Experimental Study of the Temperature Regime of Single-Layer Varnish-Protected Non-Hermetically Sealed Microwire Resistors
Abstract Generated abstract
This paper experimentally investigates the thermal behavior of single-layer varnish-protected unsealed microwire resistors under natural convection, focusing on how winding overheating depends on dissipated power and ambient temperature. Model resistors with standardized dimensions were wound with fine PEMS wire, varnished to preserve surface emissivity, heated by direct current, and measured using a calibrated differential copper, constantan thermocouple positioned at the point of maximum winding temperature. Measurements at ambient temperatures of 20, 85, 125, and 155 degrees Celsius produced design graphs relating dissipated power to overheating for several resistor sizes. These graphs are proposed for selecting resistor dimensions under specified operating limits or estimating maximum winding temperature for a given resistor type, power, and ambient temperature.
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EXPERIMENTAL STUDY OF THE TEMPERATURE REGIME OF SINGLE-LAYER VARNISH-PROTECTED UNSEALED MICROWIRE RESISTORS
A. M. Kirik, A. S. Lyashkov
(Presented by Professor, Doctor G. I. Fuks)
The temperature regime of microwire resistors has a noticeable effect on their stability, accuracy, and service life. Knowledge of the temperature characteristics of microwire resistors is absolutely necessary when using them in circuits. In this connection, we set ourselves the task of experimentally investigating the dependence of the overheating temperature on the power dissipated by the resistor and on the ambient temperature under conditions of natural convection.
The study was carried out on models of microwire resistors made by winding PEMS winding wire of diameter \(0.05\) mm tightly in one row on standardized frames of microwire resistors. Replacing the winding made of microwire by a winding of PEMS wire does not change the temperature regime of the resistor surface, which is determined by the shape, dimensions, orientation, heat release of the resistor, and the degree of blackness of the surface. To preserve the degree of blackness of the surface, the resistors were coated with the varnish used in impregnating microwire resistors.
The characteristics of the models are given in Table 1.
Table 1
| No. | Resistor index | \(D\), mm | \(L\), mm | \(l_{\mathrm{w}}\), mm | \(R\), ohm (model) |
|---|---|---|---|---|---|
| 1 | MVS-711 | 8.3 | 45.3 | 37.8 | 2551 |
| 2 | MVS-713 | 6.2 | 27.7 | 19.8 | 1038 |
| 3 | MVS-714 | 4.4 | 25.0 | 19.5 | 715 |
| 4 | MVS-716 | 4.4 | 17.3 | 12.0 | 455 |
| 5 | MVS-717 | 3.2 | 9.7 | 6.1 | 153 |
The designation of the quantities included in Table 1 is given in Fig. 1. The schematic of the experimental setup for determining the overheating temperature of the windings of the resistor models is shown in Fig. 2.
The current leads of the resistors, over a length of 10 mm, were soldered to the centers of brass plates whose temperature corresponded to the air temperature in the thermostat.
Heat generation in the winding was provided by passing rectified current through it. The power was found from the resistance of the winding and the measured voltage drop across it.
Temperature was measured with a calibrated differential copper–constantan thermocouple with thermoelectrodes 0.05 mm thick. The hot junction of the thermocouple was located at the middle of the winding on the upper generatrix of the resistor when the resistor was in a horizontal position, which ensured measurement of the maximum winding temperature. The resistor under study was placed in a thermostat in which the required air temperature was maintained.
Fig. 1.
The maximum overheating temperatures
\(\vartheta = t - t_f\) (\(t\) and \(t_f\) are, respectively, the temperatures of the winding and of the ambient medium) were determined as a function of the power \(P\) dissipated by the resistor, at ambient-medium temperatures \(t_f = 20;\ 85;\ 125\) and \(155^\circ\)C.
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Resistor
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Heat-dissipating plates
3 and 4. Hot and cold junctions of the thermocouples
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PP potentiometer
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Thermostat
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Voltmeter
Fig. 2.
The results of the investigations are presented in the form of graphs \(P = f(\vartheta, t_f)\) in Fig. 3.
The graphs in Fig. 3 make it possible:
- Under specified operating conditions for a microwire resistor—the ambient temperature \(t_f\), the maximum permissible temperature of the microwire \(t\) (and, consequently, the specified overheating \(\vartheta = t - t_f\)), and the power \(P\)—to select the dimensions of the resistor.
Fig. 3.
Legend:
- ○ \(t_f = 20^\circ\mathrm{C}\)
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- \(t_f = 85^\circ\mathrm{C}\)
- ● \(t_f = 125^\circ\mathrm{C}\)
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× \(t_f = 155^\circ\mathrm{C}\)
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For a specified power \(P\) and ambient temperature \(t_f\), to determine the overheating \(\vartheta\) (and, consequently, the maximum temperature \(t\) of the microwire) for a given resistor index.
The graphs presented are recommended as design graphs for the investigated standard sizes of single-layer varnish-protected unsealed microwire resistors.