- Sun Nov 08, 2009 2:45 pm
#84596
I am having some trouble getting my head around the PTC parameters for selecting an appropriate PTC fuse.
The terms Itrip and Ihold are what's geting me confused.
Ih - The maximum steady state current at 20°C that can be passed through a PolySwitch device without causing the device to trip
lh - The minimum current that will cause the PolySwitch device to trip at 20°C
Say I have a circuit with a regulator that can only supple 150mA, I want to limit the input current to now use anything greater.
A PTC has a Ih of 150mA and an It of 320mA. Now my question is, what really happens between Ih and It? Is this where the resistance starts to increase, until it becomes open circuit at It?
If so, my question is, is my assumption that choosing a PTC with an Ihold of 150mA the correct one for my given system?
Now for the zener diode. I would like to add overvoltage protection, currently my circuit is something like this;
Vin1----->|------[PTC]-----[VREG]-----
Vin2----->|--^
The two diodes OR the power supply to one of the other, as well as providing protection against reverse polarity connection.
Now I would like to add in a TVS Zener diode. The regulator can handle up to 16v but if Vin1 is greater than 7v, there is a problem.
If I put in over voltage protection to protect against >7v on Vin, that breakdown voltage should I look for, something a bit higher than 7v?
What's the best way to connect this in, parallel with Vin1 and GND?
Some people say that a zener when protecting against over voltage would blow up, these TVS Zeners can take 1000's of watts, so I assume that this isn't an issue?
The terms Itrip and Ihold are what's geting me confused.
Ih - The maximum steady state current at 20°C that can be passed through a PolySwitch device without causing the device to trip
lh - The minimum current that will cause the PolySwitch device to trip at 20°C
Say I have a circuit with a regulator that can only supple 150mA, I want to limit the input current to now use anything greater.
A PTC has a Ih of 150mA and an It of 320mA. Now my question is, what really happens between Ih and It? Is this where the resistance starts to increase, until it becomes open circuit at It?
If so, my question is, is my assumption that choosing a PTC with an Ihold of 150mA the correct one for my given system?
Now for the zener diode. I would like to add overvoltage protection, currently my circuit is something like this;
Vin1----->|------[PTC]-----[VREG]-----
Vin2----->|--^
The two diodes OR the power supply to one of the other, as well as providing protection against reverse polarity connection.
Now I would like to add in a TVS Zener diode. The regulator can handle up to 16v but if Vin1 is greater than 7v, there is a problem.
If I put in over voltage protection to protect against >7v on Vin, that breakdown voltage should I look for, something a bit higher than 7v?
What's the best way to connect this in, parallel with Vin1 and GND?
Some people say that a zener when protecting against over voltage would blow up, these TVS Zeners can take 1000's of watts, so I assume that this isn't an issue?