Constant-current power sources

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. 

The former method requires a constant-current power source whilst the latter require a potentiostat (see Section 19.2). 

Constant-current dc sources, called amperosiats, find application in several analytical instruments. For example, these devices are usually used to maintain a constant current through an electrochemical cell. An amperostat reacts to a change in input power or a change in internal resistance of the cell by altering its output voltage in such a way as to maintain the current at a predetermined level. 

If a cell is to be used as a potential standard, then it must be prepared as simply as possible from chemicals readily available in the required purity and, in the absence of current passage, it must have a known, defined, constant EMF that is practically independent of temperature. In this case the efficiency, power, etc., required for cells used as electrochemical power sources is of no importance. The electrodes of the standard cell must not be polarizable by the currents passing through them when the measuring circuit is not exactly compensated. 

FIGURE 2.27 Change of total impedance spectra for an electrolyte-supported cell when it was poisoned by 10 ppm H2S under (a) different constant current densities, and (b) different constant cell voltage. (From Cheng, Z. et al., J. Power Sources, 172 688-693, 2007. Copyright by Elsevier, reproduced with permission.)... 

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... 

A simple definition of a power conversion circuit is a circuit that converts a power source of a certain characteristic (e.g., 110 V AC battery voltage, spacecraft bus) into a power source with a more desirable characteristic (e.g., regulated +5V DC for digital logic, constant current sources). A wide variety of these circuits are presented in this chapter. 

This is the transient method for which most experience is available. It was introduced by Bowden and Rideal (1928). The name comes from that of Galvani4 and means, in fact, current. Thus, Galvanostatic transient means short-term constant current. The circuitry is simple. It consists of nothing more than a measurement cell in series with an adjustable resistance much larger in value than the resistance of the cell, a power source, a rapid action switch, and a cathode ray oscilloscope to record the variation in the potential of the working electrode with time. A typical potentialtime relation is shown in Fig. 8.6. 

Programmable constant-current sources can be constructed from programmable operational amplifiers or programmable power supplies. These devices coupled with the IFC produce a powerful approach to controlled-current coulometry. 

Linear loads—Electrical load which in steady-state operation presents essentially constant impedance to the power source throughout the cycle of applied voltage. A purely linear load has only the fundamental component of the current present. 

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. 

Discharging — Figure. Several types of discharge curves from power sources having the same capacity that were discharged at equal constant currents, a Flat curve b sloping curve c stepwise curve d typical capacitor discharge curve... 

Top scheme of a classical constant-current electrochemical cell, combining a high-voltage power supply with a large resistance. Bottom, an ESI source. The large resistance results here from the ion flow in the air. Reproduced with data from Van Berkel G.J. and Zhou F., Anal. Chem., 67, 2916, 1995. 

An electronically stabilized power source may show an idealized voltage-current diagram according to the right side curve. The output voltage remains constant U = U l at all load conditions I < Isc, and in the short-circuit-condition I = Isc, U drops down to zero. At point A, the output power is P UNL Isc, four times the maximum output power of a linear circuit at identical values for UNL and Isc. 

Right side electronically stabilized power source with constant output voltage UNL and current limitation at /sc, maximum output power at point A. 

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. 

---