Temperature Range: The approximate overall range of temperatures in which a sensor type can be used. Within a given temperature range, some sensors work better than others.
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Relative Cost: Relative cost as these sensors are compared to one another. For example, thermistors are inexpensive in relation to RTDs, partly because the material of choice for RTDs is platinum.
Time Constant: Approximate time required to change from one temperature value to another. This is the time, in seconds, that a thermistor takes to reach 63.2% of the temperature difference from the initial reading to the final one.
Stability: The ability of a controller to maintain a constant temperature based on the sensor's temperature feedback.
Sensitivity: The degree of response to a change in temperature.
Thermistors come in a variety of shapes-disk, chip, bead, or rod, and can be surface mounted or embedded in a system. They can be encapsulated in epoxy resin, glass, baked-on phenolic, or painted. The best shape often depends on what material is being monitored, such as a solid, liquid, or gas.
For example, a bead thermistor is ideal for embedding into a device, while a rod, disk, or cylindrical head are best for optical surfaces. A thermistor chip is normally mounted on a printed circuit board (PCB). There are many, many different shapes of thermistors and some examples are:
Figure 3: Thermistor Types
Choose a shape that allows maximum surface contact with the device whose temperature is being monitored. Regardless of the type of thermistor, the connection to the monitored device must be made using a highly thermally conductive paste or epoxy glue. It is usually important that this paste or glue is not electrically conductive.
The main use of a thermistor is to measure the temperature of a device. In a temperature controlled system, the thermistor is a small but important piece of a larger system. A temperature controller monitors the temperature of the thermistor. It then tells a heater or cooler when to turn on or off to maintain the temperature of the sensor.
In the diagram below, illustrating an example system, there are three main components used to regulate the temperature of a device: the temperature sensor, the temperature controller, and the Peltier device (labeled here as a TEC, or thermoelectric cooler). The sensor head is attached to the cooling plate that needs to maintain a specific temperature to cool the device, and the wires are attached to the temperature controller. The temperature controller is also electronically connected to the Peltier device, which heats and cools the target device. The heatsink is attached to the Peltier device to help with heat dissipation.
Figure 4: Thermistor Controlled System
The job of the temperature sensor is to send the temperature feedback to the temperature controller. The sensor has a small amount of current running through it, called bias current, which is sent by the temperature controller. The controller can't read resistance, so it must convert resistance changes to voltage changes by using a current source to apply a bias current across the thermistor to produce a control voltage.
The temperature controller is the brains of this operation. It takes the sensor information, compares it to what the unit to be cooled needs (called the setpoint), and adjusts the current through the Peltier device to change the temperature to match the setpoint.
The location of the thermistor in the system affects both the stability and the accuracy of the control system. For best stability, the thermistor needs to be placed as close to the thermoelectric or resistive heater as possible. For best accuracy, the thermistor needs to be located close to the device requiring temperature control. Ideally, the thermistor is embedded in the device, but it can also be attached using thermally conductive paste or glue. Even if a device is embedded, air gaps should be eliminated using thermal paste or glue.
The figure below shows two thermistors, one attached directly to the device and one remote, or distant from the device. If the sensor is too far away from the device, thermal lag time significantly reduces the accuracy of the temperature measurement, while placing the thermistor too far from the Peltier device reduces the stability.
Figure 5: Thermistor Placement
In the following figure, the graph illustrates the difference in temperature readings taken by both thermistors. The thermistor attached to the device reacted quickly to the change in thermal load and recorded accurate temperatures. The remote thermistor also reacted but not quite as quickly. More importantly, the readings are off by a little more than half a degree. This difference can be very significant when accurate temperatures are required.
Figure 6: Thermistor Location Response Graph
Once the placement of the sensor has been chosen, then the rest of the system needs to be configured. This includes determining the base thermistor resistance, the bias current for the sensor, and the setpoint temperature of the load on the temperature controller.
Thermistors are categorized by the amount of resistance measured at ambient room temperature, which is considered 25°C. The device whose temperature needs to be maintained has certain technical specifications for optimum use, as determined by the manufacturer. These must be identified before selecting a sensor. Therefore, it is important to know the following:
What are the maximum and minimum temperatures for the device?
Thermistors are ideal when measuring a single point temperature that is within 50°C of ambient. If the temperatures are excessively high or low, a thermistor will not work. While there are exceptions, most thermistors work best in the range between -55°C and +114°C.
