Figure 1. Typical laboratory setup for the lamp dimmer lab.

1 Introduction

Triacs and Thyristors are common devices used to control the power flow of a circuit, usually operating at a system frequency of 50 to 60 Hertz. In this lab you will gain an appreciation of the operation of a triac by doing some experiments on a residential lamp dimmer. You will discover how a triacs-firing angle affects an incandescent light bulb load by analyzing the voltage and current waveforms. You will also observe how the firing angle effects power flow, PF, DPF, CDF, total harmonic distortion, and the light bulb itself. You will also look at the gate control circuit, which determines at what firing angle the triac will operate at.

Below is a summary of what you will be working on during this laboratory.

Equipment Familiarization:

  • Fluke 43B setup.
  • Light Bulb
  • Lamp Dimmer

Circuit 1: Lamp Dimmer Circuit

Demonstrate and record the operation of a typical household lamp dimmer at different firing angles using a standard 100 watt incandescent light bulb as a load.

1.1 Equipment

1.2 Safety

  • Please watch the Safety Video before attending your lab session.

  • Remember the voltages (300VDC and 208VAC) that you are working with can cause serious harm to you if not respected. Please be careful with your hands and fingers around the circuits to avoid electrocution. If you run into any problems during your experiment disconnect power immediately.

  • Do not leave banana leads connected at only one end of the circuit with the other end floating around. This free end of the cable can potentially have voltage on it and create a dangerous hazard.

  • While working with these voltages that can possibly be exposed it is a good idea to remove any metal watches, rings, bracelets etc…

  • Understand the ratings and limitations of the equipment you are operating. Monitor your circuits closely and try to operate the equipment within specifications at all times.

  • The capacitor in the Diode Rectifier Box has a resistor in series with it. The resistors purpose is to limit the destructive inrush current that is associated with the charging of a capacitor. Therefore the switch must be in an open state to allow the current to be limited by the resistor at start-up. After the capacitor is charged the resistor must be shorted via the switch because the resistor is unable to handle the continuous power flow. Leaving the resistor in will cause errors in your results because you’re limiting the current available to the capacitor. Failure to do either step will damage the equipment!!!

  • Do not make changes to the circuit while power is applied. This doesn’t include changing load switches or moving your voltage/current probes around to make different measurements.

  • Make sure that all large capacitors are discharged before making changes to your circuit. Mainly this is the DC-Link Capacitor that needs to have a significant load on it to make sure it discharges in a reasonable amount of time.

  • Please keep your work area tidy while working on experiments.

  • Always have an Instructor or TA check your circuit changes before you apply power.

2 Pre-lab

Each student must complete a pre-lab to hand in at the beginning of your laboratory section. You must have completed all actions of the pre-lab before being allowed to participate in the lab. The laboratory is usually completed in pairs, so please try and find a partner in the same lab section as you. See the laboratory schedule to make sure you show up to the correct time and place.

2.1 Pre-lab Reading

Familiarize yourself with the lab equipment, procedures, documents and results sheet.

  1. Look at equipment pages to familiarize yourself with the equipment listed below that is used in Lab 3. Note that there are links to the lab equipment pages available.

  2. Read over the entire lab manual so you understand what you will be undertaking during the lab.

  3. Look at the provided ECE401 - Lab 3 - Results sheet to see what you will be recording as results during the laboratory.

  4. Print off a ECE401 - Lab 3 - Sign-off sheet . You only need one per group.

2.2 Pre-lab Questions

figure 2. Typical lamp dimmer circuit.

  1. Calculate the conduction time (tCOND) for firing angles 30°, 60°, 90°, 120°, 150° for each half-cycle for the circuit above. This needs to be done because the meter used in the lab doesn’t measure angles but time. Record your results in the appropriate row in the results sheet.

  2. Using the lamp dimmer circuit above and neglecting losses calculate the values to complete the “Calculations” section of the results sheet. On a separate sheet of paper do all of the calculation showing all of your work for α = 60° to hand in as your pre-lab.

3 Experimental Procedure

Danger!

The power used in this lab can cause severe electrocution that can lead to serious injury or even death. So please follow instruction carefully and be cautious with your experiments.

Have an Instructor or TA verify your circuit connections before you apply power!!!

3.1 Equipment Familiarization

It is good practice when working with equipment that has a danger associated with it that you familiarize yourself with the use, function and safety precautions necessary to operate the equipment in a manner which will keep the equipment in good working order and the user safe.

