Overcurrent protection circuit

A feature of the present invention is that an overcurrent protection circuit is constituted so as to have the voltage relationship of V.sub.2 /V.sub.1 <V.sub.3 /V.sub.2, wherein V.sub.1 is a voltage applied to a positive temperature coefficient resistor at a steady current, V.sub.2 is a voltage applied to said positive temperature coefficient resistor at the maximum current, and V.sub.3 is a voltage applied to said positive temperature coefficient at the equilibrium current. The equilibrium current is rendered less than the steady current, so that heat generation of the load is effectively suppressed and the advantage of enhanced safety of the circuit is achieved.

FIELD OF THE INVENTION 
The present invention relates to a circuit protection device including a 
positive temperature coefficient resistor (hereinafter called a polymer 
PTC element) wherein carbon black is dispersed in crystalline resin. 
BACKGROUND OF THE INVENTION 
In a series circuit composed of a load, a power source, and a polymer PTC 
element, when an overcurrent flows such as by short-circuiting the 
temperature of the polymer PTC element rises due to Joule heat generation 
and in dependence upon the temperature rise the resistance of the element 
increases. As a result, the overcurrent is limited and the circuit is 
protected. 
The polymer PTC element, as disclosed in Japanese Patent Application 
Laid-Open publication No. 216401/1986, is a resistor wherein the carbon 
black is dispersed in crystalline resin and shows a positive temperature 
coefficient. Further, it has been known that such a polymer PTC element 
has an overcurrent protecting function. 
FIG. 2 is a circuit diagram illustrating a circuit protection device 
including a polymer PTC element. In the circuit of the same drawing, when 
an overcurrent flows due to short-circuiting, and assuming that the 
resistance of the load is R and the current flowing in the load is I, the 
amount of heat generation I.sup.2 R in the polymer PTC element exceeds the 
amount of heat radiation, and the temperature of the element rises. Since 
the polymer PTC element has a resistance-temperature characteristic as 
shown in FIG. 3, the resistance thereof increases in dependence upon the 
temperature rise, the current is limited and the circuit in the form of a 
road is protected. 
FIG. 4 is a diagram illustrating a current-voltage characteristic of the 
polymer PTC element. In a region where an applied voltage to the polymer 
PTC element is low, the temperature of the element is low and Ohm's low is 
maintained. In the circuit of FIG. 2, when there is no abnormality in the 
circuit, the polymer PTC element is held in any optional regions. Such 
regions correspond to steady state, and the current at this moment is 
called a steady current. When the applied voltage to the polymer PTC 
element further increases, the resistance of the polymer PTC element will 
suddenly increase while the temperature thereof is kept substantially 
constant. After the current exceeds the largest working current which can 
flow in the non-operable range of the polymer PTC element and reaches to a 
largest value, the current attenuates in response to the voltage rise. 
This largest current is called a maximum current. When an overcurrent 
flows through the circuit, the amount of heat generation of the polymer 
PTC element exceeds the amount of heat radiation, and the temperature of 
the polymer PTC rises. The amount of heat radiation is proportional to the 
difference between the element temperature and the ambient temperature. 
Since the resistance suddenly increases and the current decreases, 
finally, the amount of heat generation and that of heat radiation become 
equal (equilibrium condition). The current at this moment is called an 
equilibrium current. The relationship between the amount of heat 
generation and that of heat radiation is expressed by the following 
equation wherein C is the heat radiation coefficient, T is the element 
temperature and T.sub.a is ambient temperature: 
EQU I.sup.2 R=C (T-T.sub.a) 
In the region where the resistance of the polymer PTC element rises, the 
temperature of the element is maintained substantially constant and the 
current I decreases until the amount of heat generation coincides with 
that of heat radiation. 
For such use, a PTC using a barium titanate system is employed widely. The 
resistance of the PTC of barium titanate is high so that the size of the 
element becomes large to increase the heat radiation coefficient. For 
example, with a polymer PTC element employed for the overcurrent 
protection use for a small size motor, it displays a current-voltage 
characteristic A as shown in FIG. 1. When a power source voltage of 10 V 
is used, the equilibrium current becomes 0.5 A which is larger than the 
steady current. For this reason the motor overheats and damage thereof is 
possibly caused. 
Further, U.S. Pat. Nos. 4,329,726 and 4,238,812 disclose, respectively, 
circuits for reducing the equilibrium current. According to the disclosure 
of these patents, when the ratio P.sub.2 /P.sub.1 between an output 
P.sub.1 of the polymer PTC element in the steady state and an output 
P.sub.2 during the PTC operation is more than 8 (or 10), it is indicated 
that the current is sufficiently limited. However, as shown by A in FIG. 
1, even though the output of the polymer PTC element at the steady current 
is 0.01 W, and the output ratio in the equilibrium state reaches up to 
400, the equilibrium current can not be limited below the steady current. 
Further, when the voltage applied to the polymer PTC element becomes 3 V, 
the equilibrium current becomes more than 1 A which exceeds the largest 
working current, and the possibility of fire occurs. Therefore, even if 
the ratio is more than 8 (or 10), it is impossible to reduce the 
equilibrium current sufficiently. 
DISCLOSURE OF THE INVENTION 
A technical problem to be solved by the present invention is to decrease 
the equilibrium current, at least below the largest working current, 
preferably less than the steady current, and further preferably less than 
one third of the steady current. 
The reason why the equilibrium current can not be limited sufficiently in A 
of FIG. 1 is that the voltage V.sub.3 at the equilibrium current comes 
near to V.sub.2 at the maximum current. Therefore, according to the 
present invention, for reducing the equilibrium current to less than the 
largest working current, the circuit is constituted so as to have the 
voltage relationship V.sub.2 /V.sub.1 &lt;V.sub.3 /V.sub.2 to solve the 
problem. 
For example, by using a power source of 10 V, and a PTC element of 
0.1.OMEGA. and 10 as an embodiment and constituting the circuit so as to 
have the voltage relationship V.sub.2 /V.sub.1 &lt;V.sub.3 /V.sub.2, the 
equilibrium current can be reduced to less than the steady current. 
Further, with a circuit constituted so that a ratio between a voltage 
V.sub.4 at the largest working current and a voltage V.sub.2 at the 
maximum current is to be V.sub.2 /V.sub.4 &lt;V.sub.3 /V.sub.2, the equibrium 
current can be limited at least less than the largest working current. 
Therefore, by reducing the equilibrium current less than the steady 
current, the heat generation of the load is suppressed and the safety of 
the circuit is enhanced. 
For satisfying the above constitution where, the voltage relationship 
V.sub.2 /V.sub.1 &lt;V.sub.3 /V.sub.2, it is necessary to decrease the 
resistance as well as the heat radiation coefficient of the element. 
The resistivity of the PTC of barium titanate which has been conventionally 
used is as high as 5.OMEGA..multidot.cm, and, further the withstand 
voltage per unit thickness is low. In order to decrease the resistance of 
the element, the size of the element has to be enlarged, and in accordance 
therewith the heat radiation coefficient increases such that an increase 
of the equilibrium current is induced. 
Therefore, in order to decrease the equilibrium current, it is necessary to 
use PTCs having a small resistivity such as polymer PTC elements. In 
particular, for uses where the maximum current reaches more than several 
amps, the polymer PTC element is indispensable.