Since thermistors are nonlinear, meaning the temperature to resistance values plot on a graph as a curve rather than a straight line, very high or very low temperatures do not get recorded correctly. For example, very small changes in very high temperatures will record negligible resistance changes, which won't translate into accurate voltage changes.
What is the optimum thermistor range?
Depending on the bias current from the controller, each thermistor has an optimum useful range, meaning the temperature range where small changes in temperature are accurately recorded.
The table below shows the most effective temperature ranges for Wavelength thermistors at the two most common bias currents.
Figure 7: Thermistor Selection Chart
It is best to choose a thermistor where the setpoint temperature is in the middle of the range. The sensitivity of the thermistor is dependent on the temperature. For example, a thermistor may be more sensitive at cooler temperatures than at warmer temperatures, as is the case with Wavelength's TCS10K5 10 kΩ thermistor. With the TCS10K5, the sensitivity is 162 mV per degree Celsius between 0°C and 1°C, and it is 43 mV / °C between 25°C and 26°C, and 14 mV °C between 49°C and 50°C.
What are the upper and lower voltage limits of the sensor input of the temperature controller?
The voltage limits of the sensor feedback to a temperature controller are specified by the manufacturer. The ideal is to select a thermistor and bias current combination that produces a voltage inside the range allowed by the temperature controller.
Voltage is related to resistance by Ohm's Law. This equation is used to determine what bias current is needed. Ohm's Law states that the current through a conductor between two points is directly proportional to the potential difference across the two points and, for this bias current, is written as:
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V = IBIAS x R
Where:
V is voltage, in Volts (V)
IBIAS is the current, in Amperes or Amps (A)
IBIAS means the current is fixed
R is resistance, in Ohms (Ω)
The controller produces a bias current to convert the thermistor resistance to a measurable voltage. The controller will only accept a certain range of voltage. For example, if a controller range is 0 to 5 V, the thermistor voltage needs to be no lower than 0.25 V so that low end electrical noise does not interfere with the reading, and not higher than 5 V in order to be read.
Assume the use of the above controller and a 100 kΩ thermistor, such as Wavelength's TCS651, and the temperature the device needs to maintain is 20°C. According to the TCS651 datasheet, the resistance is Ω at 20°C. To determine if the thermistor can work with the controller, we need to know the usable range of bias currents. Using Ohm's Law to solve for IBIAS, we know the following:
V / R = IBIAS
0.25 / = 2 µA is the lowest end of the range
5.0 / = 39.5 µA is the highest end
Yes, this thermistor will work, if the temperature controller bias current can be set between 2 µA and 39.5 µA.
When selecting a thermistor and bias current, it is best to choose one where the voltage developed is in the middle of the range. The controller feedback input needs to be in voltage, which is derived from the thermistor resistance.
Since people relate to temperature most easily, the resistance often needs to be changed to temperature. The most accurate model used to convert thermistor resistance to temperature is called the Steinhart-Hart equation.
The Steinhart-Hart equation is a model that was developed at a time when computers were not ubiquitous and most mathematical calculations were done using slide rules and other mathematical aids, such as transcendental function tables. The equation was developed as a simple method for modeling thermistor temperatures easily and more precisely.
The Steinhart-Hart equation is:
1/T = A + B(lnR) + C(lnR)2 + D(lnR)3 + E(lnR)4'
Where:
T is temperature, in Kelvins (K, Kelvin = Celsius + 273.15)
R is resistance at T, in Ohms (Ω)
A, B, C, D, and E are the Steinhart-Hart coefficients that vary depending on the type of thermistor used and the range of temperature being detected.
ln is Natural Log, or Log to the Napierian base 2.
The terms can go on infinitely but, because the error is so small, the equation is truncated after the cubed term and the squared term is eliminated, so the standard Steinhart-Hart equation used is this:
1/T = A + B(lnR) + C(lnR)3
One of the pleasures of computer programs is that equations that would have taken days, if not weeks, to solve are done in moments. Type 'Steinhart-Hart equation calculator' in any search engine and pages of links to online calculators are returned.
This equation calculates with greater precision the actual resistance of a thermistor as a function of temperature. The more narrow the temperature range, the more accurate the resistance calculation will be. Most thermistor manufacturers provide the A, B, and C coefficients for a typical temperature range.
John S. Steinhart and Stanley R. Hart first developed and published the Steinhart-Hart equation in a paper called 'Calibration curves for thermistors' in , when they were researchers at Carnegie Institution of Washington. Steinhart went on to become Professor of Geology and Geophysics, and Marine Studies at the University of Wisconsin-Madison and Stanley R. Hart became a Senior Scientist at Woods Hole Oceanographic Institution.
