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Circuit Design Tips

Using Transistors for Temperature Sensing and Control

Semiconductor temperature sensing, Temperature sensing with transistors, Thermistor transistor circuits, Transistor-based temperature monitoring, Transistors for temperature control, Utilizing transistors for temperature regulation

Thermistors are usually used to check temperatures in electronics. But, when we want to spot changes in heat, transistors and diodes are even better. As the temperature rises, a semiconductor’s forward voltage drops. Making use of this, we set up semiconductor parts to detect heat. They prove to be a great option for feeling small temperature changes. An amazing thing about these sensors is their linear response. Thermistors can’t match this feature.

Key Takeaways

  • Transistors and diodes outperform thermistors for temperature detection and monitoring in electronic circuits.
  • Semiconductor devices’ characteristics are significantly affected by temperature increases.
  • Semiconductor sensors offer more linear responses to temperature variations compared to thermistors.
  • Bipolar transistors (BJTs) can function as efficient temperature transducers when configured as diodes.
  • Metal can-type transistors provide better temperature sensing responses than those in epoxy or plastic casings.

Understanding Temperature-Dependent Characteristics of Transistors

The flow of voltage and the current across a pn junction in a BJT or a diode changes a lot with temperature. This is easy to see with a silicon diode like a 1N400X series or a 1N4148 diode. If you measure the resistance of the diode when it’s warmed, you’ll notice it changes.

Relationship Between Temperature and Semiconductor Properties

A bipolar transistor, or BJT, can work well as a temperature sensor. Especially when set up to act like a diode. The VBE, or base-emitter voltage, of the BJT depends a lot on its temperature and the current it uses. This makes BJTs very good at measuring temperatures from -55°C to +125°C.

Principles of Transistor-Based Temperature Sensing

There are several types of transistors for making temperature estimations. They use the base-emitter voltage and temperature connection. This works well when the collector’s current remains the same. Some BJT transistors work particularly well for temperature readings. Among these, metal can type transistors, such as the TO-5 and TO-18 models, are very efficient. They perform better than those with epoxy or plastic casings.

Base-Emitter Voltage (VBE) as a Temperature Indicator

Many BJT transistors have better linearity in their VBE vs Ic graph. This is compared to other types. For a high level of accuracy, a twin transistor (like MAT01, containing a pair of matched NPN silicon transistors) is used. It ensures precise results. When emitters receive different constant currents, the output is about 59 µV/°K. A differential opamp is then needed to make this into a useful reading. Calibrating the voltage range to 10 mV/°K makes readings easy with a standard voltmeter.

Delta VBE Method for Improved Accuracy

Many BJT transistors offer enhanced linearity in their VBE vs Ic relationship. To ensure accurate results, a MAT01 type twin transistor is employed. It contains two matched NPN silicon transistors in one case. When given 1 mA and 2 mA current, its output voltage is about 59 µV/°K. Again, a differential opamp is used to bring this to a practical level. Setting the voltage range to 10 mV/°K allows simple readings using a regular voltmeter.

Transistor temperature sensing

Using Transistors for Temperature Sensing and Control

Choosing the Right Transistor for Temperature Sensing

The diagram shows a precise temperature transducer. It uses a standard op amp and a 2N2222 BJT transistor. This setup is like a probe for sensing temperatures. It’s essential to have a proper housing for the probe. This could be something like a voltmeter probe grip or a metal tube.

If you’re putting this in a device, the probe might fit inside without needing extra space. But no matter where you put it, making sure it touches the surface well is super important.

Circuit Design Considerations

This design uses two voltage references of +/-6.2 volts. Diode D1 provides the +6.2 V reference, and diode D2 the -6.2 V. The BJT temperature sensor (Q1) connects to the +6.2 V. This setup ensures Q1’s emitter current reacts only to temperature changes. Op amp IC1 then turns this current into a 100 mV/°K signal. R1 is then tuned to adjust calibration.

Calibration and Linearization Techniques

To see temperatures in Celsius, you need to consider the Kelvin and Celsius relationship. They are the same, just with a 273 degrees offset. Potentiometer R3 helps switch the reading from Kelvin to Celsius. It achieves this by adjusting the current feeding back through the -6.2 V supply. By setting this correctly, amplifier IC1 outputs 0 at exactly 0° C.

Advantages of Transistor Temperature Sensors

Cost-Effectiveness and Availability

This design uses a transistor and diode together. The diode stays at the room’s temperature. It measures the voltage drop across it, which sets the reference temperature.

The transistor, T1, measures the actual temperature near a heat source. It does this based on the diode’s reference. The R1 and R2 resistor values vary with the supply voltage, Ub. Simple formulas help find these values.

Wide Operating Temperature Range

To make this design work well, the reference diode must sit in cool, room air. This should be away from the heated area, called T1, it’s monitoring.

Until it gets too hot, T1 stays off. You can set this ‘too hot’ temperature with P1. The transistor’s voltage on its base and emitter drops as it gets hotter. If this voltage drops lower than set by P1, then the transistor switches on. This makes LED D2 light up slowly.

