Constant-current mode

Fig. 4. Operational modes for stm. (a) Constant height mode, (b) Constant current mode (18).

The constant height mode of operation results in a faster measurement. In this analysis, the tip height is maintained at a constant level above the surface and differences in tunneling current ate measured as the tip is scaimed across the surface. This approach is not as sensitive to surface irregularities as the constant current mode, but it does work well for relatively smooth surfaces. 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

Figure 26. Constant current mode STM image of isolated (A), self-organized in close-packed hexagonal network (C) and in fee structure (E) of silver nanoclusters deposited on Au(l 11) substrate (scan size (A) 17.1 x 17.1 nm, f/t=—IV, /t=ltiA, (C) 136 X 136 nm, f/t = — 2.5 V, /t = 0.8 tiA, (E) 143 x 143 nm, = —2.2 V, /, = 0.72 nA). I U) curves and their derivatives in the inserts of isolated (B), self-organized in close-packed hexagonal network (D) and in fee structure (F) of silver nanoclusters deposited on Au(l 11) substrate. (Reprinted with permission from Ref. [58], 2000, Wiley-VCH.)...

Figure 2.26 The expected trajectory that would be followed by an STM tip in constant current mode over a metal sample. The surface has one alien atom more electronegative than its neighbours. From Christensen (1992).

One common use for wirewound resistors seems to be as the load for a converter. I also use that configuration when doing thermals to simulate the customer s system and for noise and ripple measurements. But rarely do I use it for anything else. I would strongly suggest you get yourself a good electronic load. But do remember to set it to CC mode (constant current mode). Because a resistor (or an electronic load set to CR mode) is just too benign. For example, rarely does it reveal any fundamental start-up issues. 

The synthesis of phtalocyanine dyes was followed by CE and the purity and composition of the end product was investigated by the same method. The chemical structure of the newly synthetized zinc phtalocyanine tetrasulphonic acid is shown in Fig. 3.163. An uncoated fused silica capillary (65cm X 75 /.an i.d.) was employed for the separation of dye components. Running buffers were 10 mM ammonium acetate (pH adjusted to 9.4 with ammonia) and 10 mM potassium dihydrogenphosphate (KH2P04) (pH = 9.0). Samples were injected hydrodynamically (50 mm for 10 s). Separation was realized in the constant current mode at 30 pA, which required about 28 kV separation voltage. Analytes... 

Capillary isotachophoresis is usually performed in constant current mode, which implies the invariable ratio between concentration and electrophoretic mobility of ions. Therefore, bands that are less concentrated than the LE are sharpened, whereas those that are more concentrated than the LE are broadened to adapt their concentration to the requested constant value between concentration and electrophoretic mobility. The consequence of this unique property of CITP is that each sample component can be concentrated to an extent that depends on its initial concentration and the concentration of the leading electrolyte. Therefore, the opportune selection of composition and concentration of the leading electrolyte allows the enrichment of diluted analytes. 

Eigure 6.18(top) displays a submolecular-resolution STM image of the afc-plane of an in-air cleaved TTE-TCNQ single crystal obtained in a constant current mode in... 

It is advantageous to operate the FET devices in a constant current mode using a feedback system to compensate for the voltage change caused by the gas molecules. 

Fig. 26a. Scanning tunneling microscopic image of a 310nmx310nm area of bare Au film epitaxially grown on a mica substrate (Au mica) measured in air constant current mode bias (Vb) of + 50 mV, tunneling current (I,) of 10 nA. b STM image of a 3.5 nm x 3.5 nm area of a bare Au/mica film constant current mode Vb = 4- 4.9 mV I, = 3.0 nA [219]...

Fig. 27a-d. STM image of a 100 nm x 100 nm a n-oetadecanethiol film coated on Au/mica b same area after 10 s of etching mode (Vb = 10 mV, I, = 10 nA) over a 10 nm x 10 nm area (within area shown by broken line% c same area after 10 min of continuous normal scanning d after 35 min normal scanning. All images were taken in constant current mode, Vb = + 1V, and I, = 1 nA [219]... 

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