Input current

Transducers The ciirrent-to-pressiire transducer (I/P transducer) is a conversion interface that accepts a standard 4-20 rnA input current from the process controller and converts it to a pneumatic output in a standard pneumatic pressure range (normally (),2-L0 bar [3-15 psig] or, less frequently, 0,4-2,0 bar [6-30 p.sig]). The output pressure generated by the transducer is connected directly to the pressure connection on a spring-opposed diaphragm actuator or to the input of a pneumatic valve positioner. 

Figure H-7 is the schematic of a basic I/P transducer. The transducer shovvm is characterized bv (1) an input conversion that generates an angular displacement of the beam proportional to the input current, (2) a pneumatic amplifier stage that converts the resulting angii-...

Id. For flyback-mode converters it is a good idea to select a power switch average current rating of about 1.5 times the maximum average input current of the supply. Another consideration is the loss. By overspecifying the current the /2i DS(on) loss (conduction loss) can be reduced with very little penalty on cost and input capacitance. 

Power factor correction circuits are intended to increase the conduction angle of the rectifiers and to make the ac input current waveform sinusoidal and in phase with the voltage waveform. The input waveforms can be seen in Figure C-2. This means that all the power drawn from the power line is real power and not reactive. The net result is that the peak and RMS current drawn from the line is much lower than that drawn by the capacitive input Alter circuit traditionally used. 

The power winding of the inductor (transformer) not only must support the maximum average input current but the output current as well. So, the wire gauge of the winding should be... 

Figure 14-5. Dashed line denotes a linear or direct relationship between input current and output horsepower for induction motor. Solid curves show how far from reality that relationship is for actual motors. Without such a calibration curve for the motor in question, determining the change in load from a measured change in current is not possible. (Used by permission Nailen, R. L. Hydrocarbon Processing, p. 205, Sept. 1973. Gulf Publishing Co. All rights reserved.)...

Demanding continuity of n at r = rg, using the Kirchhoff relation I — l = 1, and identifying (for unit input current) as the escape probability 0, we get... 

Actually, Figure 2-9 and Figure 2-10 put together clearly describe everything about the input that we need to know—how the input capacitors behave, why the load current is not the same as the input current, and also the full analysis of all the currents involved. 

But the DC input current (0.67A) cannot flow through the switch during the OFF time. Therefore it must be getting diverted as shown, charging the input capacitors in the process. Note that if this diverted current were not 0.67A, it would amount to introducing an AC component in the supposedly DC input current (which in turn would imply imperfect decoupling, which is contradictory to our initial assumption). 

Note that in a Buck, the average input current is NOT the load current (as the engineer was rather intuiAvely visuaflzing), but the average switch current. The average switch current is of course related to the load current by 7SW = Io x D (for a Buck). 

Average input current 5 [inductor ripple current, peak-to-peak value Jin Avg [i-IPP 0.93 A (0.65 A... 

Author s Comments In going from 15V to 7.4V, the duty cycle is 50%. So in fact the input current with a 1A load is only 0.5 A. This simple fact eludes many engineers, which is why I talked about it in so much detail in Chapter 2 (the missing current problem). 

The superconducting quantum interference device (SQUID) is formed from a superconducting loop containing at least one Josephson junction. Basically, a SQUID amplifier converts an input current to an output voltage with a transresistance of the order of 107 V/A. The input noise is of the order of 10-11 A/(Hz)1/2. The bandwidth of the SQUID amplifier can be up to 80kHz. The dynamic range in 1 Hz bandwidth can be 150dB. 

Optocouplers are a class of devices with input current (/ ) and output current (70) coupled optically, but isolated electrically. They are used extensively in the automation industry and in laboratory equipments where large common-mode noise/voltage or hazardous electrical shocks are present in circuits between transducers detectors and controlling equipments. The simplest optocoupler is composed of an LED (input) and a photodiode (output) as shown in Figure 1.19. 

For example, if Qi = 50 tF/cm and R = 2 fi, t = 4.6 X 10 " s (0.46 ms). Thus, in the galvanostatic transient technique, the duration of the input current density pulse is on the order of milliseconds. From a series of measurements of for a set of i values, one can construct the current-potential relationship for an electrochemical process. For example. Figure 6.20 shows the current-potential relationship for the electrodeposition of copper from acid CUSO4 solution. 

The minus sign indicates that the phase is reversed. For / fb = 100 MO, one nanoampere of input current results in an output voltage of 100 mV. In some cases, the sample must be grounded, then the noninverting input of the op-amp may be connected to a fixed dc voltage as the bias. The output voltage is the... 

Another possible circuit is a voltage amplifier, with a shunt resistor to convert the input current to input voltage (see Fig, 11.1. This type of current amplifier has more disadvantages than the picoammeter The input... 

In reducing Johnson noise, to incorporate all the amplification in one single stage is the best choice. However, a stray capacitance Cfb is always present. In this case, the relation between the input current and the output voltage is determined by the differential equation... 

First, we analyze the effect of the stray capacitance in parallel with the feedback resistance. Consider a sinusoidal input current with frequency /. From Fig. 11.1, the magnitude of the output voltage is... 

Fig. 11.4. Logarithmic amplifier, (a) Schematic of a logarithmic amplifier. A diode is used as the feedback element in a current amplifier. The current-voltage characteristics are exponential. The output voltage is then proportional to the logarithm of the input current, (b) The transfer curve of a typical logarithmic amplifier, AD757N from Analog Devices. The reference current is internally set to be 10 p,A. It is accurate up to six decades.

Very high input impedance, with a typical input current of a few pA. 

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