On the Possibility of Using Rod Vacuum Tubes as Saturation Diodes for Comparing Currents and Voltages
Abstract Generated abstract
This paper examines miniature rod vacuum tubes as measuring saturation diodes for low frequency current and voltage comparison, motivated by limits of photoelectric converters using incandescent lamps. Experimental volt ampere, dynamic, and stability measurements were made on several tube types, with EM-10, N-28, and N-37 found unsuitable, while 1Zh24B and 1Zh42A showed usable characteristics, a transfer coefficient of about 5, and optimized temporal instability as low as 0.002 percent per minute. The authors note substantial asymmetry error, about 5 to 10 percent, which prevents reliable comparison of direct and alternating currents, but they argue that the tubes can be used for comparing alternating currents under suitable waveform conditions and in differential indicators. A constructed differential indicator is described with 0.7 V input, 0.05 percent division value, and a 20 Hz to 1 MHz frequency range.
Full Text
On the Possibility of Using Rod Vacuum Tubes as Saturation Diodes for Comparing Currents and Voltages
M. S. Roitman, V. R. Tsibulsky, A. I. Kramnyuk
(Presented by the Scientific and Technical Seminar of the Faculty of Automation and Computer Engineering)
Photoelectric converters (PECs) possess high metrological characteristics and are highly promising [1]. However, the comparatively low inertia of low-power incandescent lamps limits the possibilities for using PECs at low frequencies (below 20–50 Hz) [1].
In view of the urgent need for measuring converters with a lower limiting frequency of a few hertz, we investigated the possibility of using economical miniature rod vacuum tubes as measuring saturation diodes.
Rod vacuum tubes have activated cathodes with direct heating, made in the form of a straight, uncoiled filament. They make it possible to obtain sufficient emission at a comparatively low temperature and power consumption. This circumstance is very important, since the time constant of incandescent bodies is a sharply nonlinear function of temperature and, other conditions being equal, is greater the lower the temperature.
Fig. 1
Among rod-type tubes, the most suitable are types having a comparatively low heater power consumption \((I_h = 10\text{--}15\ \mathrm{mA}\) at \(U_h = 1\text{--}1.2\ \mathrm{V})\) and a small voltage on the anode \((6 \div 60\ \mathrm{V})\). Taking these requirements into account, the following were selected for study: EM-10; N-28; N-37; 1Zh24B; 1Zh42A.
Below are the results of an experimental investigation of the indicated tubes.
The EM-10, N-28, and N-37 tubes proved to be of little suitability because of the practical absence of a sufficiently broad saturation region. For illustration, Fig. 1 shows the characteristic of the N-37 tube.
Figure 2 gives the volt-ampere characteristics of the selected rod-type pentodes 1Zh24B and 1Zh42A. The transfer coefficient of converters based on these pentodes is the same and is equal to
\[ K = \frac{\Delta I_a}{I_a}\cdot\frac{u_h}{\Delta u_h}\simeq 5. \]
The time constant of the converters was determined using the circuit shown in Fig. 3.
The experimental results are given in the table. The investigations also showed that unsaturated diodes have a sufficiently high degree of repeatability of characteristics and do not require any aging beyond the factory aging.
Table
| \(U_h\) | 0.5 | 0.7 | 0.9 | 1.1 | V | Tube |
|---|---|---|---|---|---|---|
| \(\tau_p\) | 250 | 200 | 150 | 100 | ms | 1Zh42A |
| \(\tau_z\) | 1200 | 600 | 300 | 150 | ms | 1Zh42A |
| \(\tau_p\) | 400 | 300 | 200 | 150 | ms | 1Zh24B |
| \(\tau_z\) | 1500 | 800 | 550 | 300 | ms | 1Zh24B |
The study of the temporal stability of the diodes, which is one of the fundamental questions for the converter, was carried out according to the circuit shown in Fig. 4.
As a result it was found that the stability of the diodes has an optimum with respect to anode voltage.
For 1Zh24B tubes, the optimum value of \(U_a\) lies within the range \(24\text{--}26\ \mathrm{V}\) at \(U_h \simeq 0.85\ \mathrm{V}\).
We note that the optimum value of \(U_a\) is not the same for different specimens of these tubes. An experimental check of stability in the optimum mode showed the possibility of reducing the temporal instability to \(0.002\%\) per min.
Unfortunately, all the diodes investigated have a large asymmetry error \((5 \div 10\%)\), which does not allow them to be used for comparing direct and alternating currents. However, their positive qualities can be used in comparing alternating currents or in differential indicators.
