More Op-Amp Circuits; Temperature Sensing

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Lab 5

More Op-Amp Circuits;

Temperature Sensing

Overview

In this lab we will continue our exploration of op-amps but this time in the context of a specific application: temperature sensing. One temperature sensor will use a device called a thermistor with a simple op-amp “current source”; the other will use a diode pair and differential amplifier configuration. You will also learn how to operate the oscilloscope in an X-Y mode, which is useful for characterizing the characteristics of many electronic devices and circuits.

Background Information

Thermistors

All metallic conductors have a temperature-dependent resistance. This is not always desirable but can be exploited for temperature sensing applications. Over a small range of temperature the resistance of most conductors follows an approximately linear relationship





0 0

( ) 1 _TCR( )

R T R  T T (5.1)

where R is the resistance at the reference temperature ₀ T , most commonly taken as 25ºC or ₀

0 298.15K

T  . The parameter _TCR quantifies the fractional change in resistance per degree of temperature change, and is called the “temperature coefficient of resistance”, or TCR. For ordinary metallic conductors the temperature coefficient is always positive. For example, copper has _TCR _{ }4.04 10 /K_ 3 _{. Platinum also has a similar TCR but is a better choice for}

simple resistance temperature detectors (RTDs), especially at high temperatures (e.g. above 600ºC) because it is a refractory

metal that does not oxidize easily and its resistance follows (5.1) quite well over a wide range of temperature.

Thermistors have a much greater sensitivity to temperature than platinum RTDs, but the dependence is strongly nonlinear and limited to a smaller operating temperature range, typically from -20ºC to +120ºC. Thermistors are often made from a ceramic semiconducting material, and are available in two types: positive temperature coefficient

(PTC) and negative temperature coefficient (NTC). A simple model for the temperature-dependence of the thermistor resistance for NTC devices is given by

0

/ /

0

( ) T T

R T R e e (5.2)

where R is again the resistance at the reference temperature ₀ T . In this lab we will use an ₀ NTC-type of thermistor. The datasheet provides the appropriate values for R₀ and  . Figure 5-1 illustrates the resistance-temperature dependence predicted by (5.2) for a device similar to the one we will use in our lab.

Diodes as Temperature Sensors

Figure 5-2 illustrates the current-voltage relationship for a simple silicon diode. This is also similar to the devices we used in earlier labs. In forward conduction the diodes are well modeled by

0 qV nkT/

II

e

(5.3)

where q1.6 10 19C, k1.38 10 23J/K, I is the saturation current, and ₀ n is a dimensionless “ideality” factor. Usually measurements are required to determine the Figure 5-1 – Resistance vs. temperature for an NTC thermistor with R₀ 50 k and  4400 K

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saturation current and ideality factor. For two identical devices at the same temperature but biased at different current levels, the ratio of the currents is given by

1 2 1 1 1 2 2 2 ( ) ln q V V nkT I nkT I e V V I q I      (5.4)

So the difference in voltage between two diodes should scale linearly with temperature. The key assumptions in (5.4) are that the diodes have the identical saturation currents and ideality factors, and that they both at exactly the same temperature. The only good way to satisfy these conditions is to use two diodes that are fabricated side-by-side on the same chip, using identical materials and fabrication processes. The device we will use in the lab integrates two identical devices in this fashion.

We will discuss diodes in greater depth in ECE 2B. For now, all you

have to do is accept that the I-V curves are modeled by an exponential relationship like (5.3). The important takeaway here is that the diode characteristics are temperature dependent, and anytime that happens we might try to exploit that device as a temperature sensor. In more advanced coursework like ECE 137 you will learn how circuits can be designed to deal with temperature-dependent effects and continue to function properly over a wide range of temperatures. One particular circuit, commonly called a “bandgap voltage reference”, is a clever example of how circuits can be designed to cancel out temperature effects completely. Stable voltage references are important in many applications such as data acquisition and test instruments. References http://en.wikipedia.org/wiki/Thermistor 0 5 10 15 20 25 30 0.40 0.50 0.60 0.70 0.80 0.90

Diode Voltage V, Volts

C u rr e n t I, m A 300K 280K 320K

Figure 5-2 – Temperature-dependent I-V curves for a representative silicon p-n junction.

