GB2139437A

GB2139437A - Resistive circuit detector

Info

Publication number: GB2139437A
Application number: GB08408405A
Authority: GB
Inventors: Michael P Bradford
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1983-04-18
Filing date: 1984-04-02
Publication date: 1984-11-07
Also published as: JPS59198837A; ES531669A0; GB2139437B; CA1226622A; DE3414536A1; IL71529A; IT1209530B; FR2544563A1; IT8420611A0; AU2661184A; GB8408405D0; IL71529A0; ES8502816A1; SE8401957D0; SE8401957L; AU571210B2

Abstract

A voltage drop develops across a current-limiting resistor 14 connected between a power source 10 and a load 16 in proportion to the current draw. A zener diode 18 becomes conductive in the reverse direction when the voltage drop increases to a threshold, thereby turning on a light emitting diode 22 (LED), to detect excessive current draw. A photo-SCR may be optically coupled to the LED to provide a latched fault indication. For detecting partial or complete open loads, normal load current will be high enough for LED 22 to be on, and to turn off as current falls. A thermistor may be substituted for the current limiting resistor in certain cases to provide protection and detection at higher current demand levels. <IMAGE>

Description

SPECIFICATION Resistive circuit detector Technical Field This invention relates to the protection of electric power sources and distribution equipment from faults in loads, and more particularly, to implementing said protection by solid state means.

Background Art Historically, power systems utilized fuses or circuit breakers for protection against faults in wiring or loads. Both means isolate faults by mechanically breaking contact with the power source. Both means are dependent on a thermal reaction to circuit overload currents, thus are slow and temperature dependent. The thermal element of a fuse is destroyed in the isolation process, while circuit breakers are large and heavy as compared to circuit components.

Disclosure of Invention Therefore, it is an object of this invention to protect power sources and distribution equipment from faults in wiring or loads using solid state means that are space and weight efficient. It is a further object to provide for protection without necessarily disconnecting the faulty circuit or load.

It is a still further object to provide for instantaneous current limiting. It is another object to provide for protection at low current levels. It is another object to provide means for detecting and signalling either partial or complete short circuit failures. In the main hereinafter, load faults that draw excess current are discussed, but the teachings disclosed herein are applicable to detecting partial or complete open circuits in loads.

According to the invention, a power source is connected to a load by a current limiting resistor.

Under normal operating conditions the voltage drop across the current limiting resistor is small and accounted for. When the load draws excess current (fault), such as in a short circuit, the voltage drop across the current limiting resistor increases dramatically. With a high impedance load, such as a signal circuit, the current draw is small and the current limiting resistor can have a high resistance, thereby providing for a disproportionate increase in voltage drop for a given small increase in current draw. A thermistor may be substituted for the current limiting resistor to provide similar protection at higher current demand levels. In response to the increased voltage drop across the current limiting resistor, a zener diode becomes conductive in the reverse direction (reverse breakdown) and connects the voltage drop to an indicator, such as an LED.A latching circuit may be incorporated to provide a latched fault indication or to disconnect the faulty load.

The foregoing and other objects, features and advantages of the present invention, will become more apparent in the light of the following detailed description of the invention.

Brief Description of Drawings The sole Figure is a schematic diagram of the resistive circuit protector in its basic form and with additional features.

Best Mode for Carrying Out the Invention In the sole Figure is shown a resistive circuit detector 1 01. A power source 10 provides a positive voltage on a line 12. A voltage drop means, such as a current limiting resistor 14, is connected between the power source 10 and a load 1 6. The load 1 6 represents a current drain.

Therefore, a voltage drop develops across the current limiting resistor 14, according to Ohm's Law, in response to the load current.

The source side of the current limiting resistor 14 is at all times more positive than the load side due to the voltage drop. When the load 1 6 develops a fault, such as a short circuit, the voltage drop will be large. Since the voltage drop across the current limiting resistor 14 varies with the load 16, fluctuations in the load 1 6 must be accounted for, and this invention may be most effective when practiced in situations where the load 1 6 does not vary significantly under normal operating conditions. When a fault develops, the current limiting resistor 14 instantly limits the drain on the power source 10.

A threshold conductivity means, such as a zener diode 18, is connected in series with a resistor 20 and an indicator means, such as a light emitting diode (LED) 22. They are connected across the current limiting resistor 14. The zener diode 1 8 is connected, in reverse, to the source side of the current limiting resistor 14. Therefore it will not conduct unless a threshold voltage drop, equivalent to the zener reverse breakdown voltage, is achieved across the current limiting resistor 14. When that happens, however, the zener diode 1 8 becomes conductive and allows voltage through the resistor 20 to the LED 22 which is connected in a forward direction to emit light indicative of a fault when forward biased.

Since the LED 22 requires very little forward bias (e.g., 1.5 volts) to light, the resistor 20 limits the voltage provided thereto. It should be understood that the selected of values for the resistor 14 and the zener diode are dictated by the load operating conditions and failure modes. For faults that cause abnormally high load currents, the zener diode 1 8 presents an open circuit that prevents the LED 22 from lighting during normal load operation but, in essence, "connects" the voltage drop to the LED 22 when the voltage drop exceeds a threshold. On the other hand, for detecting faults that cause abnormally low load currents, the zener diode 18 can present a closed circuit that lights LED 22 during normal operation, but "disconnects" the voltage drop from the LED 22 when the voltage drop falls below a threshold.