It should be borne in mind that the indicated statement may prove incorrect if one of the alternating voltages is strongly distorted and its spectrum contains even harmonics. In this case an error arises in the comparison of alternating voltages, caused by asymmetry.
On the basis of converters using saturation diodes, a differential indicator was developed and constructed (circuit of Fig. 4).
Figure 2.
Tube 1Zh24B
Vertical axis: \(I_a,\ \mathrm{mA}\)
Horizontal axis: \(U_a,\ \mathrm{V}\)
Legend:
○ — 1
● — 2
Curve labels:
- \(U_n = 1.2\ \mathrm{V}\)
- \(U_n = 1.2\ \mathrm{V}\)
- \(U_n = 1\ \mathrm{V}\)
- \(U_n = 0.8\ \mathrm{V}\)
- \(U_n = 1\ \mathrm{V}\)
- \(U_n = 0.8\ \mathrm{V}\)
Tube 1Zh42A
Vertical axis: \(I_a,\ \mathrm{mA}\)
Horizontal axis: \(U_a,\ \mathrm{V}\)
Legend:
○ — 977
● — 521
Curve labels:
- \(U_n = 1.2\ \mathrm{V}\)
- \(U_n = 1.2\ \mathrm{V}\)
- \(U_n = 0.9\ \mathrm{V}\)
- \(U_n = 0.9\ \mathrm{V}\)
- \(U_n = 0.7\ \mathrm{V}\)
- \(U_n = 0.7\ \mathrm{V}\)
Fig. 3
Labels in the diagram: \(R_{\mathrm{rev}}\), \(E_a\), S1-19, \(T_1\), \(V_1\), \(R\), Battery, \(+\), \(-\).
Fig. 4
Labels in the diagram: \(R_{\mathrm{rev}}\), \(E_a\), Tube, \(D_1\), \(X_1,\ \mathrm{PPTV}_1\), \(T_1\), \(X_2,\ \mathrm{PPTV}_2\), \(M\), Battery, \(U_a\).
Fig. 5. Schematic diagram of the differential indicator
Visible labels in the schematic: \(U_1\); “Meas.”; “Calibr.”; \(U_2\); \(R_1 \sim 22\text{k}\); \(R_2 \sim 22\text{k}\); \(R_3 \sim 4.7\text{M}\); \(R_4 \sim 470\); \(R_5 \sim 33\); \(R_6 \sim 33\); \(P_1\); \(\mu\text{A}\); “diode”; \(+150\); \(6.3\text{ V}\); \(a\).
The voltages being compared, \(U_1\) and \(U_2\), are fed through cathode followers to the filaments of diodes \(D_1\) and \(D_2\). The diodes, together with resistors \(R_5\), \(R_6\), form a bridge circuit, in whose diagonal a galvanometer with a sensitivity of \(10^{-8}\) A/div. is connected.
As indicated above, the magnitude and sign of the instability of saturation diodes depend very strongly on the anode voltage. Experimentally, two diodes were selected whose instability character and sign have the same value at identical voltages on the tube anodes. Stabilization of the anode voltage is carried out by two reference diodes \(D_3\), \(D_4\). Balancing of the bridge is accomplished not by changing the voltage on the tube anodes, but by changing the amplitude of the signals applied to the tube filaments.
In the calibration mode (switch \(P\) is in the “calibr.” position), the same voltage, taken as the reference, is applied to both inputs.
By means of the variable resistor \(R_1\) and by changing the amplitude of the input signal on the filament of diode \(D_1\), a voltage equal to 0.7 V is set. The bridge is balanced by changing the voltage on the filament of the second diode (with potentiometers \(R_2\), \(R_3\), \(R_4\)). After calibration, the switch is moved to the “measurement” position; in this case the voltage \(U_2\) is applied to the second input.
From the reading of the pre-calibrated galvanometer, the magnitude of the deviation of \(U_2\) from \(U_1\) is determined.
Thanks to preliminary calibration, frequency errors of the input circuits and cathode followers are eliminated. The value of one division in the developed indicator is 0.05%.
Calibration of the indicator was performed on direct current.
To eliminate the possibility of burnout of the filaments of the saturation diodes, interlocking has been applied in the anode and filament circuits. Filament voltage is applied to the cathode-follower tubes only after the anode voltage has been switched on.
Technical Characteristics
Input voltage — 0.7 V.
Indicator division value — 0.05%.
Frequency range — 20 Hz — 1.0 MHz.
The supply voltages of the anode and filament circuits are stabilized at, respectively, 150 and 6.3 V.
REFERENCES
- B. A. Perminov, M. S. Roitman, E. I. Tsimbalist, AC comparator based on photoelectric converters, “Avtometriya,” No. 5, 1965.