Indicator lamp test circuit utilizing bidirectional thyristor switching

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BACKGROUND OF THE INVENTION

This invention relates to a test circuit for an indicator lamp and more particularly to an improved circuit for simultaneously testing indicator lamps utilizing a self-triggering bidirectional thyristor.

On modern machinery there are many applications where multiple indicator lights are used. It is advantageous to have a means of rapidly testing many indicator lamps by depressing a single pushbutton. A problem with some of the prior art test circuits is that many relays or multipole pushbuttons are required in the test circuit. In some of the prior art indicator circuits, the normal signal circuits, the test circuits and the indicator lamp circuits are all operated at the same potential. It is desirable to have a circuit in which the test pushbutton and the indicator signal are at a higher normal operating voltage, such as 120 volts, while the indicator lamp is maintained at a low voltage, such as 6 volts, for system safety and cost.

In prior art U.S. Pat. No. 3,040,243 issued June 19, 1962 to I.F. Weiss a test circuit for an indicator system utilizing a single test button is disclosed. A problem with this circuit as disclosed in Weiss is that a separate transformer is required for the test circuit. The separate transformer can cause polarity and installation disabilities. In this prior art circuit the isolating diodes are disposed on the low voltage side of the indicator transformers. In a low voltage indicator lamp circuit voltage drop across the diodes can be significant. Also the disclosed circuit applies 1/2 wave rectified current to the bulb in both the normal and test conditions making it necessary to apply 8 to 10 volts or more to get a bright enough indication.

It is desirable to have a single pushbutton test circuit and operating circuit combination which is highly reliable and not susceptible to showing erroneous indications.

SUMMARY OF THE INVENTION

An indicator lamp circuit is provided in which multiple indicator lamps can be tested by depressing a single normally open pushbutton. Each indicator lamp is directly connected to the low voltage side of an indicator transformer. A test circuit and an operating signal circuit are connected to one leg of the primary high voltage side of the indicator transformer. A diode is disposed in the test circuit connection to the primary leg of each of the indicator transformers. A normally open pushbutton is connected in the test circuit to complete a circuit through the primaries of all of the indicator transformers when depressed. This allows for simultaneous testing of all the indicator lamps. When the test pushbutton is depressed a half wave rectified test signal is supplied to the indicator transformer. The operating signal circuit which is also connected to the primary leg of the indicator transformer, in parallel with the test circuit, comprises a bidirectional switchable device which permits normal alternating current to flow when activated. When the bidirectional switchable device, such as a thyristor, is activated the normal alternating current wave will cause the associated indicator lamp to burn more brightly than when energized by the half-wave rectified test signal. The diode in the test circuit, to which the normal operating signal is applied, and the diodes in the other test circuits form back-to-back rectifier circuits to prevent the alternating current signal from one signal circuit from influencing another signal circuit.

When the test pushbutton is depressed unidirectional current flows through the individual rectifiers and the associated indicator transformer, energizing it and lighting the indicator lamp, if it is not already lit due to a normal operating condition. The test circuit uses half-wave rectified current which is satisfactory for momentary testing. Lamps energized with the test signal will not show full brilliance so a check of which lamps are on in the normal mode can be made even while testing. A self-triggering bidirectional thyristor is provided in the operating signal circuit. Test current flowing through the operating signal circuit is prevented by non-conduction of the self-triggering thyristor. A diode is placed in the trigger circuit of the thyristor to prevent a signal from flowing through the thyristor and triggering it on. The bidirectional thyristor is triggered on when a low voltage low current supply is connected to the gate. If the gate is connected to one of the other leads it becomes self triggering in that one direction only. Too much gate current would ruin the device hence a resistor is required. We discovered experimentally that current, when applied to the test circuit, would flow through the coil of the device being monitored and trigger the bidirectional thyristor on. Current would then flow back through the bidirectional thyristor and energize the coil. To provide isolation a diode was added to the trigger circuit. Note that it is back-to-back with the diode in the test circuit.

It is an object of this invention to provide an inexpensive highly reliable circuit for indicator lamps using a small number of reliable components.