3.1.1 Fluke 43B setup

Setup the Fluke 43B and the current probe in the same manner that was done in the previous labs.

Figure 3. Current Probe, Fluke 43B Power Quality Analyser, Test Leads and the Power Adaptor for the Fluke 43B.

  1. Take the Fluke 43B out of the case and connect the following:

    1. The power supply to the device and an electrical outlet.

    2. The test leads with the 4mm probe tips to channel 1. Note that during the lab you may need to change the probe tips to the blade style in order to measure VOUT using the splitters recepticle.

      figure 4. Blade style probe tips

    3. The current probe to channel 2 using the supplied BNC adaptor.

  2. Turn on the Fluke 43B and configure the current probe by:

    1. Turn on the current probe to 100mV/A and make sure the Fluke 43B is setup to use the same by going to ‘Instrument Setup/Probes’ in the ‘Main Menu’.

      Note

      Note that the current probe setting used for this lab is now 100mV/A instead of 10mV/A that was used in the first 2 labs. This is because the maximum current in this lab does not exceed 10 Amps and this setting will allow us to increase the accurancy of our measurements.

      figure 5. Current probe set to 100mV/A.

    2. Zero the current probe using the dial on the current probe and also note the arrow on the current probe to specify the currents polarity.

  3. Set the function preferences as follows:

    1. Harmonics (%f) (DC .. 21).

      Note

      During the experiment you will need to change this setting back and forth between %f (for fundamental) and %r (for RMS) to obtain both of the readings.

      \[I_H=\sqrt{I_2^2+I_3^2+I_4^2+I_5^2 \cdots}\]

      \[THD_{\%f}=\frac{I_H}{I_1}\]

      \[THD_{\%r}=\frac{=I_H}{I_{RMS}}\]

    2. Power (Full)

3.1.2 Light Bulb

3.1.2.1 Light Bulb Resistance Measurement

  1. Using the fluke 43B as an ohmmeter measure the resistance across the plug of the light bulb to obtain the “cool” resistance of the light bulb. Record your measurement on the results sheet.

    Note

    Be careful when making measurements like this as you can easily include the resistance of your hands/body if your skin is in contact with any of the metal parts of the circuit.

Figure 6. Probe positions while measuring the light bulbs resistance.

3.1.2.2 Light Bulb Full-power Measurements

Danger

The light bulb is both very bright and can get very hot while operating at full power. Please be cautious while doing your experiment as you can burn yourself or melt plastic on the hot bulb!

  1. Plug the light bulbs cord directly into the receptacle on the 1ph Supply Box and measure the voltage across and the current through the light bulb. Record these measurements as well as all the others that are required to complete the appropriate column (0° = full power) on the results sheet.

    1. To measure VS and associated values insert the probes into the black (hot) and white (neutral) safety banana jacks on the 1ph Supply Box.

    2. To measure the current use the current clamp on only one wire near the light bulb socket where the wires are split.

      Figure 7. Location for the current measurment.

    3. Note that the input and output are equivalent for the full power measurement.

Note

The phase angle between the voltage and current is nearly zero during these measurements demonstrating that the light bulb is a purely resistive element. Also note that if I take my measured voltage and divide it by my measured current I do not get the resistance of the bulb I measured earlier.

3.1.3 Lamp Dimmer

Figure 8. The lamp dimmer.

3.1.3.1 Lamp Dimmer Control Circuit

The circuit below is a simplified schematic overview of the circuit that you will be using during the Lamp Dimmer Circuit experiment. It includes the lamp dimmers protection (the fuse), the triac with its gate control circuit and the LC filter components.

The Triac is the power control element, triggered via the diac. The setting of potentiometer determines the phase difference between the mains sine wave and the voltage across the trigger capacitor that is in series with the potentiometer. This in turn sets the triac triggering angle and the lamp intensity. The resistance of the diac is very high as long as the voltage across it remains within its break-over voltage limits, -VBO to +VBO. Each half cycle of the mains charges the capacitor via the charge resistors until the voltage being applied to the diac reaches one of its break-over levels. The diac then conducts and the capacitor discharges into the gate of the triac, switching it on. The triac will automatically turn off when the current through it goes to zero during the zero crossing event of the sinewave. Then the same thing happens again for the negative cycle.

Circuit 1(s). Lamp dimmer control circuit schematic.
Click here to see a simulation demo of this circuit.

