PTC or Resseteable fuses

Hello guys,

I need to place a PTC in my design with the next characteristics:

Nominal Voltage > 48V
Hold Current >1A
Trip Current > 9.3A

I am not able to find any comercial PTC with these characteristics (Mainly because the Trip Current max that I have founf is 8A. Is there any trick or tip schematic design in order to protect my design with a PTC. Maybe placing two in parallel or something lile that..

Thanks in advance!

Hi,

It seems your information is wrong:

Trip Current > 9.3A
should be
--> Trip Current < 9.3A

Klaus

Hi,

Currently I am wondering about the meaning of the trip current.

I want to break the circuit when DC Current drived trhough the track is higher than 9.3A. As i understood, in a PTC, when the current is higher than ITrip, the thermal resistor is very high, so the circuit is open... is it working so?

Regards

Hi,

Read about resettable fuses.
They have a description of every specification, and they have test conditions.

***
Usually the "hold" current is that current that never causes the fuse to trip. (Within the given ambient temperature range, maybe there are additional other conditions).

The "trip" current is the current where the fuse has to trip.. within a specified time. (Again: Within the given ambient temperature range, maybe there are additional other conditions).

****
So below the 1A the fuse must not trip.
Between 1A and 9.3A the fuse may trip or not.
Above 9.3A (and longer as the trip-time) the fuse must trip.
Above 9.3A (and shorten as the trip_time) the fuse may trip or not.

As with all resettable fuses there will be some remaining current - even when the fuse has tripped. Expect 5mA .. 50mA (depending on fuse type and load condition).

Klaus

ok in understand know.

then I need to trip the PTC when the current > 9.3A. And I did not found any PTC which is able to do this...Any suggestion?

Thanks in advance!

Hi,

as said before:

--> Trip Current < 9.3A
Not "> 9.3A"

So a fuse specified with a trip current of 6A will also safely trip with a current of 9.3A.

***
The other way round: If you specify the trip current to be >9.3A, means a fuse with a trip current of 30A meets your specification. But it will not (safely) trip with 9.3A.

***
Another explanation:
A wire and a fuse to protect the wire from overlaod. Now try to specify both to 10A current.
--> Then the wire is safe from overload if it´s current specification is "> 10A".
--> but it is safe from overlaod when the fuse trip current is "< 10A" .

Klaus

KlausST said:

Between 1A and 9.3A the fuse may trip or not.
Above 9.3A (and longer as the trip-time) the fuse must trip.
Above 9.3A (and shorten as the trip_time) the fuse may trip or not.

Click to expand...

I am thoroughly confused; please elaborate.

When we say great than 9A, we can argue: 100A (it is greater than 9A) or 10A (it is also greater than 9A) are both valid. However, the smaller one is a better value. We use minimal and maximal for such numbers.