Abstract:
A current detector/indicator for DC has a transistor ( 1 ) and an LED ( 2 ). They are arranged so that current flow through the LED causes the transistor to conduct, shunting excess current away from the LED and through the transistor. For AC, the circuit can be doubled, with transistors and LEDs arranged with inverse polarity. A triac ( 15 ) may supplant the two transistors, with parallel, opposed LEDs ( 16 ) in its gate circuit.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to current detector/indicators and in particular to improved current detector/indicators such as can be utilized by modern control systems as a means to facilitate rapid fault assessment and status of loop circuit integrity while providing circuit isolation. In other words, it is aimed at testing electrical contacts for mechanical and electrical function without breaking power to external circuits. Many uses are envisaged for this device but in particular there is the ability to supply visible information on whether a current is flowing in a circuit or not and to supply an isolated switched output of that information. 
     Light emitting diodes are excellent devices to indicate current flow but are generally limited in their current capacity to a maximum of 100 mA and are very prone to damage from reverse voltages of low magnitude. They exhibit extremely long life if subject to their design ratings of current and voltage and consume little. In this specification, it should be understood that as well as ‘ordinary’ LEDs, such as are used in many indicators, it would also be possible to use laser diode type LEDs. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a current indicator comprising a transistor and an LED in circuit therewith and arranged to have a controlling influence thereon, current flow through the LED energising it and thereby causing the transistor to conduct and shunt excess current away from the LED and through the transistor. 
     In a preferred, simple form for indication of DC or one polarity of AC, there is one transistor and the LED is connected across its collector and base. The LED can be part of an opto-isolator capable of providing a remote indication of current flow, in which case another LED in circuit with the transistor might be arranged to be energised when the transistor is conducting, thereby to provide local indication of current flow. This other LED can be connected across the emitter and collector. 
     For indicating both polarities of AC, the indicator can be doubled with two transistors connected in inverse parallel. 
     In another convenient form the transistor is a triac in an AC power line which may pass current selectively full wave or either half wave, and wherein there are two LEDs, in parallel with reversed polarities, in the triac&#39;s gate circuit arranged to trigger the triac into conduction when power is on the line, each LED being energized with the gate trigger current during the associated half wave conduction through the triac but extinguishing when the triac avalanches into conduction. 
     Again, each LED can be part of an opto-isolator capable of providing a remote indication of current flow, in which case another LED in series with each first LED may provide local indication of current flow. 
     Whenever there are two LEDs for visible indication of current they may be combined into a bi-color LED, AC being indicated by a mixture of the colors. More usefully, four different conditions may be delivered via two wires by way of two switches isolated by two inversely polarised diodes. 
     PNP and NPN transistors are usable. Where the circuit is doubled, two transistors of the same type may be used, or complementary PNP and NPN transistors. 
     Preferably, the transistor(s) will have reverse voltage protection, for example by a diode across the collector and emitter. 
     In one arrangement, a load may be energized through a power diode, across which is connected the emitter and base of a transistor, the collector being connected through the LED to the other side of the load. There could be two LEDs in parallel or series, one being for local indication and the other being part of an opto-isolator. It is important that the power diode and the transistor share a common thermal environment for reliable operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, some embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIGS. 1 to  11  and FIGS. 13 to  15  are circuit diagrams of current indicators, and 
     FIG. 12 shows a waveform associated with FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In each of FIGS. 1 to  3 , there are two versions of the circuit, one with an NPN transistor, the other with a PNP transistor. They each work in a similar fashion and will be described as one circuit. 
     A basic current indicator is shown in FIG.  1  and consists of a transistor  1  and an LED  2  coupled across the collector and base of the transistor, which is between terminals A (positive) and B (negative). 
     A resistor  3  or diode  4  between the base and emitter of the transistor  1  can enhance the sensitivity of the device, but neither is essential to its working. They are therefore not illustrated with definite connections, but the arrows indicate their position, if used. The polarity of the diode  4  matches that of the transistor junction. 