Applications of Transistor Temperature Sensing

Thermal Management in Electronic Systems

Many devices by Linear Technology, like LTC3880, rely on an external PNP transistor for temperature sensing. They include features for accurate temperature readings. This method helps protect the devices. An example is the LTC3880, which stops writing to memory if it gets too hot inside.

Inductors are influenced by the temperature, which can affect the current measurements. To fix this, devices like the LTC3880 use the inductor’s temperature for better current readings.

Industrial Process Control

Linear Technology devices often have a lowpass filter that reduces interference. Yet, sometimes, this can cause a shift in the data. But, using a certain method, this shift can be undone, reducing measurement errors. So, a good design is vital for accurate results.

Automotive Temperature Monitoring

An LTC3880 in Figure 10 shows how signals change with different applied currents. The difference in signals during the high and low points is how the temperature is determined. This setup can show measurement errors due to coupling effects, unless carefully managed.

Noise and Error Sources in Transistor Temperature Sensing

A diode sensor includes a 2N3906 transistor, 10μF capacitor, and both current and voltage sensors. It calculates small signal impedance at 52Ω up to 10MHz. This means, high-speed signals might add noise to the readings if the signals’ strength is near this 52Ω. A simulated source found a filter capacitor cuts down on noise well.

A 2A current in a 10mΔ trace causes a 20mV error. If this occurs during a 10% duty cycle, it could lead to a 2mV DC shift.

Ground Impedance and Noise Coupling

A magnetic field and its loops form a third error type. Think of this as like energy between two inductors. A 3cm PCB trace over ground may have about 10nH inductance. If a trace next to it gets 2A and 1.0% transfers, 30mV noise could arise. This might create a 3mV DC shift.

Linear Technology’s devices often use lowpass filters to cut down on noise. But in some cases, this filtering causes a big DC shift. If noise is much larger than ΔVBE and not the same in each measurement, ΔVBE won’t remove it.

Filtering Techniques for Noise Mitigation

Linear Technology’s products often use lowpass filters to minimize noise. But sometimes, this method leads to a notable DC shift. If the noise level is high compared to ΔVBE, and the noise varies between current readings, ΔVBE can’t offset it.

Practical Implementation and Layout Considerations

Using the right routing and grounding helps a lot in making transistors work for temperature sensing. The Linear Technology data sheets give clear steps. These help guide returning the current from the transistor back to the device. The current often goes back through a sense ground (SGND) or the negative input of an amplifier. This step must be done carefully. The current should follow its own path without mixing with other electrical flows. This reduces errors and follows the instructions in the data sheet closely.

The job of capacitor C99 is key. It makes sure there is a clear path for electrical signals, stopping any harmfully different voltage levels. A detailed example can be seen in Figure 12. Here, a LTC3883 module is shown with a part called a 2N3906 PNP connected. Inside this part, there’s a temperature sensor, marked as Q10. C99 helps keep the sensor’s signal steady by filtering the path it takes.

When adding temperature sensing to a device, you might not need to change its outer design. The transistor can be placed inside the device’s body. It’s important that the sensor touches the spot you want to measure well. This ensures an accurate reading no matter where the sensor is placed inside.

transistor temperature sensor layout

Temperature Compensation Techniques

Software-Based Temperature Compensation

Look at Figure 10 and the LTC3880 example. Signal 1, TSENSE, changes with the LTC3880’s actions. It sends a 32μA for a higher signal and 2μA for a lower signal. This happens at the waveform’s high and low points.

Signal 2, VOUT, is pulled by the 32μA, and sometimes by 2μA too, like in Figure 11. However, not both measurements get affected by this.

This issue appears because one measurement is influenced by external factors. Proper design or setting wider limits is how this is often dealt with.

Hardware-Based Temperature Compensation Circuits

Preventing errors like these involves two main steps. They both deal with how the PCB is designed. The first step is to get rid of any shared paths for the ground. The second is to correctly lay out how the signal traces run.

Linear Technology’s datasheets help with where the current should go. Current goes to a sense ground (SGND) or a minus (-) input. The important part is that it shouldn’t share a path with high current lines. It should return in its own sense trace. This goes back to the device without any sharing of current paths, going to specific pins as the datasheets say.

Emerging Trends and Future Developments

The world of temperature sensing and control is changing fast. Researchers and engineers are finding new ways to solve problems. One exciting breakthrough is the thermal transistor, created at the University of California, Los Angeles. This device can control heat flow precisely, which is a big step forward in how we manage heat.

The thermal transistor can switch heat on and off quickly, at over 1 MHz. It’s also very good at its job, with a thermal conductance ratio over 1300%. This means it’s efficient in controlling how heat moves. Plus, it’s durable, able to switch more than a million times, showing its usefulness in real applications.

Scientists are making these thermal transistors better by changing their designs and materials. Using them in power circuits could lead to computers that work even better and are more reliable. They could be used in electronics, 3D circuit packaging, clean energy systems, making things, and helping with medical treatments. This idea also opens doors to better understand how to manage heat at the smallest levels in nature and in our bodies.

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