UNIVERSAL CURRENT SENSORABSTRACTThe measurement of electric current strength is not always easy especially when the measured signal requires further electronic conditioning. Simply connecting an ammeter to an electrical circuit and reading out the value is no longer enough. The current signal must be fed into a computer in which sensors convert current into a proportional voltage with minimal influence on the measured circuit. The basic sensor requirements are galvanic isolation and a high bandwidth, usually from DC up to at least 100 kHz. Conventional current measurement systems therefore tend to be physically large and technically complexEarly Solutions The oldest technique is to measure the voltage drop across a resistor placed in the current path. To minimize energy losses the resistor is kept very small, so the measured voltage must be highly amplified.
This is a training module on the Honeywell Current sensor; Welcome to the training module on the Honeywell Current sensor. This training module introduces different types and features of a current sensor and its application. Honeywell offers a wide variety of current sensors to monitor alternating (ac) or direct (dc) current.
The amplifiers offset voltage must be as small as possible and its supply voltage must be at the potential of the circuit, often 110 V mains (230 V in Europe) with high parasitic peaks from which its output must be isolated. This requirement increases overall system cost.
Principle is the transformer. Its construction is much simpler widespread, but it doesnt allow the measurement of DC signals. Isolation between primary and secondary sides is implicitly given. A problem is the limited frequency rangeHall sensors also measure the magnetic field surrounding the conductor but, unlike current transformers, they also sense DC currents.
A circular core of soft magnetic material is placed around the conductor to concentrate the field. The Hall element, which is placed in a small air gap, delivers a voltage that is proportional to the measured current.
This sensor also offers a galvanic isolation. The very small output voltage of the Hall element must be highly amplified, and the sensitivity is temperature dependent and requires adequate compensation. There is an inevitable offset, i.e., a small DC voltage at zero current; the offset amplitude and temperature coefficient are subject to significant fluctuations.
The smaller the current to be measured, the higher the offset-induced relative error. Also of note is sensitivity to short current peaks in the circuit: according to the hysteresis properties of the core material, these peaks can cause a static magnetization in the core that results in a permanent remanence, and finally to an offset alteration of the Hall element. Of Hall effect current sensor are open loop and closed loop. In the former, the amplified output signal of the Hall The element is types directly used as the measurement value. The linearity depends on two that of the magnetic core.
Offset and drift are determined by the Hall element and the amplifier. The price of these sensors is low, but so is their sensitivity. Closed-loop Hall sensors are much more precise. The Hall voltage is first highly amplified, and the amplifiers output current then flows through a compensation coil on the magnetic core (see Figure 1). It generates a magnetization whose amplitude is the same but whose direction is opposite to that of the primary current conductor. The result is that the magnetic flux in the core is compensated to zero.
Magnetoresistive field (The principle is similar to that of an op amp in inverter half or full bridge. The barber poles mode, for are positioned such that in thepresence of a magnetic field the value of the first resistor increases and that of the second decreases. Sensors are usually configured as aThe following figure shows the closed loop type hall sensorMagneto resistive sensorPractical magnetic field sensors based on the magnetoresistive effect (see The Magnetoresistive Effect) are easily fabricated by means of thin film technologies with widths and lengths in the micrometer range. They have been in production for years in many different executions 1,2,3,4. To reduce the temperature dependence, they are usually configured as a half or a full bridge. In one arm of the bridge, the barber poles are placed in opposite directions above the two magnetoresistors, so that in the presence of a magnetic field the value of the first resistor increases and the value of the second decreases (see Figure 2).
For best performance, these sensors must have a very good linearity between the meas uredquantity (magnetic field) and the output signal. Even when improved by the barber poles, the linearity of magnetoresistive (MR) sensors is not very high, so the compensation principle used on Hall sensors is also applied here. An electrically isolated aluminum compensation conductor is integrated on the same substrate above the permalloy resistors (see Figures 3 and 4). The current flowing through this conductor generates a magnetic field that exactly compensates that of the conductor to be measured. In this way the MR elements always work at the same operating point; their nonlinearity therefore becomes irrelevant. The temperature dependence is also almost completely eliminated.
