Current Sensor and It’s Application

Sensing variable current flow is a major requirement in frequent electronics systems and the strategies to do so are as an assortment of the applications themselves. A sensor is a unit that can determine a physical phenomenon and compute the latter, in other words, it gives a measurable demonstration of the wonder on a particular scale or range. A current sensor is a device that recognizes electrical current in a wire or a system whether it is high or low and creates an indicator relative to it. It might be then used to presentation the measured current in an ammeter or might be archived for further classification in a data acquisition system or might be used for control purposes. The current sensor is “disturbing” as it is an incorporation of some of the sensors, which may cause system performance.

There is a wide variety of current sensors to monitor alternating or direct the current and its measurement is required in many applications be it in industrial, automotive, or household fields.

Principle:

The current sensor is a device that detects and converts current to get an output voltage, which is directly proportional to the current in the designed path. When current is passing through the circuit, a voltage drops across the path where the current is flowing. Also, a magnetic field is generated near the current-carrying conductor. These above phenomena are used in the current sensor design technique.

Current Sensing Element- Sense resistor:

Current sensing refers to the 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 sensitive. Then we can place the sensed resistor anywhere in series with the circuit it maybe 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 power losses:

The currently 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 coefficient:

Temperature is the main coefficient factor of resistance in terms of accuracy. A resistor with temperature coefficient resistance closer to zero, in the entire operation that should be used. The 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

Pros:

• The 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

Cons:  

• 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 of 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. The 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. The 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 the power systems of all kinds, such as power supplies, motor controls, lighting controls.

Current Transformer:

These sensors provide critical information for system control and safety. And generate an output signal proportional to the 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 a 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.

Photo Credit

• Current Transformer by wikimedia
• primary and secondary side current curve by gstatic
• Current Transformer by gstatic