Consider the following. A high impedance load 16, such as a signal circuit draws 1.5 milliamps through a 4000 ohm limiting resistor 14. The power dissipated in the limiting resistor 1 4 is small, on the order of 10 milliwatts. The voltage drop across the limiting resistor 14 is 6 volts (.0015 amps x 4000 ohms). A load fault causes a change in the voltage drop of 4 volts for every milliamp of current change (.001 amps x 4000 ohms). This disproportionate ratio makes load changes very noticeable, thereby facilitating the detection of load faults and making the use of a resistor as the voltage drop means very suitable for low current applications. For higher current applications, a high resistance is not desirable and a thermistor may be substituted for the limiting resistor 14 as the voltage drop means.A thermistor's resistance can be low (e.g. 3 ohms) at current levels on the order of 100 milliamps (e.g.) and increase dramatically at current levels indicative of a load failure, such as a short circuit.

This characteristic of the thermistor makes its use ideal, in the context of this invention, for higher current circuit applications, such as driving lamps or relays.

Although the LED 22 itself indicates a fault, a latched indication of the fault can be provided as shown at a latching portion 1 02. Therein a latching means, such as a photo SCR 26 is optically coupled to the LED 22. The SCR 26 is connected in a forward direction to the source side of the current limiting resistor 14, but is not conductive until it is irradiated (gated) by the LED 22. Once the SCR 26 becomes conductive, a voltage is provided through a switch 28 on a line 30 to light an indicator lamp 32. It should be understood that the indicator lamp 32 could also be a relay, or the like, for disconnecting the load 16. The latching portion 102 remains latched until the fault is removed and the SCR 26 is reset by momentarily breaking the circuit at the switch 28.

As shown in a portion 103, an OR circuit 36 is connected for response to any number of resistive circuit protectors RCD2, Rod3 ... RCDn, such as exemplified by the combination of the resistive circuit protector 101 and the latching portion 102 to provide a common indication (FAULT) of a fault. It should be understood that, through appropriate logic, various combinations of resistive circuit protectors could be interconnected using various logic to provide a fault indication based on various combinations of load failures.

As shown in a portion 104, a plurality of zener diode/LED "legs", patterned after the series combination of the zener diode 18, resistor 20 and LED 22, are connected in parallel across the current limiting resistor 14. The values for the zener diodes are selected to provide discrete fault indications in each leg at various current draw levels to provide a more quantitative indication of current draw.

It should be understood that numerous variations may be utilized for achieving the same or equivalent functions as disclosed herein.

Similarly, although the invention has been shown and described with respect to an exemplary embodiment thereof, it should be understood by those skilled in the art that foregoing and various other changers, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the invention.

Claims

1. A resistive circuit detector, for protecting a power source that supplies current to a load from a load fault that causes an excessive current draw and for providing an indication of said load fault, that comprises: voltage drop means, connected to provide the current from the power source to the load, for limiting said current, and for providing a voltage drop indicative of said current; indicator means for providing a signal indicative of the load fault in response to the voltage drop; and threshold conductivity means, connected for response to the voltage drop means and connected to the indicator means, for connecting the voltage drop to the indicator means when the voltage drop exceeds a threshold.

2. A resistive circuit detector according to claim 1 wherein the threshold conductivity means is a zener diode.

3. A resistive circuit detector according to claim 1 wherein the voltage drop means is a resistor.

4. A resistive circuit detector according to claim 3 wherein the resistor has a high resistance such that a slight increase in the current draw produces a disproportionately large increase in the voltage drop.

5. A resistive circuit detector according to claim 1 wherein the voltage drop means is a thermistor, the resistance of which increases with increased current draw.

6. A resistive detector according to claim 1 characterized by latching means for providing a latched indication of the fault in response to the signal indicative of a load fault.

7. A plurality of devices according to claim 1 wherein a plurality of signals indicative of faults are provided to means for providing a signal indicative of a combination of faults in response to a combination of signals indicative of load faults provided thereto.

8. A resistive circuit detector, for providing an indication of the current draw by a load from a power source, that comprises: resistive means, connected to provide current from the power source to the load, for providing a voltage drop indicative of the current draw by the load; indicator means for providing a signal in response to the voltage drop; and threshold conductivity means, connected for response to the resistive means and connected to the indicator means, for connecting the voltage drop to the indicator means when the voltage drop exceeds a threshold.

9. A resistive circuit detector according to claim 1 wherein the signal provided by the indicator means is indicative of a partial or complete short circuit in the load that causes an excessive current draw.

1 0. A resistive circuit detector according to claim 1 wherein the signal provided by the indicator means is indicative of a nominal current draw and the non-provision of the signal by the indicator means is indicative of a partial or complete open circuit in the load that causes a reduced current draw.

11. A plurality of devices according to claim connected in parallel to provide discrete indications of various levels of current draw.