Current Sensing Element- Sense resistor:
Current sensing refers to generation of the voltage signal which is related to the current passing in the circuit. A conventional way of sensing current is to insert a resistor in the path of current to be sense. Then we can place the sensed resistor at anywhere in series with the circuit it may be load or switch. Therefore current sensing devices are to be considered as current to voltage converter.
Factors on which the functioning of the sensing element depends
· Values must be taken low in order to minimize the power losses:
The current sensed values usually depend on the threshold voltage of the circuit whose operation is completely based on the sensed current information.
· To increase the accuracy we must consider low temperature co efficient:
Temperature is the main coefficient factor of resistance in terms of accuracy. Resistor with temperature coefficient resistance closer to zero, in the entire operation that should be used. Power derating curve provides allowable power at different temperatures. But peak power capability is a function of energy; hence energy rating curve should be taken into account
Pros and Cons of current sensing resistors consist of
· Cost is very low when compared to other devices.
· High dimension inaccuracy
· Computable current range from very low to medium
· Capability to determine DC or AC current
· Introduces supplementary resistance into the measured circuit path, which may increase source output resistance and outcome in objectionable loading effect.
· Power gets lost due to the direction of power dissipation. Consequently, current sensing resistors are rarely used away from the low and medium current sensing applications.
Two methods of current sensing:
1. Direct current sensing:
Direct current sensing is dependent upon Ohm’s law. By putting a shunt resistor in arrangement with the system load, a voltage is generated across the shunt resistor that is proportional to the system load current. The voltage over the shunt could be measured by differential amplifiers for example current shunt amplifiers, operational amplifiers or difference amplifiers. It is typically implemented for load currents <100A.
2. Indirect current sensing:
Indirect current sensing is dependent upon Ampere’s and Faraday’s laws. By putting a loop around a current carrying conductor, a voltage is induced over the loop that is proportional to the current. This type sensing method is utilized for 100A – 1000A load currents.
Low-Side Current Sensing:
It is a low input common mode voltage. Low-side current sensing connects the sensing resistor between the load and ground. This is desirable because the common-mode voltage is near ground, which takes into consideration the utilization of single-supply, rail to rail input /output op-amps. Load is giving to the single supply and resistance is grounded. The drawbacks to low-side sensing are disturbances to the system load’s ground potential and the inability to detect load shorts.
High Side Current Sensing:
High-side current sensing connects the sensing resistor between the power supply and load.
High-side sensing is desirable because it directly monitors the current delivered by the supply, which considers the identification of load shorts. The test is that the amplifier’s input common mode voltage range must have as a feature the load’s supply voltage. Finally out is measured across the current sensed device, and load is grounded. Figure below is representing the primary and secondary side current curve:
Current Transformer (CT):
Current transformer (CT) is a transformer is used to measure electric currents. CT is the most widely recognized sensor around today’s high current solid-state energy meters. It can measure up to extremely high current and consumes little power. It is also very useful in measuring or monitoring high current, high voltage and high power circuits. These are used in power system of all kinds, such as power supplies, motor controls, lighting controls.
These sensors provide critical information for system control and safety. And generate an output signal proportional to measured current.
Features of Current Transformer:
· Measures AC only
· Electrical Isolation
· No Power Supply
· Lower Cost
These sensors are these days being used widely in almost all the industries because of their vast applications and the type of output they provide which can be controlled and can be used for different applications.
Current Sense the voltage drop proportional to the load current across a resistor of 10R is taken and is stepped up by a current transformer (CT) to feed to a bridge rectifier to generate pulsating dc for the comparator to develop current sense. The comparator generates the zero crossing pulses from a pulsating D.C.
Applications of current sensor:
· Open loop current sensor using TLE4998S.
· Current sensor using TLE4998S in range selection mode.www.ctsensorducer.com