Selection of resettable fuse (PTC) for the board

I am create a board that will have:

1. MCU Atmega328P - x1
2. SK6812 RGBW LED - x9

The maximum current according to my calculations is 650mA (at full brightness of all LEDs, taking into account the margin).

I want to protect the circuit with a resettable fuse (PTC)

Example: Littelfuse 1206L075/13.2WR

• Current hold: 750mA
• Current trip: 1.5A
• Max voltage: 13.2V
• Max current: 100A

Questions:

1. It's a good choice?
2. I can not understand what the current trip means? Is this for inrush current or trip current?
3. Passive anging: +85°C, 1000 hours-/+5% typical resistance change. Is this a normal? The board will be operated in the temperature range: -25C - + 35C. How is it possible to recalculate the aging time at a lower temperature?
• 1. No-one can tell you that because there might be other requirements. 2. Trip current: This is the maximum current that the fuse can conduct without activation of the fuse. 3. Check datasheet or ask the manufacturer. – Stefan Wyss Aug 19 at 7:41

2 Answers

1. This is your job.
2. Defined on the datasheet: "Itrip = Trip current: minimum current at which the device will trip in 20°C still air. In other words it is guaranteed to (eventually) open at that current under the specified conditions. It may open at less current, depending on unit-to-unit variations and depending on ambient temperature. Refer to the derating curve. For typical time to open at fixed ambient refer to the preceding graph (this is important for figuring out how it behaves with short overloads)
3. There is really no basis to extrapolate the data, especially since it's not even in a specified direction. Typically that kind of drift is not linear but might be similar (cumulative) for 10,000 hours as for 1,000 hours, but as I said, there's really no information. What you can glean is that this is not a precision device and you need to allow a lot of safety margin.

In general you should try to make the difference between "must carry" and "must trip" as great as possible. That will make it easier to specify a fuse that will not open under extreme conditions (maximum ambient temperature, worst-case current and surge) and yet will protect what it is intended to protect under the opposite worst-case conditions.

The purpose of such a device is typically to protect the copper traces and maybe some other things such as wiring, so making the traces and wires fatter than necessary to carry the normal current will increase the available margin.

Note that the 750mA device will typically carry perhaps only 500mA at 70°C ambient, for example. If you expect the device to operate more than once, there might also be other aging factors involved, suggesting even more margin.

Also, SMT devices are sensitive to the amount of surrounding copper, far more so than through-hole types.

Think like a professional.

1. What you need? want to avoid?
2. What do you expect? And do if too hot?
3. What must you start with?
4. Write input/function/output specs!
5. Got everything? if not, go to 1.
6. Start design. Does it meet 4? Missing specs? Go to 4.
7. start design.
8. Any assumptions uncertain? Read part specs/graphs, verify.
9. Design Validation Test plan, complete. Review 1 to 8.
10. Meet all your criteria in 4? Perfect,! Done.
11. Not done? Do a better job next time on 1 to 4.
• Thanks for the checklist! – Алекс Гарисон Aug 19 at 16:04
• @АлексГарисон y’er welcome and these are the basics. You grow it, with wanting to avoid errors. – Sunnyskyguy EE75 Aug 19 at 16:07