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Pre-lab Preparation

Before Coming to the Lab

□ Read through the details of the lab experiment to familiarize yourself with the components and testing sequence.

□ One person from each lab group should obtain a parts kit from the ECE Shop.

Required Equipment

■ Provided in lab: Bench power supply, Function Generator, Oscilloscope, Decade Box ■ Student equipment: Solderless breadboard, and jumper wire kit

Parts List

Qty Description

1 MSD6100G Dual switching diode, TO-92 common cathode 1 NTC Thermis tor - 50K Ohms @25C, -4.7/C

1 20 Ohm 5W, Cement Resistor

1 Large Binder Clips; Clip size 2”; Cap. 1-1/16” 1 100-Ohm 1/4 Watt resistor

1 1.2 k -Ohm 1/4 Watt resistor 1 12 k-Ohm 1/4 Watt resistor 4 100 k-Ohm 1/4 Watt resistor

In-Lab Procedure

5.1 Op-Amp Current-Source; Thermistors

First Step: DC Biasing

The lab uses op-amps again, so remember our advice: always make setting up the bias circuit your first step. This time we will use ±12V bias:

□ Insert the op-amp in your breadboard and add the wires and bypass capacitors as shown in Figure 5-3. To avoid problems later you may want to attach a small label to the breadboard to indicate which rails correspond to

s V

 ,  , and ground. V_s

□ Next, set the power-supply to produce ±12V and attach the supply and COM connections to the terminals on

your breadboard. The use jumper wires to power the rails as shown. Remember, the power-supply COM terminal will be our circuit “ground” reference. Once you have your bias connections, monitor the output current on the power-supply to be sure that there aren’t any inadvertent short-circuits. You may wish to use a DMM to probe the IC pins directly to insure that pin 7 is at +12V and pin 4 is at -12V.

+

-+

-+V_s -V_s _0.1μF 0.1μF 2 3 4 7 6

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A Simple Thermistor Circuit

As described in the background section the thermistor resistance varies with temperature in a predictable way. Our goal is to design a circuit that uses the thermistor to generate a voltage proportional to temperature. One way to do this is to drive the thermistor with a known constant current, and adjust the magnitude of this current so that the voltage varies in a prescribed manner over temperature. Figure 5-4 illustrates one way of doing this. The current I is set by resistor R and the voltage ₁  , which Vs is most conveniently taken as the negative

supply voltage, -12V. In this case the output voltage at each temperature will be

1 ( ) ( ) 12 T out R T V T R  (5.5)

The circuit resembles an inverting amplifier, but when used in this fashion it is essentially a voltage-controlled current source, since the current through the “load” (the thermistor) is a constant, independent of the value of R_T, and set by the input voltage  . V_s

The next step is to choose the range of output voltage. In a real application the output voltage of the sensor is often fed into a digital panel meter or other data acquisition system, in which case there might be some constraints on the allowable voltages. But in this lab we can choose the voltage range to be anything we like. We will be content to simply monitor the voltage with our DMM and perform the mathematical conversion between voltage and temperature by hand.

Let’s design the circuit to produce V_out 1V when the thermistor is at room temperature (T 25 C ). In most cases the thermistors are specified to have a nominal resistance R at ₀ this temperature; you can look this up in the data sheet or just measure it manually using your ohmmeter:

□ Find or measure the room-temperature resistance of your thermistor, and with the power-supply disconnected or turned off, construct the circuit shown in Figure 5-4. Use the bench decade box for R₁.

□ Using (5.5), calculate the approximate value of R needed to make ₁ V_out1V at room temperature, and set your decade box to this value.

□ Turn on power and fine-tune the value of R to make ₁ V_out 1V.

