GB2088161A

GB2088161A - Bulb Driver with Short Circuit Protection

Info

Publication number: GB2088161A
Application number: GB037498A
Authority: GB
Original assignee: Ultra Electronic Controls Ltd
Current assignee: Ultra Electronic Controls Ltd
Priority date: 1980-11-21
Filing date: 1980-11-21
Publication date: 1982-06-03

Abstract

A bulb driver circuit is described in which a small current is fed to the bulb 10 through a resistor 24. The bulb voltage is monitored by a comparator 28 and only if there is no short circuit will the output of the comparator 28 permit transistor 20 to switch on when the circuit receives a control signal (22). Transistor 20 in turn drives power transistor 12 to provide power directly to the bulb 10. In an alternative embodiment, transistor 20 is connected to act as a comparator. <IMAGE>

Description

SPECIFICATION Bulb Driver with Short Circuit Protection The present invention relates to a bulb driver that is to say a circuit for applying power to a bulb and is specifically concerned with a bulb driver circuit in which the supply voltage is protected in the event of a short circuit at the bulb.

In certain applications, for example an aircraft cockpit, there are numerous bulbs to be lit and there is a risk to the wiring harness if any of the bulb circuits should develop a short circuit. The present invention provides a bulb driver which enables the power supply to be isolated in the event of a short circuit.

In accordance with the present invention, a bulb driver circuit comprises an electronic power switch for applying power to a bulb in response to a control signal, means for causing a quiescent current to flow through the bulb the quiescent current being insufficient to enable the bulb to glow and a comparator for sensing the voltage across the bulb and enabling energisation of the bulb by the power switch only when the voltage across the bulb exceeds a predetermined value.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures is a circuit diagram of a first embodimentfof the invention, and: Figure 2 is a circuit diagram of a second embodiment having greater power dissipation that the circuit of Figure 1.

In Figure 1, there is shown a bulb 10 which may be energised by switching on a transistor 12.

The transistor 12 has an emitter resistor 14 which provides negative feedback so that the voltage across the bulb 10 when energised is relatively stable even if the supply voltage should fluctuate within limits of say 1 8 to 32 volts. The base voltage for the transistor 12 is taken from between two resistors 1 6 and 1 8 which form the collector load of a driver transistor 20. The emitter of the driver transistor 20 is earthed while its base is connected through a switching transistor 22 to earth. The switching transistor 22 may, for example, form part of an optical coupler. A resistor 24 and a diode 26 serve to drive a quiescent current through the bulb 10 from a second voltage supply which may, for example, be 10 volts.The voltage developed at the junction between the diode 26 and the bulb 10 is applied to one end of a comparator 28 through a current limiting resistor 30. The input of the comparator 28 is further connected to earth through a diode 32. The other input of the comparator 28 is connected to a point at a reference voltage which is derived from a potential divider constituted by resistors 34 and 36 connected in series with one another between the 10 bulb supply and earth.

The output of the comparator 28 is fed by way of a current limiting resistor 38 to the base of the transistor 20.

The above circuit operates as follows: Assuming that the bulb has not yet been energised and that only a small quiescent current is flowing through it, then a voltage is present across the bulb which exceeds the reference voltage taken from tlhe potential divider 34, 36.

Consequently the output of the comparator 28 is high and if the transistor 22 is not conductive the transistor 24 is switched on. The drop in potential at the junction between resistors16 and 18 switches on the power transistor 12 which now becomes conductive and applies power directly to the bulb 10 with the emitter resistor 14 acting to stabilize the voltage. If it is desired to switch off the voltage then the transistor 22 is rendered conductive which brings the base potential of transistor 20 down to earth thus switching off the transistor 20 and with it the transistor 12.

It will be noted however that the transistor 22 can only be used to switch the transistor 12 on and off as long as the output of the comparator 28 is high, that is to say while a voltage is developed across the bulb 10. If the bulb 10 should develop a short circuit then the output of the comparator 28 will immediately go low and prevent the bulb from being energised.

In the above embodiment the function of the diode 26 is to prevent power from being fed back into the 10 volt power supply from the nominal 28 volt power supply. The purpose of the diode 32 on the other hand is to protect the input of the comparator 28. The voltage drop across the forward biased diode 32 exceeds the voltage developed across the volt 10 by the quiescent current so that this diode does not act to short circuit the voltage to be detected but only to protect the comparator 28 when the nominal 28 volt power supply is connected to the bulb.

Because of the gain of the comparator 28 only a relatively small quiescent current need flow through the bulb 10. It has been found in practice that the comparator 28 can be dispensed with if a higher quiescent current is made to flow through the bulb 10. In this case the transistor 20 in addition to acting as a driver, acts as a comparator, the circuit shown in Figure 2 is based on this simplification.

The bulb 10 in this embodiment receives a quiescent current from the same nominal 28 volt power supply which provides the power to drive the bulb. The quiescent current flows through a resistor 24 which has a lower resistance than the resistor 24 in the first embodiment. As there is no risk of current flowing from one power supply to the other the diode 26 of the first embodiment is no longer necessary. The bulb is additionally connected by way of a current limiting resistor 38' to transistor 20' the base of which is once again connected via a transistor 22 to earth.

Resistors 16' and 18' which may have different values from the corresponding resistors in the first embodiment provide base bias for the power transistor which is provided with an emitter resistor 14 to stabilize the voltage supplied to volt 10.

In the embodiment in Figure 2, if the bulb 10 should develop a short circuit then the base of transistor 20' is earthed preventing the latter transistor and transistor 12 from switching on.

However if the voltage exceeds a predetermined threshold then the current flowing through the resistor 38' is sufficient to render the transistors 20 and 12 conductive to supply power directly to the bulb 10 through power transistor 12 and the current stabilization resistor 14. When it is desired to switch off the bulb, the transistor 22 is rendered conductive thus reducing the voltage at the base of transistor 20' to cut off the transistor 20' and the transistor 12. In this embodiment there is positive feedback between the collector of transistor 12 and the base of transistor 20' which enables very fast switching times.

Claims

1. A bulb driver circuit comprising an electronic power switch for applying power to a bulb in response to a control signal, means for causing a quiescent current to flow through the bulb the quiescent current being insufficient to enable the bulb to glow and a comparator for sensing the voltage across the bulb and enabling energization of the bulb by the power switch only when the voltage across the bulb exceeds a predetermined value.

2. A bulb driver circuit as claimed in claim 1 wherein the electronic power switch is a transistor having an emitter resistor, the bulb forming the collector load of the transistor.

3. A bulb driver circuit as claimed in claim 1 further comprising a driver transistor having a collector load connected to the base of the power transistor.

4. A bulb driver circuit as claimed in claim 3 wherein a transistor is connected between the base of the driver transistor and a point at reference potential to enable the bulb to be switched on and off.

5. A bulb driver circuit as claimed in claim 4, wherein the transistor connected to the base of the driver transistor forms part of an optical coupler.

6. A bulb driver circuit as claimed in claims 3, 4 or 5 wherein the base of the driver transistor is connected to the output of a comparator the inputs of which are connected respectively to the bulb and to a point at reference potential.

7. A bulb driver circuit as claimed in any of claims 3, 4 and 5 wherein the said driver transistor acts as a comparator being connected to the bulb and operative to change states when the bulb voltage exceeds the said predetermined value.

8. A bulb driver circuit constructed arranged and shaped to operate substantially as hereinbefore described with reference to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.