Thermistors are temperature-dependent resistors, changing resistance with changes in temperature. They are very sensitive and react to very small changes in temperature. They are best used when a specific temperature needs to be maintained, and when monitoring temperatures within 50°C of ambient.
Thermistors, as part of a temperature control system, are the best way to measure and control heating and cooling of a Peltier device. Their ability to adjust in minute increments allows the greatest overall system stability. Thermistors can be embedded in or surface-mounted on the device needing temperature monitoring. Depending on type, they can measure liquids, gases, or solids.
Wavelength supplies a variety of bead and cylindrical head thermistors. For a listing of the current selection, click here.
-09-04
In the world of electronics, maintaining signal integrity and safety is paramount. Isolation amplifiers play a crucial role in achieving this by separating input and output circuits while allowing the desired signals to pass through. This not only prevents interference and electrical hazards but also ensures accurate amplification, even for weak signals. This article delves into the workings of isolation amplifiers, exploring their components, applications, and benefits. What Is an Isolation Amplifier? An isolation amplifier is a type of amplifier used to send an analog signal across a barrier between the input and output without any direct electrical connection between them. The barrier can be a transformer, an optical barrier or even a capacitive barrier. The main purpose of the isolation amplifier here is to prevent high voltage or current from passing from one section to another, protecting the circuit from danger and preventing interference with the signals. Basic Components of an Isolation Amplifier Below are the key parts of an isolation amplifier: 1. Input Stage The input stage of an isolation amplifier is responsible for receiving the analog signal that needs to be isolated and amplified. This stage typically includes: Signal Input Terminals: These are the points where the analog signal enters the amplifier. Input Protection Circuitry: This ensures that any excessive voltage or current at the input does not damage the amplifier. It may include components like diodes, resistors plus fuses. Buffer Amplifier: A buffer amplifier is used to prevent the input signal from being loaded down by the next stages of the circuit. It ensures that the signal strength is maintained. 2. Isolation Barrier The isolation barrier is the main and core component that provides the electrical isolation between the input and output stages. Different types of these barriers include: Transformer Isolation: It uses a transformer to transfer the signal magnetically. This method provides excellent isolation and is commonly used in power applications. Optical Isolation: This employs optocouplers, where the signal is converted into light on the input side and back to an electrical signal on the output side. This method is effective for high speed signal transmission. Capacitive Isolation: Uses capacitors to transmit the signal while blocking direct current and low frequency noise. This method is often used in digital isolation amplifiers. Also Read: How Optocouplers Work 3. Output Stage The output stage of an isolation amplifier reconstructs and amplifies the isolated signal to the required level. Components in this stage include: Output Buffer Amplifier: This amplifies the isolated signal to usable level. It also serves to match the impedance of the isolation barrier with the output circuitry. Output Terminals: These are the points where the isolated and amplified signal is outputted to the next stage of the system. Output Protection Circuitry: Similar to the output protection, this ensures that the output components and connected devices are protected from excess voltages or currents. 4. Power Supply The power supply for an isolation amplifier is designed to maintain the isolation between the input and output stages. This often involves: Isolated Power Supply: Each side of the isolation barrier which is output and input has its own isolated power supply to prevent any electrical connection that could compromise the isolation. DC-DC Converters: These are commonly used to provide isolated power to different parts of the amplifier. 5. Feedback Mechanism In some isolation amplifiers, a feedback mechanism is used to ensure the accuracy and stability of the signal. This can be implemented through: Optical Feedback: Light signals are used to provide feedback across the isolation barrier. Magnetic Feedback: Uses magnetic fields to provide feedback while maintaining isolation. How Isolation Amplifiers Work Now that we understand the basic components of isolation amplifiers, let’s see how they work. Source: https://www.watelectronics.com The diagram above is a perfect illustration of an isolation amplifier. It operates in the following steps: Input Signal Acquisition: The input signal connects to the isolation amplifier. The input stage of the amplifier is designed to have a high input impedance. This means it resists the flow of current so it only draws a very small amount of current from the source of the input. Because of this, it doesn’t strain the signal source. This feature is important because it ensures the amplifier doesnt interfere with or change the original signal. In simpler terms, it allows the amplifier to listen to the signal without disturbing it keeping the signal’s quality intact. Signal Conversion Once an input signal has been received and safeguarded from outside interference, further processing is required so that it can travel across the isolation. This next step requires transforming the signal to a form that can be “transferred” through the barrier without a direct electrical