Protection of electrical measuring instruments against overload by semiconductor rectifiers
Abstract Generated abstract
This paper examines methods for protecting sensitive electrical measuring instruments from overload by using semiconductor rectifiers as nonlinear circuit elements. It derives equivalent circuits and calculation formulas for direct rectifier shunting and for unbalanced bridge arrangements, comparing their transmission coefficients and maximum overload currents under small and large input voltages. The analysis indicates that an unbalanced bridge of nonlinear resistances can provide more effective current limitation than simple shunting, while maintaining low scale nonlinearity within the measuring range, and gives example calculations using copper-oxide rectifiers and notes possible use of germanium diodes for galvanometer protection.
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PROTECTION OF ELECTRICAL MEASURING INSTRUMENTS FROM OVERLOAD BY SEMICONDUCTOR RECTIFIERS
M. S. Roitman
(Presented by the scientific seminar of the Radio Engineering Faculty)
When electrical measuring instruments are used, it is very often necessary to protect them from possible overloads. For example, the use of a highly sensitive instrument as a bridge-balance indicator requires that measures be taken to protect it under large unbalance voltages.
It is therefore expedient to limit the magnitude of the current through the measuring instrument by using a nonlinear shunt [1] or a nonlinear series resistance.
Semiconductor rectifiers—most often copper-oxide ones—are used as nonlinear elements.
The current-voltage characteristic of copper-oxide rectifiers and its piecewise-linear approximation are shown in Fig. 1.
Fig. 1.
Connecting rectifiers in parallel with the instrument makes it possible to implement automatically operating current limiters (Fig. 2). It is obvious that, for small voltage drops across such a nonlinear shunt (the equivalent circuit of Fig. 3), its resistance is large \((r_0/2)\), and the shunting effect is small. The ratio of the voltage across the instrument to the source voltage (the transmission coefficient) is equal to:
\[ k=\frac{I_2 R_2}{E} = \frac{r_0 R_2} {r_0\left(R_{\mathrm{в}}+R_0+R_2\right)+2R_{\mathrm{в}}\left(R_0+R_2\right)} . \tag{1} \]
As the voltage increases above 200 mV \((e_0 \approx 200\ \text{mV})\), the resistance of the nonlinear element drops sharply and the greater part of the current branches into the shunt, bypassing the instrument.
The equivalent circuit of a nonlinear shunt for large input voltages is shown in Fig. 4.
Fig. 2.
The maximum current is determined by the expression
\[ I_{g\max}= \frac{E_{\max} r_g + e_0 R_{\mathrm{d}}} {r_g\left(R_{\mathrm{v}}+R_{\mathrm{d}}+R_g\right)+R_{\mathrm{v}}\left(R_{\mathrm{d}}+R_g\right)} . \tag{2} \]
Fig. 3.
As follows from (2), the magnitude of the current through the instrument increases with increasing voltage.
It is more expedient, for the purpose of limiting the current, to use an unbalanced bridge based on nonlinear elements (Fig. 5). The equivalent circuits for small (a) and for large (b) input voltages are shown in Fig. 6.
Fig. 4.
The transfer coefficient in this case is determined by the formula
\[ K=\frac{U_{cdxx}R_2}{E\left(R_{cd}+R_2\right)}= \]
\[ =\frac{r_0R_2}{4R_vR_2+r_0(R_2+R_v+2r_g)+2r_g(R_2+R_v)} \simeq \frac{r_0R_2}{4R_vR_2+r_0(R_2+R_v+r_g)}, \tag{3} \]
Fig. 5.
Fig. 6a.
and the maximum overload current by formula (4)
\[ I_{2\max}=I_1+I_2-I_3=-\frac{e_0}{r_g+R_2}. \tag{4} \]
Figure 7 gives the characteristics of the dependence of the voltage across the instrument on the source voltage for direct shunting by rectifiers (a) and when an unbalanced bridge of nonlinear resistances is used (b) for the following specific data: \(R_v=200\ \Omega\); \(E_{\max}=1.5\ \mathrm{V}\), \(R_2=800\ \Omega\), \(I_{2\,\mathrm{nom}}=100\ \mu\mathrm{A}\). Copper-oxide rectifiers of type MZV-3 were used as the nonlinear elements; for them \(r_0\simeq 60\ \mathrm{k}\Omega\), and \(r_g\simeq 60\ \Omega\).
For convenience in comparing the characteristics, equal sensitivity of the electrical measuring instrument in both variants was adopted (\(R_d=122\ \Omega\)) at low voltages.
Table 1
| Circuits of unbalanced bridges with rectifiers for limiting the current through the measuring instrument | Formulas for calculating the transmission coefficient and the maximum overload current |
|---|---|
| Circuit diagram shown with a measuring instrument in the bridge diagonal, rectifiers in opposite arms, resistances \(R_g\), \(R_2\), and voltage \(U_b\). | $$K=\frac{R_2(R_g-2R_0)}{R_2(R_g+2R_0)+4R_0R_g}$$ $$I_{2\max}=\frac{E_0}{R_g+R_2}$$ |
| Circuit diagram shown with a measuring instrument in the bridge diagonal, rectifiers in the arms, resistances \(R_g\), \(R_2\), and voltage \(U_b\). | $$K=\frac{r_0R_g-4R_0r_g}{r_0r_g(2R_0+R_g)+2R_0R_g(r_0+2r_g)+R_2(2R_0+R_g)(r_0+2r_g)}$$ $$I_{2\max}=\frac{E_0-e_0}{2R_2+R_g+r_g}$$ |
It should be noted that the nonlinearity of the instrument scale within its measuring range (80 mV), caused by connecting the MZV-3 rectifiers, does not exceed 2%.
Fig. 6b.
Fig. 7.
Fig. 8.
Two further variants of automatic current limiters are possible, using the circuit of an unbalanced bridge with rectifiers. The circuits of both variants and the corresponding calculation formulas are given in the table.
Protection of galvanometers can be provided by connecting germanium diodes of types DG-Ts21–DG-Ts27 as an additional nonlinear resistance; their averaged current-voltage characteristic in the nonconducting direction is shown in Fig. 8. The magnitude of their resistance at small voltages \((r_0)\) ranges from 2 to 5 kΩ.
LITERATURE
- Karandeev K. B., Semiconductor Rectifiers in Measuring Technology. Publishing House of the Academy of Sciences of the Ukrainian SSR, 1954.
- Karandeev K. B., Bridge Methods of Measurement. Gostekhizdat of the Ukrainian SSR, 1953.