The current in the compensation conductor is strictly proportional to the measured amplitude of the field; the voltage drop across a resistor forms the electrical output signalMagnetoresistive sensors, as are Hall elements, are very well suited for the measurement of electric currents. In such applications it is important that external magnetic fields do not distort the measurement. This is achieved by forming a full bridge made of four MR resistors, where the two arms of the bridge are spatially separated. The barber poles have the same orientation in the two arms, so that only a field difference between the two positions is sensed. This configuration is insensitive to external homogeneous perturbation fields.
The primary current conductor is Ushaped under the substrate, so that the magnetic fields acting on the two arms of the bridge have the same amplitude but opposite directions. This way the voltage signals of the two half-bridges are added. The sensors have been in production for several years 5. In the examples shown in Photo 1A, B, and C, a ceramic plate is used as the substrate, onto the back of which the primary conductor is glued to achieve an isolation of several kilovolts. The sensors require neither a core nor a magnetic shielding, and can therefore be assembled in a very compact and cheap way. The have a linearity error of.
Temperature is the most often-measured environmental quantity. This might be expected since most physical, electronic, chemical, mechanical, and biological systems are affected by temperature. Certain chemical reactions, biological processes, and even electronic circuits perform best within limited temperature ranges.
Temperature is one of the most commonly measured variables and it is therefore not surprising that there are many ways of sensing it. Can be done either through direct contact with the heating source, or remotely, without direct contact with the source using radiated energy instead. There are a wide variety of temperature sensors on the market today, including Thermocouples, Resistance Temperature Detectors (RTDs), Thermistors, Infrared, and Semiconductor Sensors.
5 Types of Temperature Sensors. Thermocouple: It is a type of temperature sensor, which is made by joining two dissimilar metals at one end. The joined end is referred to as the HOT JUNCTION.
The other end of these dissimilar metals is referred to as the COLD END or COLD JUNCTION. The cold junction is actually formed at the last point of thermocouple material.
If there is a difference in temperature between the hot junction and cold junction, a small voltage is created. This voltage is referred to as an EMF (electro-motive force) and can be measured and in turn used to indicate temperature. Thermocouple.
The RTD is a temperature sensing device whose resistance changes with temperature. Typically built from platinum, though devices made from nickel or copper are not uncommon, RTDs can take many different shapes like wire wound, thin film.
To measure the resistance across an RTD, apply a constant current, measure the resulting voltage, and determine the RTD resistance. RTDs exhibit fairly linear resistance to over their operating regions, and any nonlinearity are highly predictable and repeatable. The PT100 RTD evaluation board uses surface mount RTD to measure temperature. An external 2, 3 or 4-wire PT100 can also be associated with measure temperature in remote areas.
The RTDs are biased using a constant current source. So as to reduce self-heat due to power dissipation, the current magnitude is moderately low. The circuit shown in figure is the constant current source uses a reference voltage, one amplifier, and a PNP transistor. Thermistors: Similar to the RTD, the thermistor is a temperature sensing device whose resistance changes with temperature. Thermistors, however, are made from semiconductor materials.
Resistance is determined in the same manner as the RTD, but thermistors exhibit a highly nonlinear resistance vs. Temperature curve. Thus, in the thermistors operating range we can see a large resistance change for a very small temperature change.
This makes for a highly sensitive device, ideal for set-point applications. Semiconductor sensors: They are classified into different types like Voltage output, Current output, Digital output, Resistance output silicon and Diode temperature sensors. Modern semiconductor temperature sensors offer high accuracy and high linearity over an operating range of about 55°C to +150°C. Internal amplifiers can scale the output to convenient values, such as 10mV/°C. They are also useful in cold-junction compensation circuits for wide temperature range thermocouples. A brief details about this type of temperature sensor are given below. Sensor ICs There are a wide variety of temperature sensor ICs that are available to simplify the broadest possible range of temperature monitoring challenges.
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These silicon temperature sensors differ significantly from the above mentioned types in a couple of important ways. The first is operating temperature range. A temperature sensor IC can operate over the nominal IC temperature range of -55°C to +150°C. The second major difference is functionality. A silicon temperature sensor is an integrated circuit, and can therefore include extensive signal processing circuitry within the same package as the sensor. There is no need to add compensation circuits for temperature sensor Ics. Some of these are analogue circuits with either voltage or current output.
