Proximity detector with a stable current source

A proximity detector having a power switch 30 which is switched as a function of the approach of an object. The detector includes a MOS depletion transistor T1 which connects one of the terminals 15 of the detector to a control input of a power switch 30 through a control switch T2. A regulator stage 45 is coupled to T1 in order to keep T1 saturated in the closed condition of the detector and to regulate the gate voltage of T1 when the detector is in a open state.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a proximity detector of the type with two 
terminals able to be connected to a charging circuit which includes a load 
put in series with an external, AC or DC, electrical power source in such 
a way that this source is in a position to supply the load and the 
detector. 
2. Discussion of Background 
Such detectors generally include, on the one hand a detector unit that may 
be inductive, capacitive, photoelectric, etc. to which a processing 
electronic circuit is connected that is capable of supplying a switch 
signal in response to the approach or the presence of an object or target 
and, on the other hand, an electronic power switch connected to the 
terminals of the detector to determine the opening and the closing of the 
charging circuit. The switch is commanded by the switch signal in such a 
way that it is off in the open condition of the detector and on in the 
closed condition of the detector. 
To supply power to the electronic circuits of the detector, a current 
source is provided that must operate when the detector is open. The 
current then supplied by the source must be low and stable whatever the 
conditions of supply voltage and temperature. Furthermore, even when the 
detector is closed, it must be supplied with a residual voltage which must 
remain low and well controlled whatever the switched current passing 
through the power switch and which must vary only slightly with 
temperature. 
SUMMARY OF THE INVENTION 
The purpose of the invention is to make the current source for a detector 
with two terminals, of the type described, stable and precise, while at 
the same time preserving the low cost nature of the detector. 
According to the invention, the current source comprises 
an MOS type transistor whose drain-source path, in the closed condition of 
the detector, forms part of a conduction path linking one terminal of the 
detector to a control input for the power switch, 
a regulator stage coupled to the first transistor in order to keep it 
saturated in the closed condition of the detector and to regulate its gate 
voltage in the open condition of the detector, 
the regulator stage including two bipolar transistors preferably paired in 
one and the same package, one of which has its base connected to the 
source of the MOS transistor through a reference voltage device and whose 
main path connects the gate of the MOS transistor to an output of the 
current source, and the other of which connects the source of the MOS 
transistor to said output via a reference resistance. It is advantageous 
to be able to use discrete electronic circuits in the detector conforming 
to the invention, particularly for its current source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The proximity detector 10 illustrated in the Figures is of the type with 
two terminals B1, B2, each able to be connected through respective wires 
11, 12 to a load L arranged in series with a source of electrical energy 
13. This source 13 is provided in order to supply power to the detector, 
whether it be open or closed, as well as to the load; it can be a DC or AC 
voltage source, the detector, in effect having at its input a dual 
half-cycle rectifier bridge 14. 
The proximity detector includes a detector unit 20, an electronic power 
transistor switch 30 and a command device 40 for the switch and power 
supply control. The detector unit can be an inductive, capacitive, 
photoelectric etc. type, capable of supplying a switch signal S in 
response to the approach or the presence of an object or a target K; the 
electronic power switch 30 is arranged on a conductor 31 that connects the 
DC voltage terminals 15, 16 of the rectifier bridge 14, for example by 
means of a high voltage branch 31a at the potential V and a low voltage 
branch 31b at the potential 0V, and it is fixed to a control switch 41 for 
the device 40. Tche detector unit 20 is connected to an electronic 
processing circuit that is not shown. 
In addition the device 40 includes a DC current source 42 and a voltage 
increasing circuit 43. 
The current source 42 is connected between the high voltage branch 31a of 
the conductor 31, that is to say the terminal 15 of the bridge 14, and the 
voltage increasing circuit 43, to which it is connected via a diode D1; it 
has a regulator unit 44 constituted, according to the invention by an MOS 
depletion transistor T1, which allows adaptation to a broad range of power 
supply voltages, for example from 20 V to 264 V, and a regulator stage 45 
which co-operates with T1. The drain of transistor T1 is connected to the 
branch 31a of the conductor 31 and its source A is connected to the 
regulator stage 45 and to a main terminal of the control switch 41, the 
other main terminal of which is connected to an input E of the voltage 
increasing circuit 43. The switch 41 is, for example, an MS transistor T2. 
