Temperature constant current source

In order to determine the thermal time constant of the microhotplate in dynamic measurements, a square-shape voltage pulse was applied to the heater. The pulse frequency was 5 Hz for uncoated and 2.5 Hz for coated membranes. The amplitude of the pulse was adjusted to produce a temperature rise of 50 °C. The temperature sensor was fed from a constant-current source, and the voltage drop across the temperature sensor was amplified with an operational amplifier. The dynamic response of the temperature sensor was recorded by an oscilloscope. The thermal time constant was calculated from these data with a curve fit using Eq. (3.29). As already mentioned in the context of Eq. (3.37), self-heating occurs with a resistive heater, so that the thermal time constant has to be determined during the cooHng cycle. 

To demonstrate temperature effects, we will look at two circuits that can be used as constant current sources. One circuit will be greatly affected by temperature and the other is designed to be relatively independent of temperature. The circuits use transistors and resistors. The temperature dependence of transistors has already been discussed in detail in Section 4.F. Before we look at the circuits, we will look at how PSpice handles temperature characteristics of resistors. 

The op-amp constant current source below is designed to eliminate the effects of temperature on the BJT used in the current source. This current source is a very accurate and temperature-independent current source ... 

Even with resistor temperature dependence, the collector current changes by only about 13 pA over the entire temperature range. This circuit is a very temperature-independent constant current source, as long as the temperatures of R1 and R2 are the same and they have similar temperature coefficients. ... 

EXEHC15E 143 The circuit below is a current mirror and is a constant current source of about 50 pA. Display how the collector current of Q2 varies with temperature. Let resistors have a linear temperature coefficient of 200 ppm. The part name of both transistors is LM3046-Q. 

Six grams (0.053 mole) of iron(III) fluoride is mixed thoroughly with 60 g. (0.1 mole) of purified-grade sodium meta-phosphatet in a beaker and the contents are transferred to the carbon crucible. The crucible is placed in the reaction chamber and is heated to 925°C. at the rate of 75°C./hour in a flowing argon atmosphere. The crucible is allowed to equilibrate at 925°C. for 1 hour. The center electrode is inserted to a depth of in. and a current of 250 mA. (61 mA./cm.2) is used. A constant current source is used to maintain the desired current. The electrolysis should be carried out from 12 to 24 hours to give a sufficient yield. After electrolysis the melt is cooled to room temperature at the rate of 75°C./hour. The crucible is cut to expose the reaction boule, and the excess melt is removed by leaching in hot, dilute hydrochloric acid. Free carbon is removed by flotation with methylene iodide(diiodomethane). The product is in the form of metallic needles and small crystallites with a yield of 0.5-1 g. 

The operation of a photovoltaic system is governed by the current-voltage characteristic curves of the photovoltaic module. Such a set of curves, for different values of the incident solar irradiance and constant photovoltaic module temperature, is shown in Figure 2.4. The curves consist of two parts. In the first part the photovoltaic module behaves as a constant-current source, with amplitude proportional to the solar irradiance level. In the rest of the curve, current decays... 

The inverters are either voltage source or current source (see Figure 7-7a and b). There are other variations, but they apply to drivers smaller than the ones used with compressors. However, pulse-width-modulated (PWM) (see Figure 7-7c), transistorized units are less complicated and are relatively maintenance-free with reliable units available to at least 500 hp. For all but the smaller compressors, the current source inverter is the one typically used. With a six-step voltage source, a rule of thumb has been to size the motor at two-thirds of its rating so as not to exceed the insulation temperature rise. For current source motors, the output torque is not constant with decreased speed, which fortunately is compatible with most compressors, as torque tends to follow speed. For current source drives, one needs to upsize the motor captive transformer by approximately 15% to account for harmonic heating effects. 

The considerations so far rely on constant heating power, and the way how this power is applied to the microhotplate does not play a role. In fact, a monolithically integrated control circuitry does not apply constant power but acts as an adjustable current source. Moreover, for measuring the thermal time constant experimentally, either a rectangular voltage or rectangular current pulse is applied. Analyzing the dynamic temperature response of the system leads to a measured time constant, which... 

There will be a time interval between the application of a voltage to a TSR and the establishment of its equilibrium temperature and resistance. Thus NTC resistors can be used to delay the establishment of a final current and power level, while PTC units can be used to give an initially high current that falls back to a required level. PTC units can be used to maintain a comparatively constant current from a source of variable voltage since the increase in resistance resulting from power increase due to a voltage increase may be sufficient to inhibit any current increase. 

Calculated Power and Efficiency. The simplified analytical models of thermionic characteristics have been used to project the converter efficiency and power density with the barrier index as a parameter. These projections are shown in Figures 8 and 9 as functions of the emitter temperature. The dashed lines in these two figures are for a constant current density of 10 A/cm. If the current density is adjusted to maximize the efficiency at each temperature, the calculated performance is represented by the solid lines. Typical present generation themionlc converters operate with Vg near 2.0. Ignited mode converters in laboratory experiments have demonstrated practical operation with 1.85 < Vg < 1.90. Other laboratory devices with auxiliary sources of ions and/or special electrode surfaces have achieved Vj < 1.5, but usually not under practical operating conditions. 

For most applications, an alternative is employed. Recall that, in measuring the resistance of a thermistor, a fixed resistor is normally connected in series with the sensor. If a constant-voltage source ( s) is used, the circuit current is inversely proportional to the total resistance. Then the relationship between the measured voltage drop across the fixed resistor and the thermistor temperature can be almost linear over a range of temperature. The linear part of this curve can be shifted along the temperature scale by changing the value of the fixed resistor. 

Fig. 3.4. Kinetics of temperature variations in pyrolytic ceQs of different types. (A),(B) = in filament-type cells directly heated by electric current (A) 1,300°C 2,500 C 3,800°C pyrolysis time 10 sec from ref. 48 (B) 1, with constant-voltage source, heating time (HT) = 10 sec 2, with constant-voltage source and additional source of special powerful discharge for rapid heating, HT = 15 msec diameter of heated platinum wire 0.25 mm pyrolysis temperature 800°C reprinted with permission from ref. 57. (C) In Curie-point cell for certain ferromagnetic materials with wire diameter of 0.5-0.6 mm. 1 =CoNi (60 40) 2 = Fe(Zn) 3 = Fe 4 =CoNi (33 67) 5 = NiFe (60 40) 6 = NiCrFe (51 1 48) 7 = NiFe (45 55) 8 = Ni oscillator frequency 0.45. MHz from ref. 65. (D) In Curie-point cell for wire (1) 0.05 mm and (2) 0.5 mm in diameter pyrolysis time 1 sec HT = 0.02 and 0.1 sec from ref. 65. (E) In Curie-point cell for wire (filament) 0.5 mm in diameter. 1, 30-W Philips oscillator, HT = 1.3 sec 2, 2.5 kW oscillator, HT = 120msec reprinted with permission from ref. 57.

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