□ Now pinch the thermistor between your thumb and finger and wait until the output voltage reaches a steady value. Record this number. From (5.5) you can estimate the thermistor resistance and hence its temperature. You can use a plot like that of Figure 5-1 or the figure in the datasheet to compute the temperature, or alternatively compute it from (5.2) which can be inverted to give

ln T T R         (5.6)

+

-V_out -V_s R_T R₁ LM741 Thermistor I

+

-+

-V_out -V_s R_T R₁ LM741 Thermistor I

Figure 5-4 – Thermistor driven by a constant-current as set by R . 1

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Testing with a Resistive Heater

Next we will create a very simple heat source using a power resistor. Such resistors are designed to dissipate more power than the conventional ¼ Watt resistors that we normally use, but as with all resistors that power is dissipated as heat. Many commercial heaters operate on similar principles: water heaters, space heaters for small rooms, toaster ovens, etc. all use resistive filaments (usually a nickel-chrome alloy) as the heat source. So the idea here is to use the power resistor to create a programmable heat source, and put the thermistor in contact with it so that we can sense how hot the resistor becomes. The power resistor that we have chosen has a large rectangular shape which gives us a nice, flat, “hot” surface:

Power resistor (heat source)

+

-V_out -V_s R_T R₁ LM741 20Ω/5W +V1 Thermistor and power resistor in thermal contact Power resistor (heat source)

+

-+

-V_out -V_s R_T R₁ LM741 20Ω/5W +V1 Thermistor and power resistor in thermal contact

Figure 5-5 – Using a power resistor as a heat source for temperature sensing.

□ Construct the circuit shown in Figure 5-5, with the power resistor and thermistor clamped together using the large binding clip that is included in your kit. Use the remaining independently adjustable output on your power supply for V . Use jumper wires as ₁ needed for the electrical connections, but take care not to allow the thermistor leads to electrically touch the power-resistor leads. Those components should be thermally connected, but not electrically connected.

□ The 20Ω power resistor is rated for maximum power dissipation of 5W. What value of supply voltage will result in exactly 5W dissipation? Turn up supply V to the voltage ₁ you just calculated and monitor the ammeter on the power supply to verify that the current drawn is approximately 0.5A.

□ Allow about 3 minutes for the temperature to stabilize, and then record the output voltage out

V . Since the heat flow from the power resistor to its surroundings is not managed well its temperature may not be completely stable; if so, just note the fact in your lab notebook and record the average value at the 3-minute mark. Again use the charts or (5.6) to estimate the temperature of the power resistor.

□ Turn V back to 0V and allow the power resistor to cool. Monitor ₁ Vout to see when it has returned to 25C (V_out=1.00V). Fanning or blowing on the device may speed this up.

Oscilloscope X-Y Mode

So far in ECE 2 we have used the oscilloscope in a time-base or “Y-T” mode for displaying the input signals as functions of time. But most oscilloscopes can also be configured to display one signal as a function of another. This is called the X-Y mode, and is useful for studying the current-voltage (I-V) characteristics of devices, as well as phase relationships of

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sinusoidal signals. We will use this mode to display the current-voltage characteristic of the thermistor.

□ The desired input signal is a triangle wave at 1 kHz, with a 10V amplitude (peak-to-peak voltage of 20V). To verify and/or change the scope's display mode, press the "DISPLAY" button, then look at the "Format" field. The two possibilities are YT (time base), or XY. For now we want YT.

□ Disconnect R from the -12V supply ₁ and reconnect it to the output of the function generator, as indicated in Figure 5-6. Monitor V_in using channel 2 of the scope, and monitor the op-amp output V_out with channel 1. It should be obvious by now that Ch1 will represent the voltage across the thermistor. Remember that this is an inverting amplifier configuration, so the relationship between thermistor current and input voltage is I V_in /R₁. To

obtain a non-inverted measurement of the thermistor current we cannot simply reverse the black and red clip leads from channel 2 of the scope, since the black lead is connected to ground inside the scope (remember Lab #3!). We can, however, invert the measured signal using the scope's invert function. On the Ch2 menu screen, press the Invert button to obtain “Invert On”. The two traces, Ch1 and Ch2, should now be in phase.