It is another object of this invention to provide an indicator lamp circuit utilizing a single transformer for operating each indicator light at a low voltage for safety and to permit use of long life indicator lamps.

It is a further object of this invention to provide a circuit when the test current is a 1/2 wave rectified signal and the operating circuit signal is a normal ac wave.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention reference may be had to the preferred embodiment exemplary of the invention shown in the accompanying drawing in which:

FIG. 1 illustrates the test circuit utilizing the teaching of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the figure there is shown an indicator light circuit utilizing the teaching of the present invention. A plurality of indicator lights 10 are connected to the low voltage side 12 of indicator transformer 14. Each indicator lamp 10 is supplied power from a single indicator transformer 14. Operation of the indicating lamp 10 at a low voltage, such as for example 6 volts, allows for the use of inexpensive bulbs and provides for safe operation. The high voltage side 16 of transformer 14 can be supplied with a higher voltage level, such as normal 120 volts ac. One leg 18 of the primary connection of each indicator transformer 14 is connected to a common line 20. At test signal line is connected between the primary of transformer 14 and a common test line 30. When test pushbutton 34 is depressed, closing the test circuit, the test current travels through the test circuit energizing the signal lamp 10. A diode 26 is disposed in each of the connections from the primary transformer leg 19 to the common test line 30. Diode 26 rectifies the test current which flows through indicator transformer 14 to illuminate signal light 10. When test pushbutton 34 is depressed half wave rectified current flows through all of the indicator transformers energizing all indicator lamps 10.

An operating signal circuit is also connected to the primary leg 19 of each transformer 14. A bidirectional thyristor 22 is disposed in each of the operating signal circuits. The bidirectional thyristor provides isolation and prevents current from reaching the coil and energizing its starter or relay. Bidirectional thyristor 22 comprises an input connection 24, an output connection 25, and a trigger connection 28. The output 25 of bidirectional thyristor 22 is connected to a line 40 through which current flows after the start pushbutton 42 is depressed, closing contacts 44 which seal in relay 46. Self-triggering bidirectional thyristor 22 is triggered on, energizing transformer 14 with alternating current and lighting the lamp 10, when current flows through line 40. The trigger 28 of bidirectional thyristor 22 is connected to the output 25 through resistor 48 and diode 50 connected in series. The diode is placed in the trigger circuit to prevent a signal from flowing through the coil and triggering the bidirectional thyristor 22 ON.

The disclosed circuit provides a novel and improved circuit for simultaneously testing the operativeness of several indicating lamps. This is accomplished by placing a self-triggering bidirectional thyristor circuit in series with the indicator transformer 14. When current flows in the line 40 it triggers bidirectional thyristor 22 on, energizing transformer 14 and lighting lamps 10. A path parallel to the indicator circuit is provided through the common test line 30. Current flow through this parallel path is blocked by back bias diodes 26 in the other test connections. In effect the diodes of adjacent units form a back-to-back rectifier pair with the diode in the test circuit.

When the test pushbutton 34 is depressed current flows through the individual rectifiers 26 energizing transformers 14 and lighting lamps 10 which under normal conditions are lit by operating signals. The test signal is one-half wave rectified ac which for extended operation could have a deleterious effect on the transformer but for momentary test conditions will be satisfactory. The lamps 10 will not show full brilliance when energized by the test signal so an indication of which lamps are energized by a normal operating signal can be determined even during test. The path through the operating signal circuit is blocked by bidirectional thyristor 22 when it is turned off. When bidirectional thyristor 22 is turned on current flows through primary leg 19 due to the operating signal. Diode 50 is placed in the trigger circuit of bidirectional thyristor 22 to prevent a signal from flowing through coil 46 and energizing triac 22.

The disclosed circuit offers advantages over the prior art such as being less complicated and requiring simpler wiring. The disclosed circuit also has the advantage of having a test signal which is of a lower magnitude than a normal operating signal so that normal operation of the lamps can be discerned even during circuit tests. When the transformer 14 is energized with a normal operating signal bidirectional thyristor 22 conducts normal alternating current and puts no abnormal strain on transformer 14 as might occur when it is energized with one half wave rectified current.

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