connection. To achieve this, the signal goes through specific converters. One of the most commonly employed methods is a voltage-to-frequency converter. In this process, the input voltage is converted into a frequency signal, whereby the frequency to the output signal is proportional to the input voltage level. This frequency can then be easily transmitted across the isolation barrier, usually using an optical or a transformer-based system. Another method is a current-to-voltage converter. Here, the input signal is normally in the form of a current and this is converted to a voltage output. This conversion helps in passing the signal across the barrier without compromising the accuracy and integrity of the information. Signal Transmission: The converted signal proceeds to the output stage by crossing the isolation barrier. Transmission can occur in a number of forms, such as optical coupling, where the signal is converted to light, magnetic coupling where the signal is transmitted through a magnetic field or capacitive coupling involving capacitors. These methods help to guarantee that the signal is free from any kind of electrical interference. Signal Reconstruction: At the output stage, the signal is reconstructed. This is often done with a frequency-to-voltage converter, which changes the frequency signal back into a voltage or with a voltage to current converter which converts a signal into a current. These conversions bring the signal to be used with no further degradation. Output Signal Amplification: Now that the signal is reconstructed, it is then amplified by the output stage of the isolation amplifier. This stage is designed with low output impedance allowing it to drive a load effectively. Low output impedance ensures that the amplified signal can be delivered to the connected device or circuit without losing quality or being affected by the load. To better understand this, here’s an example. Let’s consider an isolation amplifier with a gain of 100. We want to amplify a signal with a peak-to-peak voltage of 1 V. The output will be: Vout = Gain x Vin = 100 x 1 = 100 V So, the output voltage will be 100 V. Now, let’s say we want to calculate the common-mode rejection ratio (CMRR) of the isolation amplifier. The CMRR is the ratio of the differential gain to the common-mode gain. The differential gain is the gain of the amplifier when the two input signals are different. The common-gain is the gain of the amplifier when the two input signals are the same. The CMRR can be calculated using the following formula: CMRR =20 x log10 (Ad/ Ac) Where: Ac represents the common mode gain Ad represents the differential gain Let’s say we have an isolation amplifier with a differential gain of 100 and a common gain of 0.01. The CMRR will be: CMRR = 20 x log10 (100/0.01) = 80 dB So, the CMRR of the isolation amplifier is 80 dB Applications of Isolation Amplifiers Isolation amplifiers are useful in many different areas where it’s important to keep signals clean and protect people or equipment. Here’s how they are commonly used: Medical Equipment: In devices like ECG and EEG machines, isolation amplifiers keep patients safe by separating them from the electronics that monitor their vital signs. This prevents any electrical shocks and makes sure that the readings are accurate, without any interference from other electronic devices nearby. Industrial Automation: In factories, there are often lots of machines and electrical equipment that can create noise and interference. Isolation amplifiers help by separating and simplifying signals from sensors and control systems, so the information remains accurate even in these challenging environments. Power Supply Monitoring: When monitoring high voltage power supplies, it’s important to do so safely. Isolation amplifiers allow for the measurement of voltage and current without exposing the measuring instruments or people to dangerous high voltages, keeping everything safe and the data reliable. Data Acquisition Systems In identification research and testing, data needs to be collected accurately from various sensors. Isolation amplifiers help by ensuring that the signals from these sensors are clean and free from interference providing precise data for analysis. Benefits of Using Isolation Amplifiers Below are the main benefits of isolation amplifiers: Improved data accuracy Enhanced equipment lifespan Reliable communication network protection Patient safety in medical equipment Reduction of signal interference Limitations of Isolation Amplifiers Isolation amplifiers offer many benefits, but they also have certain limitations. Here are some of the key drawbacks: Higher cost Power consumption Limited isolation voltage FAQs People often have questions about isolation amplifiers. Here are some of the most common ones. What are the key requirements for an isolation amplifier? An isolation amplifier must handle a specific signal voltage range, usually from millivolts to several volts, and support a particular signal frequency range, typically up to Kilohertz. On what principle does an isolation amplifier actually operate? Isolation amplifiers operate on the principle of attenuation, where a high barrier impedance acts in series between the input and output to prevent direct electrical connection and ensure isolation. Conclusion Isolation amplifiers come in handy in order to ensure that input and output circuitry are not interfering with each other while at the same time providing paths for the required signals. By fully eliminating interference, they shield against high voltage dangers. Though they have disadvantages such as a relatively higher cost involved, their merits of precise data transmission and equipment safeguards make them desirable for different uses.
#Isolation Amplifiers
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