Others combine analogue-sensing circuits with voltage comparators to provide alert functions. Some other sensor ICs combine analogue-sensing circuitry with digital input/output and, making them an ideal solution for microprocessor-based systems. Digital output sensor usually contains a temperature sensor, analog-to-digital converter (ADC), a two-wire digital interface and registers for controlling the IC’s operation. Temperature is continuously measured and can be read at any time.
If desired, the host processor can instruct the sensor to monitor temperature and take an output pin high (or low) if temperature exceeds a programmed limit. Lower threshold temperature can also be programmed and the host can be notified when temperature has dropped below this threshold. Thus, digital output sensor can be used for reliable temperature monitoring in microprocessor-based systems. Temperature Sensor Above temperature sensor has three terminals and required Maximum of 5.5 V supply. This type of sensor consists of a material that performs the operation according to temperature to vary the resistance. This change of resistance is sensed by circuit and it calculates temperature. When the voltage increases then the temperature also rises.
We can see this operation by using a diode. Temperature sensors directly connected to microprocessor input and thus capable of direct and reliable communication with microprocessors. The sensor unit can communicate effectively with low-cost processors without the need of A/D converters. An example for a temperature sensor is LM35.
The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Celsius temperature. The LM35 is operates at -55˚ to +120˚C. The basic centigrade temperature sensor (+2˚C to +150˚C) is shown in figure below.
Features of LM35 Temperature Sensor:. Calibrated directly in ˚ Celsius (Centigrade).
Rated for full l −55˚ to +150˚C range. Suitable for remote applications.
Low cost due to wafer-level trimming. Operates from 4 to 30 volts.
Low self-heating,. ±1/4˚C of typical nonlinearity Operation of LM35:. The LM35 can be connected easily in the same way as other integrated circuit temperature sensors.
It can be stuck or established to a surface and its temperature will be within around the range of 0.01˚C of the surface temperature. This presumes that the ambient air temperature is just about the same as the surface temperature; if the air temperature were much higher or lower than the surface temperature, the actual temperature of the LM35 die would be at an intermediate temperature between the surface temperature and the air temperature. The temperature sensors have well known applications in environmental and process control and also in test, measurement and communications. A digital temperature is a sensor, which provides 9-bit temperature readings. Digital temperature sensors offer excellent precise accuracy, these are designed to read from 0°C to 70°C and it is possible to achieve ±0.5°C accuracy.
These sensors completely aligned with digital temperature readings in degree Celsius. Digital Temperature Sensors: Digital temperature sensors eliminate the necessity for extra components, such as an A/D converter, within the application and there is no need to calibrate components or the system at specific reference temperatures as needed when utilizing thermistors.
Digital temperature sensors deal with everything, empowering the basic system temperature monitoring function to be simplified. The advantages of a digital temperature sensor are principally with its precision output in degrees Celsius. The sensor output is a balanced digital reading. This intends no other components, such as an analogue to digital converter and much simpler to use than, a simple thermistor which provides a non-linear resistance with temperature variation. An example for a digital temperature sensor is DS1621, which provides a 9 bit temperature reading. Features DS1621:. No external components are required.
Temperature range of -55⁰C to +125⁰C in 0.5⁰ intervals is measured. Gives temperature value as a 9-bit reading. Wide power supply range (2.7V to 5.5V). Converts temperature to digital word in less than one second.
Thermostatic settings are user definable and Non volatile. It is as 8-pin DIP. Pin Description:. SDA – 2-Wire Serial Data Input/ Output. SCL – 2-Wire Serial Clock. GND – Ground.
TOUT – Thermostat Output Signal. A0 – Chip Address Input. A1 – Chip Address Input. A2 – Chip Address Input. VDD – Power Supply Voltage.
Working of DS1621:. When the temperature of the device exceeds a user-defined temperature HIGH then the output TOUT is active. The output will remains active until the temperature drops below user defined temperature LOW. User defined temperature settings are saved in nonvolatile memory so it may be programmed prior to insertion in a system.
The temperature reading is provided in a 9-bit, two’s complement reading by issuing the READ TEMPERATURE command in the programming. A 2 wire serial interface is used for input to the DS16121 for the temperature settings and for output of temperature reading from the DS1621 Photo Credit:. Temperature Sensor.