     When conducting, the voltage drop Vf across the base-emitter junction of the transistor is typically 0.6 V while the forward voltage of the LED varies with colour and type between 1.0 V for infra-red types to 4.0 V for certain colors. The voltage drop that appears across terminals A and B when there is current flow is the sum of the transistor emitter-base Vf plus that of the LED and will therefore be of the order 1.6 to almost 5 V. 
     With terminal A positive in relation to terminal B, the LED  2  will pass current through the base-emitter junction of the transistor  1  and force the transistor into collector-a emitter conduction. This diverts current from the LED, which is typically limited to a maximum capacity of 100 mA. 
     The circuit will control the voltage drop across itself and of course the presence of current is indicated by the illumination of the LED. 
     FIG. 2 is essentially the same as FIG. 1, but with the addition of a diode  5  across the transistor oriented to provide reverse volts protection. LEDs are very prone to damage from reverse voltage even of low magnitude. 
     FIG. 3 extends this arrangement in two respects. First there is another LED  6 , in series with a resistor  7 , in parallel with the diode  5  across the transistor. The resistor  7  will be of the order of 10 ohms and serves to control the current in the LED  6 , which will illuminate when current flows from A to B and serve as a local indicator. 
     The LED  2 , however, is now part of an opto-isolator  8 , and when conducting and radiating it will produce an output at  9  which can be fed to a remote station. 
     These circuits are useful for DC or where there is interest only in one polarity of an AC current. An arrangement for detection or indication of AC across terminals C and D is shown in FIG. 4, using NPN transistors. A PNP version is equally possible or a combination of PNP and NPN transistors. FIG. 4 is essentially a doubling of FIG. 2, with similar transistors  1  connected in reversed polarity arrangement, collectors to emitters, with their associated LEDs  2  as in FIG. 2, and with the diodes  5  combined into a transient suppressor  10 . It will be appreciated that the LEDs illuminate alternatingly in synchronism with the half cycles of the AC and enable four different conditions to be indicated by different combinations of light. 
     For improved sensitivity a single diode  4  may be connected between the base and emitter of each transistor, as in previous figures, or a diode  11  and resistor  12  in series, as illustrated. The diode provides a minimal current path for the associated LED without turning the transistor on, but enough to exploit the minimum current illumination of the LED, which may be of the order of 0.5 mA. 
     FIG. 5 shows a combination of the circuits of FIGS. 3 and 4. The LEDs  2  each form part of an opto-isolator  8  as in FIG. 3, while instead of two separate LEDs  6  and separate resistors  7 , these can be merged into a single bi-color LED  13  and a single resistor  14  in parallel with the transistors. The different colors will illuminate alternately, but with anything other than very low frequencies they will appear as two side by side continuous colours when AC is present. 
     Instead of two separate transistors a triac  15  may be used as shown in FIG.  6 . Its gate stimulation is via two inversely connected LEDs  16 , and here they are used to detect and relay information on two different loads  17  which, with associated diodes  18  (inversely arranged) and switches  19 , are in parallel in the power circuit. Only two wires are necessary, thereby reducing wiring costs to both input and output circuits of a control system. The LEDs  16  each serve in turn to pass current to the triac gate until the triac switches on. Thereafter the triac  15  assumes the full load current (LED turns off) with its Vf falling to less than one volt (insufficient to maintain an LED “on”) until the supply passes through the 0 volt part of its supply curve. 
     This routing of the gate current for the triac  15  through two inversely polarized LEDs  16  causes it to trigger into conduction at a point determined by the Vf of the LED and the polarity of each half cycle. Because the supply is an alternating current they will light at a frequency determined by the supply frequency but 180 degrees opposed. Together they effectively double the supply frequency. The LEDs mirror the action of the switches  19  and illuminate to indicate whichever or both switches are passing current. 
     An alternative isolated system can be realised to act as a feedback to the control system as shown in FIG. 7, utilising opto-isolators  20  instead of visible LEDs. 
     The systems of FIGS. 6 and 7 could be combined as shown in FIG. 8, where the parallel branches in the gate circuit each have an LED  16  and an opto-isolator  20 . 
