JPS58101557A - Overcurrent protection circuit - Google Patents

Abstract

PURPOSE:To protect a subscriber circuit from overcurrent due to a failure in a subscriber line, by providing a temperature detection element near a feeding transistor (TR) of a electronic type subscriber circuit. CONSTITUTION:A subscriber telephone set (not shown) is fed from a power supply VBB via feeding TRs6, 13 controlled with current limiting resistors 8, 16 and a call current driving circuit 11. A temperature detection element 1 such as PTC thermister is provided near the TRs 6, 13. If a feeding current (i) is excessive due to a failure such as grounding of a subscriber line, the temperature of the TRs6, 13 is risen, the resistance value of the element 1 is increased, and the potential of a connecting point 17 between the said element 1 and a resistor 3 is increased. A comparator 2 controls the call current driving circuit 11 to limit the call current so as not to make the current (i) excessive.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overcurrent protection circuit in an electronic subscriber circuit used in a telephone exchange.

If an overcurrent flows through a subscriber line to which an electronic subscriber circuit is connected, the subscriber circuit components may overheat or catch fire. Possible causes of this fault 11IL- are contact with the earth on the All side of the joining tuple and contact with the battery on the Bli side. The subscriber circuit usually has an internal resistor 44
Since it is designed with 0ρ (2209 on each side of All B edge sides), the maximum current flowing through the subscriber S* during normal times is
20mA (SST (power supply) 74
40ρ (internal resistance)), but the maximum current flowing through the subscriber line during the above fault is 240mA (55V / 22
0Ω) overcurrent. Therefore, to prevent subscriber circuit components from heating up and catching fire due to stiffness.

A large heatsink was added to the power supply transistor, and the external resistor was made of a large component that can withstand four times the normal power in order to withstand the load in abnormal situations.

SUMMARY OF THE INVENTION An object of the present invention is to provide an overcurrent protection circuit which eliminates the above-mentioned drawbacks and prevents electronic shore circuit components from heating up and catching fire due to overcurrent.

The present invention detects temperature in the vicinity of the heat sink (downwind of the heat flow) of the power supply transistor, which is one of the components for the electronic subscriber circuit, which performs Harukame-style maintenance. When the temperature around the transistor exceeds a specified temperature, the device controls the drive circuit that drives the transistor to stop or limit the power supply to the subscriber line and protect the subscriber circuit.

Hereinafter, embodiments of the present invention will be described in detail using -.

Figure 1 shows a circuit that detects the temperature of the power supply transistor using a temperature detection element, compares it with a reference value using a comparator, and drives the drive circuit to stop the function of the power supply transistor. be.

The second reason is as shown in FIG. 1, which shows a circuit in which a control transistor is added to the circuit shown in FIG. 1 to limit the current flowing to the power supply transistor. FIG. S is a diagram showing the temperature characteristics of the temperature detection element (PrC sensor).

In the figure, 1 is a temperature detection element (P'IIC therister), 2
is a comparator, black is a resistor, 45 is a resistor that determines the reference voltage, 41s is a power supply transistor for A and B lines, 8.14 is a resistor in the communication current loop, and 15 is a power supply transistor 4 .15 is a resistor that determines the bias, 7.12 is a communication current control transistor for controlling the amount of communication current, 10.14 is a resistance for limiting communication current 14I, 11 is a power supply transistor 4. 1! , or a drive circuit that drives the control transistor 7112.

First, the subscriber circuit having the eleventh current stop +a function will be explained.

When the temperature of the power supply transistor 6113 rises due to overcurrent during a fault, the temperature detection element 1 mounted near the transistor 6.1B
is detected. By kneading, the potential at point 17 increases due to this temperature characteristic, as shown in Figure S, which shows the temperature characteristic of the PTOt-'' mister, which is a temperature detection element. The reference potential and the potential at point 17 are compared by comparator 2, and when the potential relationship at each point satisfies equation (1), the output level of comparator 2 is inverted.

(Potential of point 17)〉(Potential of point 18) - (
1) Based on the output level signal of the comparator 2, the communication current drive circuit 1
1, and the power supply transistor 6.15 is driven by Towa.
to prevent subscriber circuit components from overheating and catching fire.

Next, a subscriber circuit having a second current limiting function will be explained.

This current 11tl limited circuit, like the first prisoner, detects the temperature rise of the power supply transistor 6.13 due to the amount of communication current with the temperature detection cable 1, and detects the change in the potential level at point 17.
It is monitored by the comparator 2, and the output level signal of the comparator 2 drives the communication current drive circuit 11, thereby operating the communication current control transistor 7.12. If the control transistor 7.12 operates, the power supply transistor 6.1B
A resistor 10°14 is connected in parallel to the resistor 9.15 which determines the Peianu point of The potential applied to both ends decreases. In other words, the communication current i has decreased. Therefore, by controlling the communication current, heat generation and ignition of subscriber circuit components can be prevented.

As explained above, according to the present invention, it is possible to prevent subscriber circuit components from generating heat and catching fire due to overcurrent, and furthermore, from spreading fire to others.

[Brief explanation of drawings]

1s11 is a diagram showing the first embodiment of the overcurrent protection IS circuit according to the present invention, FIG. 2 is a diagram showing the second embodiment of the present invention, and FIG. 5 is a diagram showing the temperature characteristics of the temperature detection element. The illustration is tuna. Be 4t 5-resistor, 11.-talking current drive circuit. 6.15-Power supply transistor, 7.12-Control transistor, ? f fI 2nd figure 30 Continuation of page 1 ■Applicant NEC Corporation No. 1, 5-3 Shiba, Minato-ku, Tokyo ■Applicant Fujitsu Ltd. 1015-255 Kamiodanaka, Nakahara-ku, Kawasaki-shi

Claims (1)

[Claims] In a subscriber circuit having a power supply transistor that supplies power to a subscriber and a drive circuit that drives this transistor,
By providing a temperature detection element for detecting the temperature of the power supply transistor, a comparison means for comparing the potential of the armpit temperature detection element with a reference potential, and a means for controlling the drive circuit of the transistor with the output of the temperature detection element, the power supply can be improved. An overcurrent protection Si circuit characterized by stopping the function of a transistor L or limiting the current.