# Design / parameters of PTCs / NTCs. Help with protection circuit.

Discussion in 'The Projects Forum' started by esm., Mar 18, 2012.

1. ### esm. Thread Starter Member

Jan 20, 2012
30
0
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:
http://documents.tycoelectronics.co...SCD25063HpdfEnglishENG_SS_SCD25063_H.pdf

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 |

Last edited: Mar 18, 2012
2. ### evilclem Member

Dec 20, 2011
118
16
You plan to use the PPTC fuse as a current limiter rather than it's intended purpose as a fuse? Not really that simple.

My understanding is that hold current is the current you can safely pass through the PPTC fuse without tripping it.

Last edited: Mar 19, 2012