Sense resistor

A schematic of the apparatus is shown in Figure 2. The series resistor senses current. When the overall circuit RC (R and C are circuit resistance and capacitance, respectively) time constant is short compared with the voltage developed on the sensing resistor is proportional to the instantaneous... 

Another concept is brushless excitation, in which an ac generator (exciter) is direc tfy coupled to or mounted on the motor shaft. The ac exciter has a stator field and an ac rotor armature which is directly connected to a static controllable rectifier on the motor rotor (or a shaft-mounted drum). Static control elements (to sense synchronizing speed, phase angle, etc.) are also rotor-mounted, as is the field discharge resistor. Changing the exciter field adjusts the motor field current without the necessity of brushes or slip rings. Brushless excitation is suitable for use in hazardous atmospheres, where conventional brush-type motors must have protective brush and slip-ring enclosures. 

Thyristors have been replacing saturable reactors they are small, efficient, and easily controlled by a wide variety of control systems. A modern crane control drive uses fixed secondary resistors and two sets of primaiy thyristors (one set for hoist, one for lower). With tachometer feedback for speed sensing, the control for the motor provides speed regulation and torque hmiting in both directions, all with static-devices. A wide variety of control systems is possible the control should be designed for the specific application. 

Instantaneous current sensing is used to proteet the power semieonduetors. A eurrent sensing resistor is plaeed in the power switeh eurrent path and its voltage represents the instantaneous eurrent flowing through the power deviee. This... 

The seeond method of overeurrent proteetion is pulse-to-pulse overcurrent protection. This method guarantees a maximum safe power switeh eurrent. A eurrent sensing element (resistor or eurrent transformer) is plaeed in series with the power switeh(es). It views the instantaneous eurrent flowing through the power switeh and will instantly eutoff the power switeh if a preset instantaneous eurrent limit is exeeeded. This eireuit must be very fast and will proteet the power switeh from all forms of instantaneous overeurrents ineluding eore saturation. It is not a form of eurrent-mode eontrol sinee this proteetion limit is fixed and not infiueneed by external parameters. 

To begin the proeess, one deeides how mueh sense eurrent is to be drawn through the output voltage resistor divider. For the sake of reasonable values to be ealeulated for the error amplifier eompensation values, resistanee values in the range of 1.5 to 15 K should be used in the upper leg of the resistor divider. Tet us use a sense eurrent of 1 mA as the resistor divider sense eurrent. This makes the lower resistor in the divider (Ri)... 

Continuing the design example, let us use the elosest f pereent resistor value -2.49 K. This makes the aetual sense eurrent... 

Multiple output sensing is done by using two resistors in the top of the voltage sensing resistor divider. The top-end of eaeh resistor goes to a different positive output voltage as seen in Figure 3 4. 

The eenter node of the resistor divider beeomes a eurrent summing node where a portion of the total sense eurrent eomes from eaeh of the sensed output voltages. The higher power output, and usually the output that requires a tighter output regulation, require the majority of sense eurrent. The lighter loaded... 

First select a voltage divider sense current—a nominal 1mA. Determine the lower resistor (RIO + Rll) in the voltage divider ... 

Knowing the upper resistor in the +5 V voltage sensing divider (4.7 K),... 

The eurrent sense resistor in the souree eonneetion of the power MOSFET is approximately... 

It is necessary to add a small leading edge spike filter to the current sensing output. To keep the time delay to a reasonable value, I will use a 1K ohm resistor and a 470 pF capacitor. 

Starting on the secondary side of the power supply, assign the sense current through the voltage sensing resistor divider to be approximately 1 mA (or 1K per volt). Using the closest resistor value for the lower resistor a 2.7K resistor yields and actual sense current of 0.926 mA. One can immediately calculate the value of the upper resistor (R3) ... 

Current sense resistor (R15) and voltage sensing resistor divider (Rll and R13)... 

The IC provides only a shutdown pin with an aetivation threshold of 0.4 V minimum. I plan to enter a hieeup mode of overeurrent proteetion as a form of baek-up proteetion. To minimize the size of the eurrent sensing resistor, I will employ a variation of the eurrent foldbaek sensing eireuit. Here 0.35 V will be eontributed by a resistor in the voltage sensing resistor divider (R14). The value of R15 is then... 

I will assign a sense eurrent of about 1.0 mA through the voltage sensing resistor divider. This make the resistanee represented by the sum of R13 and R14... 

I am going to use a sense eurrent of 1.0 mA. This makes the lower resistor in the voltage sensing resistor divider (R9) a 2.49 K 1 pereent resistor. 

The voltage-sense voltage divider is started by selecting a sense current through the divider. I will use 1 mA. The lower resistor (R3) becomes... 

The minimum speeified maximum linear limit of the input to the multiplier (pin3) is 2.5 V. This level should be the peak value of the divided reetified input waveform at the highest expeeted ae input voltage at the erest of the sinusoid (370 V). If a sense eurrent of 200jUA is seleeted at this point the resistor divider beeomes... 

The eurrent sense resistor should be sized in order to reaeh the 1.1 V eurrent sense threshold voltage at the low ae input voltage. The value then beeomes... 

For the output voltage sense resistor divider, seleeting the sense eurrent as 200jUA, the lower resistor beeomes... 

This makes the true sense eurrent 2.5 V/12K = 208 jUA. The upper resistor is... 

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