Voltage limited constant current

Most systems are equipped with power supplies that can be operated in constant voltage, constant current, and constant power modes. Common limits are 30 kV and 300 liA for constant voltage and constant current, respectively. Software-controlled polarity switching and programmable gradients are conveniences. 

The lUIa characteristics (Figure 13.6) is composed of an lU characteristic with an attached charging phase. Upon reaching a certain lower current limit, charging is continued at slightly increased voltage with constant current (I-charging phase). 

A lower limit of bias is given by the onset of unstable macropore formation. This is shown in Fig. 9.13, which shows the pore morphology and the corresponding formation conditions in the current density-voltage plot The current density J is held constant by the intensity of the backside illumination, so that the influence of the applied bias can be studied independently. At -0.4 V, versus a platinum wire as a pseudoreference electrode, the current density is constant over the... 

Regulated direct current (DC) power supplies designed for electrophoresis allow control of every electrophoretic mode. Constant voltage, constant current, or constant power conditions can be selected. Many power supplies have timers and some have integrators allowing runs to be automatically terminated after a set time or number of volt-hours (important in IEF). All modes of operation can produce satisfactory results, but for best results and good reproducibility some form of electrical control is important. The choice of which electrical parameter to control is almost a matter of preference. The major limitation is the ability of the chamber to dissipate the heat generated by the electrical current. 

The mean capacitance, Cn, is determined from the time necessary to discharge the capacitor between two voltage limits with a constant current, I, with the relation (Equation 11.2). 

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

---