Design / parameters of PTCs. Help with current limiting circuit.

[esm.]

Hi.
I have a PicKit3 MCU programmer from Microchip.
There is a PTC sensor close to the USB connector. Thats the PTC:
http://search.digikey.com/us/en/products/MINISMDC050F-2/MINISMDC050FCT-ND/1045862

Datasheet:
**broken link removed**

Somebody could explain the meaning of each of these parameters below? These are the parameters of the PTC above:

- PTC resettable fuse
- Current - Hold (Ih) (max): 500mA
- Current - Trip (It): 1A
- Time to Trip: 0,15s
- Current - Max: 100A
- R mim/Max: 0.150 ~ 1000 Ohm

I would like to know how to calculate a PTC value (determine its parameters) to make a over-current protection device, in a way the PTC begins to increase its resistance considerably only when the current flowing through it reaches a determinated value (I want to choose this value, in mA).


For example, I have some GPS modules that have a pin dedicated to connect the antenna power supply (I am putting 4,5V at this pin in my tests, and the GPS supports up to 5V at this pin). This pin have a maximum consumption of 50mA (maximum peak current), according to the GPS datasheet.
I would like to design a over-current protection against short circuit between the antenna wires (one wire is GND and the other is 4.5VDC), the choosen electrical current limit is 80mA to 100mA, for example.

Is it possible to design a current limiting circuit with the parameters below just by using a PTC in series with the antenna supply pin?

1) If the electrical current flowing through the PTC is below 60mA, it has very low series resistance (There is a very low voltage drop over the PTC)

2) If the electrical current flowing through the PTC is above 80mA~100mA, it has considerably high series resistance and limits the electrical current to a safe value? (lower than 120mA, for example, or some value near to this).


| 4,5V power supply | ------> | PTC | ------> | Antena power supply pin of my GPS module |

 

[skogsjanne]

It is probably not possible to design a current limiting circuit with the parameters below just by using a PTC in series with the antenna supply pin.

A PTC depend on generating heat to trip it. You will probably have to design it with a trip point far above 100mA to get reliable operation at 60mA.

If you need precise behaviour like this you should use electronic current limiting.

 

[FvM]

It's rather unlikely, that a 500 mA PTC will ever trip in PICkit 3. I guess, it has been provided to avoid a risk of fire in case of serious damage of PICkit 3, if it's connected to a not correctly current limited USB port. There are some PCs, where VUSB is directly connected to +5V rail, which can supply several 10 A.

But the 500 mA PTC can't protect the associates semiconductors, e.g. transistor Q4.

Referring to your application, a 50 mA PTC (which is also used in PICkit 3 to protect the debug interface against reverse voltage feed) can protect low power circuits in some cases. They are unsuitable however for precise current limiting.