     FIG. 9 shows a PNP transistor  21  and an NPN transistor  22  with collectors to terminal C and emitters to terminal D. A transient suppressor  23  and LEDs  24  and  25  with inverse polarity and each with a series resistor  26  are in parallel across these terminals. Across the base and collector of each bi-polar transistor there is a MO-SFET, the PNP transistor  21  having an N-channel device  27  and the NPN transistor  22  having a P-channel device  28 . Their gates are photo coupled to respective LEDs  29  and  30  of opposite polarity across the terminals C and D, each in series with a resistor  31 . Whichever LED  29  or  30  is conductive illumines and switches on the associated MOS-FET  27  or  28 , which in turn makes the associated bipolar transistor  21  or  22  conductive. 
     FIG. 10 can be regarded as a simpler version of FIG.  9  and corresponding parts are similarly referenced. Two NPN transistors  32  and  33  are connected across terminals C and D with reverse polarity and the associated LEDs  29  and  30  photo couple directly to the bases of these bipolar devices. 
     FIG. 11 shows the circuit of FIG. 4 (less the transient suppressor) across an inductor  34  which is energised with alternating positive and negative pulses of low voltage (less than one volt) but significant current, as shown by the full lines of FIG.  12 . In this case, the LEDs  2  will light out of phase with the actual current in the inductor  35 , over the periods indicated by the dotted lines in FIG.  12 . They will be energized by the collapsing magnetic field generating a reverse current. The LEDs  2  therefore indicate that a current has just been present in the inductor. 
     All the above circuits are intended to be inserted into a current carrying conductor and to derive their power for operation from the line. Adequate precautions will be taken to protect the devices against current overload and transient voltage spikes. 
     In FIG. 13 a simple DC arrangement is shown where the load  36  is energised through a power diode  37  with a reverse volts protection diode  38  in parallel. A PNP transistor  39  has its emitter connected to the positive (terminal A) side of the diodes and its base to the load side. The collector circuit has an LED  40  and resistor  41  in parallel with an opto-isolator  42  and resistor  43 , both connected to the zero volts terminal B. 
     The voltage drop across the power diode  37  when the load is carrying current makes the transistor  39  conductive and generates a response in the LED  40  and from the opto-isolator  42 . A variable resistor  44  of 0.6 to 600 ohms could be included across the diodes  37  and  38  as shown as a means of establishing an adjustable threshold for the LEDs to illuminate at different current levels. Any lack of integrity of the load circuit will indicate locally and feed back to a remote station. 
     FIG. 14 shows a generally similar circuit but with the LED  40  in series with the opto-isolator  42 , and with a single resistor  45 . 
     Similar arrangements are possible using an NPN transistor. 
     The indicator of FIGS. 13 and 14 require that the return power line is close at hand to power the L.E.D. and that the arrangement can tolerate another circuit path shunted elsewhere without degrading the information in that circuit. 
     FIG. 15 shows a further circuit suitable for use when “intrinsically safe” regulations apply. This is where power to the field circuits is mandatorily low, negating the need for power transistors but exploiting the ability of the device to interpret four bits of information form two wires. 
     Between terminals A and B to which AC is applied there is a bi-color LED  46  in series with two opto-isolators  47  in parallel, their LEDs being arranged with inverse polarity. The circuit continues beyond terminal B to a double switch assembly  48  and  49  with associated diodes  50  and  51 . 
     The switches may be coupled to some device such as a valve, and in the position shown this valve is in an intermediate position, neither open nor closed. Both parts of the LED  46  will be on, and both opto-isolators energized. 
     If the valve is fully opened, the upper switch  48  closes, bringing the diode  50  into circuit. The corresponding part of the LED  46  and one of the opto-isolators will be on. Likewise, if the valve is fully closed, the lower switch  49  closes and the other pat of the LED  46  and the other opto-isolator  47  will be on. 
     If there is no illumination from the LED, there must be a fault. 
     This arrangement can be applied to the circuits of FIGS. 4,  5 ,  9 ,  10  and  11 . 
     Whilst the invention has been described above, it extends to any inventive combination of the features set out in the introduction or the description.