current protection.

Bear in mind that normal fuses can pass 1.6 times their rated current for some considerable time before they operate, so even this would require careful selection. Depending on your USB source, this may have integral over current protection to protect from an external overload.

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