Diode constant current

A "constant current diode" (lower left of Fig. 14.11) also has two exterior wires but is more complex inside. It changes resistance according to the current passing through it, within reasonable limits, which tends to keep the current constant. (More in Chapter 18.) It is used to make sawtooth waves (see index) with straight ramps, instead of the otherwise exponential voltage buildups. 

The top of the sawtooth is not straight, since the capacitor charges in a curved, exponential manner, as shown back on page 94. Therefore, for use in making horizontal sweeps for cathode ray tubes, a constant current diode is usually put in series with the resistor (lOK as shown in this diagram), to force the charging curve to become linear. 

Figure 3.4. Breakdown of device performance at Tg The luminance for a constant current density of 25 mA/cm2 is plotted for two light emitting diodes that were made with hole transport materials of different Tg. Triangles 75-nm TPD/65 nm Alq3 Circles 75-nm Spiro-TAD/65 nm Alq3. The dotted lines mark the respective Tg values TPD 63°C, Spiro-TAD 133°C. At these temperatures, the materials become soft, which results in a steep decrease in the efficiency.

The constant current source keeps the emitter current constant at 10 mA. Since the collector current is approximately equal to the emitter current, the constant current source keeps the collector current approximately constant at 10 mA. Since the base is grounded, the emitter voltage is a diode drop below ground ... 

Electrical Resistivity. The electrical resistivity of these materials was measured from room temperature to 15 K at 5 K intervals on bars, 6-10 mm long and 1 mm square, using a DC four-probe method. The electrical contacts were made with fine platinum wire and silver paste. A constant current of <10 mA was used. The voltage difference was measured to 0.1 fiV. The temperature of the specimen was maintained by a closed-cycle cryogenic system and measured using a calibrated silicon diode. 

Electrical effects. Electrical methods are convenient because an electrical signal can be easily processed. Resistance thermometers (including thermistors) and thermocouples are the most widely used. Other electrical methods include noise thermometers using the Johnson noise as a temperature indicator resonant-frequency thermometers, which rely on the temperature dependence of the resonant frequency of a medium, including nuclear quadrupole resonance thermometers, ultrasonic thermometers, and quartz thermometers and semiconductor-diode thermometers, where the relation between temperature and junction voltage at constant current is used. 

An important application of a JFET is as a constant current source or as a current regulator diode. When a JFET is operating in the active region, the drain current Ins i relatively independent of the drain voltage Vbs- The JFET does not, however, act as an ideal constant current source since Ins does increase slowly with increases in Vbs- The rate of change of Ins with Vps is given by the drain-to-source conductance gds = dIns/dVns- Since Ins is related to the channel length L by Ins oc l/I, the drain-to-source conductance can be expressed as... 

In Fig. 7.20 a diode-connected JFET or current regulator diodeis shown. Since Vbs = 0, Ins = loss- The current regulator diode can be modeled as an ideal constant current source in parallel with a resistance ro... 

The current regulator dynamic output resistance will be pq = rconstant current source in parallel with a 10-kS2 dynamic... 

Figure 5.3 shows a homemade constant-current power source for LEDs and laser diodes. The screen displays the intensity supplied to the LEDs in mA. Sources have been designed to power one, two or four LEDs simultaneously but independently. Those having two or four separate outputs are especially useful for RGB LEDs. 

Homemade constant-current power source for LEDs and laser diodes powering a diode mounted on a threaded cell holder head. 

Constant-current source for independently powering three laser diodes or LEDs and a low-voltage, low-power tungsten lamp. 

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