ECET-402 Lab1b

24 Jun ECET-402 Lab1b

Objective:
The objective of this exercise is to use a temperature sensor to turn an output device ON or OFF at a given temperature.

 

Parts Needed:

1.      Multisim Software

2.      2.   1-kW thermistor, [RSR-21T1K] [may be labeled as TTC102]

3.      741 Op-amp

4.      Red and Green LEDs

5.      5.   (2) 5.1 kW, (2) 470 W, (1) 1 kW Resistors

6.   Wires, wire cutter, breadboard

7.    Dual adjustable power supply

8.   10 W, 10W resistor as a heat source (optional)

 

Procedure:

 

Part A: Simulation using Multisim software:

 

Using an RTD sensor, design a circuit that would turn a heater ON whenever the temperature drops below 68° F.

1.      A Resistance Temperature Detector (RTD) is a sensor that its resistance increases as the temperature rises. We will learn more about RTDs in Chapter 6, but for time being let’s select one from the list of available RTD sensors in Multisim.

 

 

a)      Open the Multisim software. From the components toolbar, select:
Misc ®  TRANSDUCERS  ®  HEL-700-T0A.  Place this component on the worksheet area by clicking OK. The following symbol will appear on screen:

 

Note that by pressing the “T” key, temperature will increase in increments of 5° C. Pressing the Shift and “T” keys together will decrease the temperature in 5° C steps. Double click on the symbol and change the “Increment Value” from 5 to 1. Now, you may have to use a different key to increment or decrement the temperature! Construct the following circuit:

b)      Determine the resistance of the RTD for a range of temperatures from 0 to 30° C and complete Table 1 on the worksheet.

 

c)      Graph the temperature-resistance characteristics of the RTD on Figure 1 provided on the worksheet.

 

2. Op-amp comparator circuits have been discussed in earlier courses. We will briefly review some of their properties.

 

a) From the menu on the components toolbar, select

 

b) Wire up the Op-amp as shown below (X₁ is a red probe):

 

3.      We are now ready to design our comparator circuit. We were asked to turn the heater on anytime the temperature dropped below 68° F. The red probe will represent the heater. It will be ON For temperatures below 68° F and it should turn OFF for temperatures 68° F or above.

 

 

a)      Since our RTD sensor uses Celsius degrees, we need to convert 68° F to Celsius.

 

b)Next we will build the following voltage divider circuit using a 5 V DC source:

 

 

c)      From Table 1 on the worksheet, determine the resistance of RTD at 20° C.

 

d)We decided to have a reference voltage of 2V across the RTD sensor at 20° C. Using the Ohm’s law, determine the current in the voltage divider circuit

 

 

e)     If we would like to have 2 V across the RTD at 20° C, then what would be the voltage across R₁?

f)   Knowing the voltage across R₁, and the current found in part (d), we can use Ohm’s law to determine

 

4)  Now let’s put everything together and build the following circuit in Multisim:

 

Paste a copy of your Multisim circuit on the worksheet.  At 20° C, the voltage across the RDT will be 2 volts. Therefore the red probe will be ON. As temperature increases above 20° C, the RTD resistance will increase. Therefore the voltage applied to the inverting terminal of op-amp will be more than 2 volts. As we mentioned earlier, anytime the voltage on the inverting input of the op-amp is more than the voltage on the non-inverting input, the probe will be OFF.

 

5.      Lower the temperature below 20°C and notice that the red probe stays ON. Increase the temperature above 20 and the red probe should be OFF. If that is not the case, adjust R₁ slightly to achieve the goal. What is the adjusted value of R₁?      R₁ =

 

 

6.      Repeat steps 4 and 5 above for an air-conditioner to be turned ON anytime the temperature is 68° F or higher. Use a green probe to represent the air-conditioner.  Describe the process on the worksheet.

 

Part B: Hardware Lab Experiment:

 

In this part we will use a thermistor as a temperature sensor to build a simple circuit so that a green LED is ON when the temperature is 77° F or less, and a red LED is on for temperatures above 77° F.

Note: The reason for selecting 77 ° F is that it translates into 25° C and the resistance of thermistors is usually given at this temperature on the datasheets. Otherwise we need to use Eq. 1 given below to approximate the resistance of the thermistor for other temperatures.

 

Pre-lab:

 

We will use a 1 kW thermistor included in your parts kit for this experiment. Thermistors are not generally used to measure temperature but rather to detect changes in temperature. They do not display the same accuracy as other temperature sensors. The temperature-resistance characteristics data of a thermistor are not linear. Unlike RTDs the resistance of thermistors decreases as temperature rises.

 

Thermistors are rated by the value of their resistance at 25° C. For example, your thermistor (which may be labeled as TTC102) is a 1-kW thermistor.

 

The following formulas can be used to determine the resistance and temperature of a thermistor with good approximation.

 

Eq. 1

Eq.2

 

In the above equations, and must be in °K, [°K = °C + 273.15]; and  are in Ohms.

is generally 25° C or 298.15° K

is the temperature in °K

is the resistance of thermistor at 25° C (provided on datasheets)

is the resistance of thermistor at temperature

is the Beta value (provided by the manufacturer on the datasheet)

 

The datasheet for the thermistor used in this experiment may have the following information

 

Lab experiment:

 

 

Build the following circuit on a breadboard.

 

In above circuit, R₁ and R₂ are used as a voltage divider. The voltage across R₂ is a constant 2V, used as a reference at the inverting input of the op-amp. We could have used an external 2V source.

 

If room temperature is less than 77° F, the green LED will be on. Hold the thermistor between your thumb and index finger. You may generate enough heat to turn the red LED on. If that didn’t work, you may use a 10 W, 10W resistor as a source of heat. Apply 5 VDC across the resistor and place the thermistor very close to it. Be careful not to touch the resistor, since it could get very hot. As the temperature increases to around 77° F, the green LED is turned off and the red LED is turned on. Of course if the room temperature in your lab is more than 77° F, the red LED will be on unless somehow you cool off the thermistor.

 

 

Complete the worksheet provided in the iLab tab and submit to week 1 dropbox.