Overheat protection device and electrical system having same
The present invention provides an overheat protection device which is capable of effectively preventing a secondary battery from overheating. In the overheat protection device ( 20 ), a variable resistive element(s) ( 11 ) of which resistance varies depending on a temperature is located on and thermally combined with a certain position(s) of an electrical system ( 1 ), and a switching element ( 15 ) for controlling a current depending on an applied voltage thereto is arranged so as to control the current flowing through the electrical system ( 1 ). When the certain position of the electrical system ( 1 ) comes to be under a high temperature condition, the variable resistive element ( 11 ) interrupts the current flowing through the electrical system ( 1 ) by changing the applied voltage to the switching element ( 15 ).
1. An overheat protection device comprising a variable resistive element for which resistance varies depending on temperature and which is a polymer PTC element, characterized in that the device further comprises a switching element which controls a current flowing through an electrical system depending on an applied voltage thereto, and a resistor having a resistance R, said electrical system comprising a secondary battery, and the variable resistive element is composed of a plurality of variable resistive elements at least some of which are electrically connected in series with each other, the variable resistive element being located on and thermally combined with a certain position of the electrical system and interrupts the current flowing through the electrical system by changing the applied voltage to the switching element when the certain position comes to be under a high temperature condition, the variable resistive element having a resistance in a high temperature state P H and a resistance in a low temperature state P L , the relationship between the resistance of the variable resistive element and the resistor being R/P L >10 and R/P H < 1/10.
2. The overheat protection device according to claim 1 , wherein the variable resistive element is located on and thermally combined with the secondary battery.
3. The overheat protection device according to claim 1 , wherein all of the plurality of variable resistive elements are electrically connected in series with each other.
4. The overheat protection device according to claim 1 , wherein
the variable resistive element and the resistor are electrically connected in series with each other and in parallel to the electrical system, and
the switching element is electrically connected in parallel to the resistor.
5. The overheat protection device according to claim 4 , wherein
the switching element is a field effect transistor (FET),
a gate of the FET is electrically connected to a position between the variable resistive element and one end of the resistor,
a source of the FET is electrically connected to another end of the resistor,
the source and a drain of the FET are electrically connected to form a part of an electric circuit comprising the electrical system, and
when a voltage between the gate and the source of the FET becomes not greater than a threshold value, the current does not substantially flow between the source and the drain of the FET so that the current flowing through the electrical system is interrupted.
6. The overheat protection device according to claim 5 , wherein a value of the voltage between the gate and the source is expressed as formula (1):
V
GS
=
R
P
+
R
V
0
(
1
)
wherein V GS is the voltage between the gate and the source, V 0 is a voltage across the variable resistive element and the resistor, P is a resistance of the variable resistive element, and R is a resistance of the resistor.
7. An electrical system comprising the overheat protection device according to claim 1 .
8. The electrical system according to claim 7 , wherein the secondary battery is electrically connected to an electrical element to form an electric circuit, and the overheat protection device is connected in parallel to and between the secondary battery and the electrical element.