GB1604435A - Electrical circuit with load continuity detector - Google Patents

Description

(54) ELECTRICAL CIRCUIT WITH LOAD CONTINUITY DETECTOR (71) We, LUCAS INDUSTRIES LIMITED, a British Company, of Great King Street, Birmingham B 19 2XF, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an electrical circuit with a load continuity detector.

In such applications as road vehicle lamp failure monitoring it is conventional to detect the continuity of the lamp filament by detecting the voltage drop across a series resistor. Such resistors create an additional voltage drop in the system, must be rated to carry the full load current and in high current systems, will therefore tend to run hot and may be bulky, expensive components. It is an object of the present invention to provide an electrical circuit with a lamp filament or other load continuity detector in which the need for a series resistor is eliminated.

An electrical circuit in accordance with the invention comprises a drive transistor, a load connected to the emitter of the drive transistor so that the latter acts as an emitter follower. a resistor connected between the base and emitter of the drive transistor, a control circuit connected to control the base current of the drive transistor to turn it on and off. a detector transistor having its baseemitter path responsive to the potential drop across the resistor, and means for ensuring that the full voltage drop across the baseemitter path of the drive transistor when the drive transistor is conductive is reduced before application to the base-emitter path of the detector transistor.

The drive transistor may be a compound transistor of the Darlington type.

In the accompanying drawings, Figures I to 6 are circuit diagrams of six examples of the invention.

In Figure 1. the load 11 is connected between the emitter of a drive transistor 12 and the circuit earth, the collector of the transistor 12 being connected to a positive supply rail 13. A resistor 14 is connected between the base and emitter of the transistor 12 and its base is connected to a control circuit 16 which controls the base current of the transistor 12. The circuit 16 may include mechanical and or electronic switching components.

A detector transistor 17 has its base connected to the base of the transistor 12 through a protection resistor 20 and its emitter connected to the emitter of the transistor 12, so that the resistors 14 and 20 are connected across its base-emitter. The collector of the transistor 17 is connected to a warning circuit 18 which is controlled by the transistor 17.

The protection resistor 20 is useful when the transistor 12 is a high power type having a large base-emitter saturation voltage drop, V,. The resistor 20 protects the base-emitter of the transistor 17 which may be a small signal type.

Whenever the control circuit 16 is supplying sufficient current to turn on the transistor 12, a voltage equal to the base-emitter voltage of the transistor 12 will appear across the reistor 14, which acts when no such current is being supplied to ensure that transistor 12 is turned off. The voltage across the resistor 14 is sufficient to turn on the transistor 17 so that the warning circuit 18 is operated. Should the load be disconnected or fail due to an open circuit fault there will be no path for current through the resistor 14 and transistor 17 will not turn on.

Figure 2 shows another arrangement. In this case a protection resistor 21 is connected between the emitters of transistors 12 and 17.

The arrangement shown in Figure 3 is similar to that shown in Figure I (and like part are denoted by the same reference numerals preceded by the number "1") except that the drive transistor 112 is a Darlington transistor (or a discrete Darlington pair). A resistor 120 is connected between the bases of the transistors 112 and 117.

Turning now to Figure 4, the output transistor 112 is again a Darlington transistor. In this case a diode 122 is used to interconnect the bases of the transistors 112 and 117.

In the arrangement shown in Figure 5 the base of the Darlington transistor 112 is connected to its emitter by two resistors 123, 124 in series, the base-emitter of the detector transistor 117 being connected across only one of these resistors to provide the required degree of protection.

The arrangement of Figure 6 is similar to that of Figure 5 except that the resistor 123 is replaced by a diode 125, the base-emitter of the detector transistor 117 being connected across the resistor 124.

WHAT WE CLAIM IS: 1. An electrical circuit comprising a drive transistor, a load connected to the emitter of the drive transistor so that the latter acts as an emitter follower, a resistor connected between the base and emitter of the drive transistor, a control circuit connected to control the base current of the drive transistor to turn it on and off, a detector transistor having its base-emitter path responsive to the potential drop across the resistor, and means for ensuring that the full voltage drop across the base-emitter path of the drive transistor when the drive transistor is conductive is reduced before applicaton to the base-emitter path of the detector transistor.

2. An electrical circuit as claimed in Claim 1 in which the drive transistor is a compound transistor of the Darlington type.

3. An electrical circuit as claimed in Claim I or Claim 2 further including warning means controlled by the detector transistor.

4. An electrical circuit substantially as hereinbefore described with reference to and as shown in Figure 1, or Figure 2, or Figure 3, or Figure 4, or Figure 5 or Figure 6 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. In the arrangement shown in Figure 5 the base of the Darlington transistor 112 is connected to its emitter by two resistors 123, 124 in series, the base-emitter of the detector transistor 117 being connected across only one of these resistors to provide the required degree of protection. The arrangement of Figure 6 is similar to that of Figure 5 except that the resistor 123 is replaced by a diode 125, the base-emitter of the detector transistor 117 being connected across the resistor 124. WHAT WE CLAIM IS:

1. An electrical circuit comprising a drive transistor, a load connected to the emitter of the drive transistor so that the latter acts as an emitter follower, a resistor connected between the base and emitter of the drive transistor, a control circuit connected to control the base current of the drive transistor to turn it on and off, a detector transistor having its base-emitter path responsive to the potential drop across the resistor, and means for ensuring that the full voltage drop across the base-emitter path of the drive transistor when the drive transistor is conductive is reduced before applicaton to the base-emitter path of the detector transistor.

2. An electrical circuit as claimed in Claim 1 in which the drive transistor is a compound transistor of the Darlington type.

3. An electrical circuit as claimed in Claim I or Claim 2 further including warning means controlled by the detector transistor.

4. An electrical circuit substantially as hereinbefore described with reference to and as shown in Figure 1, or Figure 2, or Figure 3, or Figure 4, or Figure 5 or Figure 6 of the accompanying drawings.