BEST MODE FOR CARRYING OUT THE INVENTION 
Hereinbelow, an embodiment of the present invention is explained with 
reference to the drawings. 
In the circuit according to the embodiment which is composed of, as in FIG. 
2, a power source of 10 V, a load in the form of a small sized motor of 
2.OMEGA., and a polymer PTC element which has carbon black dispersed in 
polyvinylidene fluoride, a diameter of 10 mm, the resistance-temperature 
characteristic in FIG. 3, and the current-voltage characteristic of B in 
FIG. 1, by constituting the circuit so as to have the voltage relationship 
of V.sub.2 /V.sub.1 &lt;V.sub.3 /V.sub.2, the equilibrium current is possibly 
reduced less than the steady current. Namely, in the present embodiment, 
the voltage relationship of V.sub.2 /V.sub.1 &lt;V.sub.3 /V.sub.2 is achieved 
as explained above, the equilibrium current is reduced down to 0.1 A with 
respect to the steady state current of 0.16 A so that the overheating of 
the small sized motor is possibly suppressed. In contrast in the circuit 
which is composed of the same power source of 10 V, the load in the form 
of the small sized motor of 2.OMEGA. and a barium titanate PTC having the 
same largest working current as that of the above polymer PTC element, the 
equilibrium current, becomes 0.5 A with respect to the steady current of 
0.16 A the amount of heat generation of the small sized motor in an 
abnormal condition reaches of 25 times of that using the polymer PTC 
element.