□ As a convenience in reading the thermistor current, change the decade resistor to 100kΩ. This will make the Channel 2 voltage exactly 100,000 times the thermistor current. □ Verify that both traces are clean triangle waves with reasonable amplitudes. Adjust the

two traces on the scope so that they are both centered vertically, i.e., their zero levels are both exactly on the middle horizontal gridline. Now change to XY mode, using the DISPLAY menu. In this mode you can still adjust the Volts/Div sensitivity of each channel. You can also adjust the offsets, but be careful if you do, since that moves the (0,0) position away from the center of the display. Channel 1 is the horizontal variable (X), and Channel 2 is the vertical variable (Y).

□ Sketch the trace from the scope in your notebook. For the vertical scale, indicate both the actual volts/div for channel 2, and the corresponding current in the thermistor (V/100k) □ A resistor has a linear I-V characteristic with slope equal to (1/R). What resistance do

you calculate from your display for the thermistor?

□ Turn the voltage V back up to 10V and watch the slope of the XY trace change as the ₁ power resistor heats up. What resistance value do you calculate after the temperature has settled? Turn V₁ back to zero when you are finished.

5.2 Differential Measurements; Diode Sensors

The second method for measuring temperature will take advantage of the inherent

Power resistor (heat source)

+

-V_out R_T R₁ LM741 20Ω/5W +V1 Thermistor and power resistor in thermal contact Vin Power resistor (heat source)

+

-+

-V_out R_T R₁ LM741 20Ω/5W +V1 Thermistor and power resistor in thermal contact Vin

Figure 5-6 – Same circuit as in Figure 5-5, but with a function generator as the stimulus for current in R . ₁

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circuit. The diode pair device chosen for this lab is the MSD6100 shown in Figure 5-7. The two devices are fabricated on the same chip and hence are very well matched in terms of their I-V characteristics.

□ Leaving the ±12V power supply connections to the op amp intact, and verifying that V₁ is still set to 0, modify the circuit to that of Figure 5-8. Couple the MSD6100 to the power resistor as shown, similar to the method we used with the thermistor. Notice that the two resistors connecting the diodes to the +12V supply are different by a factor of 10. This will make the currents in the two diodes different by about the same factor. Measure the two resistors using your ohmmeter prior to inserting them in the circuit.

□ Turn on power and make sure V is set to 0V. Record the voltage drops across the diode ₁ biasing resistors (the 1.2k and 12k resistors). From this information and the actual measured resistances, determine the current flowing in each diode.

+

-+

-

V_out LM741 20Ω 5W 100 kΩ +V₁ 100 kΩ 100 kΩ 100 kΩ 1.2kΩ 12kΩ +12V _{Differential Amplifier} MSD6100 thermally coupled to power resistor

Figure 5-8 – Temperature sensor using diode-pair and differential amplifier.

□ Now measure V_out with the DMM. The op-amp circuit is a simple differential amplifier with a nominal differential gain of one, so the output voltage should be approximately the same as the difference in the voltage drops across the two diodes. You should use your DMM to confirm this by recording the diode voltage drops directly. The op-amp circuit is not perfect, however, owing to the tolerances in the resistor values and the internal offset voltage of the op-amp.

□ Now turn up V to 10V. Again allow 3 minutes for the temperature to settle. What is ₁ Vo? Using the information in the background section to calculate the temperature of the diodes and power resistor. For the MSD6100, the information in the datasheet is consistent with an ideality factor of 1.75.

Congratulations!

You have now completed Lab 5

As noted in the previous lab: keep all your leftover electrical components!

Figure 5-7 – MSD6100 integrated diode pair in a TO-92 package.

More Op-Amp Circuits; Temperature Sensing

Lab 5