When the detector is quiescent (in the open or "not switched" condition), 
T2 is off and the current I1 which passes through transistor T1 is routed 
via the regulator stage 45 and a diode D1 to an input F of the voltage 
increasing circuit 43; when the detector is activated (in the closed or 
"switched" condition), T2 is on and passes a current I2 to the input E of 
circuit 43, this supplying a voltage that is higher at H than at E. The 
voltage generated by the circuit 43 at its output H is applied to the 
detector unit 20 and notably to its electronic circuit. The power switch 
30 command signal is supplied through the output J of the voltage 
increasing circuit. 
Part of the detector diagrammatically shown in FIG. 1 is represented in 
FIG. 2 in a way that better shows the constitution of the regulator stage 
45 and the voltage increasing circuit 43 and their relationship with the 
other components. It should be noted that the power switch 30 is formed by 
a Darlington assembly, T3 cascaded with an MOS transistor T4. The MOS 
depletion transistor T1, constituted like T4 by a high voltage component, 
taking up little space has its drain connected to branch 31a and its gate 
also connected to branch 31a via a gate polarisation resistance R1. The 
point A to which the source of T1 is joined, is connected on the one hand, 
via a reference resistance R2 to the collector of a bipolar transistor T5 
with a base-collector loop, the emitter of which is connected to a 
conductor 32 connected to the anode of the diode D1; on the other hand, 
the point A is connected via a reference voltage device 47, for example an 
active circuit or a Zener diode, to the base of a bipolar transistor T6 
and via a resistance R3 to the conductor 32. It is advantageous that the 
transistors T5, T6 form part of one and the same component in both being 
paired in a single package 46, for example of an SOT 363 type. 
The voltage increasing circuit 43 has an inductance L1 connected on the one 
side to the input E of the circuit which is connected to T2 and via a 
capacitance C1 to the branch 31b; on the other side, the inductance L1 is 
connected on the one hand to a diode D2, the cathode of which is connected 
at output J4 of circuit 43 to the gate of the MOS transistor T4 and on the 
other hand to the drain of an MOS transistor T7 for operating the voltage 
increasing circuit; the gate of T7 is actuated by a logic signal S' 
supplied by a clock and its source is set at the low potential of 31b. The 
cathode of D1 is connected to the point H, itself connected through a 
capacitance C2 to the low potential of 31b. Finally, the point E is 
connected at output J3 of the circuit 43 to the base of the Darlington 
transistor T3 by a conductor 33 that includes an electroluminescent diode 
D3. 
The detector described operates in the following way. 
In the detector's open condition (not switched), the signal S turns off the 
control transistor T2 and the signal S' turns off T7. The transistor T1 is 
on and the current I which passes through it and which mainly passes 
through R2 and T5 stays constant since the transistor T6 regulates the 
gate voltage of T1. T5 compensates for the voltage drop in T6. The current 
passing through the voltage reference circuit 47 and R3 is low in relation 
to that flowing through R2 and T5. The constant stable current I is routed 
via D1 to the point H to supply the detector unit. 
In the detector's closed condition (switched), the signal S turns the 
transistor T2 on and the clock signal S' is active and permits excitation 
of T7. As T2 is on, the transistors T5 and T6 are off and T1 becomes 
saturated. The voltage drop in transistors T1 and T2 is small and the 
current that passes through them controls the power switch 30 through the 
outputs J3 and J4 of circuit 43, more precisely via a conductor 33 
connecting the point E to the base of the Darlington transistor T3 and via 
the conductor 34 connecting the cathode of D2 to the gate of T4. The 
residual voltage of the detector remains small and in the form of an 
alternating square-wave voltage. 
The current source described has several advantages. It can operate just as 
well under very low voltage as under high voltage thanks to the regulator 
stage 45. The transistors T5 and T6 of stage 45, paired in one and the 
same component package, make the current source very precise and stable, 
particularly in relation to temperature.