PHOTOELECTRIC CONVERTER WITH A KFDM-TYPE SILICON PHOTODIODE

M. S. Roitman, V. R. Tsibulsky

(Presented by the scientific seminar of the Department of Radio Engineering)

The basic element determining the metrological characteristics of standard verification and measuring alternating-current apparatus is the measuring converter. In metrological organizations in this country and abroad, thermoelectric converters are used as such an element.

As a result of investigations carried out by the authors and by E. I. Tsimbalist, photoelectric converters (FECs) based on low-power incandescent lamps and photoresistors are finding ever wider application in verification apparatus [1], [2]. Unfortunately, the photoresistors currently produced by domestic industry do not possess sufficient time stability; the “breathing” effect in a photoresistor reaches 0.1% in 5 min. This is quite permissible for the overwhelming majority of practical applications, but is unacceptable for precision comparators.

In connection with the above, the search for a light-sensitive element with sufficiently high time stability becomes of great importance. The present article gives the results of an experimental investigation of FECs based on special experimental low-power incandescent lamps and KFDM photodiodes. The current-voltage characteristics of the KFDM for different lamp currents are shown in Fig. 1 (reverse connection of the diode).

The transfer coefficient of the photoconverter is equal to

\[ K=-\frac{\Delta R_{\text{d}}}{R_{\text{d}}}\left|\frac{\Delta I_{\text{l}}}{I_{\text{l}}}\right|, \]

where \(R_{\text{d}}\) is the resistance of the photodiode;
\(I_{\text{l}}\) is the current through the lamp;
\(\Delta I_{\text{l}}\) is the increment of the current through the lamp;
\(\Delta R_{\text{d}}\) is the corresponding increment of the resistance of the photodiode. The change in \(K\) with increasing illumination is shown in Fig. 2.

The circuit for measuring the temperature instability of the converter is shown in Fig. 3.

Figure 4 gives graphs of the temperature instability of the photodiode (I) and of the converter as a whole (II). (The lamp operates in the specified-voltage mode; the temperature coefficients are given in absolute value.)

Fig. 1.

Fig. 2.

Fig. 3. SK—step compensator—KL 0.02%; \(B_1\)—acid accumulator \(E = 2\ \mathrm{V}\); \(R_1\)—wire manganin rheostat \(r = 10\ \Omega\); \(R_2\)—standard resistance \(r = 1\ \Omega\), “Norma” No. 1606044, class 0.01%; \(L_1\)—incandescent lamp NSM; \(D_1\)—KFDM photodiode; \(R_3\)—wire manganin resistance \(r = 10\ \mathrm{k}\Omega\); \(R_4\)—load for SK, \(r = 100\ \Omega\); R309—potentiometer, class 0.005%, No. 000080.

Fig. 4.

Fig. 5.

The temperature coefficient of the KFDM, according to the data obtained, is equal to

\[ K_{t^\circ}=0.3 \div 0.6\%/\text{deg}; \]

and, referred to the input,

\[ \frac{K_{t^\circ}}{K}=0.03 \div 0.06\%/\text{deg}. \]

The time instability of the converter (Fig. 5) is determined according to the circuit shown in Fig. 3, but taking into account the instability of sources \(B_1\) and \(B_2\).

Figure 6 gives the instability of the NSM lamp used in this converter. A comparison of the curves shows that the fluctuation resistances of the lamp depend on the filament temperature. The magni-

Fig. 6.

tude of the converter asymmetry [3] depends strongly on the mutual orientation of the lamp filament and the photodiode. When the filament is perpendicular to the photodiode, the asymmetry is minimal and less than \(0.005\%\).

The sensitivity of the converter can easily be brought to \(10\ \text{mV}\) per \(0.01\%\).

It should be noted that the instability of TBB and TEM reaches \(0.01\%\) in 5 min, while the sensitivity of thermoconverters is within \(5\ \mu\text{V}\) per \(0.01\%\).

Thus, in terms of their metrological characteristics (sensitivity, asymmetry, temperature and time instability), photoelectric converters based on low-power incandescent lamps and silicon photodiodes are substantially better than thermoelectric converters and can be successfully used to construct precision ac comparators.

REFERENCES

1. M. S. Roitman, E. I. Tsimbalist, Yu. M. Fomichev. A standard ac voltage source. Proceedings of the VIII All-Union Conference on Automatic Control and Methods of Electrical Measurements, 1966.

2. B. A. Perminov, M. S. Roitman, E. I. Tsimbalist. An ac comparator using photoelectric converters, Avtometriya, No. 5, 1965.

3. T. B. Rozhdestvenskaya. Electrical Comparators for Precise Measurements of Current, Voltage, and Power, Publishing House of the State Committee of Standards, Measures, and Measuring Instruments of the USSR, 1964.