Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1997-02-25
1998-08-04
Le, Que
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
250214L, 356226, H01J 4014
Patent
active
057897387
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The invention concerns a circuit for setting an operating point of at least one opto-electronic element, which generates current during a short-circuit operation and voltage during a no-load operation, wherein the opto-electronic element constitutes a component of a receiver for alternating light useful signals and the circuit adjusts the operating point below the saturation voltage of the opto-electronic element with a resistive circuit connect parallel to the opto-electronic element, at least for the compensation of constant light signals.
PRIOR ART
A circuit of this type is known, for example, from DE-C 39 30 415 and is used to measure changes in light incidence and to obtain usable signals from this. Since there is, in principle, a logarithmic connection between the intensity of illumination and the no-load voltage of a photodiode, a saturation voltage is quickly reached with increasing illumination, which is reached at approximately 0.5 V, depending on the photodiode used. Above the saturation voltage, there is a linear relationship between illumination intensity and photoelectric current over many powers of ten. If the intent is to convert alternating light useful signals, superimposed by constant light, into a signal voltage, it is necessary that the photodiode be operated in a range below this saturation voltage in order to be able, at all, to detect a signal pattern and thus set the operating point.
In DE-C 39 30 415, a compensation voltage is generated for this purpose by means of a second photodiode without requiring current from the supply voltage. The voltage from the second photodiode is supplied to the actual photodiode as a counter, or external voltage via a coil with constant internal resistance. As a result of this, two photodiodes are initially necessary, of which only one actively receives the alternating light. However, for a compensation of the photoelectric voltage with the same type of voltage generated by the second photodiode, both diodes must be illuminated evenly, which is not always ensured. In order to solve this problem, it is suggested that the interference signal photodiode is designed larger than the useful signal photodiode, which results in higher costs for the circuit. In this case, the voltage is varied while the resistance remains the same. The use of a coil in the direct current path additionally has the disadvantage that this circuit cannot be integrated into an IC (integrated circuit). In addition, this integration is made even more difficult in that it is not possible to switch to a symmetric input because of the capacitor, which is parallel-connected to the interference signal photosensor.
In order to hold the photodiode below the saturation voltage, it is also known to operate the photodiode during the diode operation with bias voltage. For this purpose, a bias voltage is fed to the diode in order to block it (opposite polarization to the saturation voltage). Alternatively, the diode can also be operated by a cell without bias voltage. In that case, the photodiode functions as a current source. For this purpose, the photodiode is kept at a zero potential wile the photoelectric current is compensated via a resistor.
Both types of operation, meaning diode or cell operation, have in common that they need an external power source in order to compensate for the photoelectric current that appears during illumination. Depending on the type of diode and illumination, this compensation current can amount to several umpteen .mu.A to several mA. This current must be taken from the supply voltage, which can lead, in the case of battery operation, to a quick failure of the battery. This current, which must be supplied by the battery, can possibly amount to a multiple of the current taken up by the complete circuit.
The invention is suited in a preferred way for use in connection with a battery-operated water sensor, such as is known from DE-U 93 09 837.5. Operating a water sensor with a battery is desirable, for example, in the field of boat constructi
REFERENCES:
patent: 5578815 (1996-11-01), Nakase et al.
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