3.2 Lamp Dimmer Circuit

  1. Connect the lamp dimmer circuit which is shown in Circuit 1.

    1. Use the 1ph supply box as the source to power the light bulb. Remember to first plug the 1ph Supply Box into a receptacle at the bench and then use the light switch available on the box to turn the 1ph supply available on the recepticle and safety banana jacks ON and OFF.

    2. Use the lamp dimmer in series with the light bulb so it can control the power flow to the light bulb. The power is controlled by moving the slider on the lamp dimmer where one extreme position turns it off and the other opposite extreme position applies something close to full power.

    3. Use the splitter so a voltage measurement can be made across the load (light bulb) while the light bulb is plugged in to the circuit.

    4. Use the 100 watt incandescent light bulb as a load for this circuit. The brightness of the light coming from the light bulb will visually indicate the electrical power being comsumed by the light bulb.

Circuit 1(d). Lamp dimmer circuit diagram.
Click here to see a simulation demo of this circuit.

Measurement Notes

Note that voltage and current labels on the diagram indicting where the measurements can be made.

  • Measure VS using the safety banana plugs available on the 1ph Supply Box.

  • Measure VOUT using flat blade probe tips inserted into the appropriate unoccupied slots of the splitter.
    Danger: Be careful while inserting the probes into the splitter as there the potential for exposed metal. figure 9. Measuring Vout using the flat banana probe tips.

  • Measure IS using the current probe on only one of the wires near the light bulb where the wires are split.

  • As the light bulb and the triac are in series IOUT will equal IS. The control circuit draws a very small negligible current and can be ignored.

  1. To become familiar with how the lamp dimmer circuit operates, first while measuring VS and IS and again with VOUT and IS. Slide the lamp dimmer control back and forth while the Fluke 43B is in several different modes of operation. Particularly, when in the Harmonics mode of voltage, current and power. Notice how the input and output differ.

  2. To obtain a specific firing angle connect the fluke 43B to measure both VOUT and IS as shown in the above diagram. Note the tθcond that you calculated in the pre-lab and use the scope to measure the conduction time by counting divisions while having a half cycle displayed on the screen Adjust the lamp dimmer firing angle by adjusting the slider until the triac conduction time matches the one desired.

  3. For firing angles 30°, 60°, 90°, 120° and 150° complete the required measurements in the results sheet.

  4. Before you move on to the next section verify your results with an Instructor or TA and get them to sign your results sheet.

3.3 Clean-Up

  1. Return the safety banana leads neatly to the wall and place the probe tips in the appropriate box which should be available on the counter somewhere. Put the Fluke 43B away in its case as shown.

Note

Please make sure that the Fluke current probe” is turned OFF, leaving the probe on will drain the 9V battery with-in a day and will then need to be changed before the next class.

Figure 10. Return safety banana lead and the test leads to the wall, place the Fluke 43B back in the case as shown and return the 4mm test probes to the box.

  1. Place the remaining equipment tidy on the workbench.

Figure 11. Leave the remaining equipment tidy on the workbench.

  1. Remember to back-up and share any computer files you may have created during the laboratory and that you log-off or shutdown the laboratory desktop.

  2. Have a lab instructor or TA sign your Sign-off sheet to ensure that you have cleaned everything up properly before you leave.

4 Post-Lab

The following is what you are expected to hand-in approximately one week after completion of the lab, check eClass for the exact time and date. All reports need to be submitted to the appropriate link on eClass. You only have to hand-in one copy per group. Please have your pages in a single pdf file in the following order:

  • Use a scanned/picture of your ECE401 - Lab 3 - Sign-off sheet as your cover sheet. Make sure that you have obtained the required lab sign-off signatures at the bottom of the page. Also make sure that all of your group members names, CCID’s and lab section are visible in the table at the top of the page.

  • The completed ECE401 - Lab 3 - Results sheet .

  • The answers to the post-lab questions.

4.1 Questions

  1. Looking at the two graphs where you plotted the calculated and measured values of POUT and POUT,pu; Explain why the error increases with firing angle with your actual measured values but doesn’t with your per-unit values? Explain the advantages of using the per-unit system to compare results? (5 marks)

  2. With this lamp dimmer circuit the current harmonics don`t contribute to any real power flow on the input side but do on the output side. Explain in your own words why? (5 marks)

  3. The lamp dimmer circuit is a series circuit where the input current equals the output current. However the input and output voltage can look vastly different from each other and also have significant differences in their rms values depending on the firing angle. Considering this, explain how the Input power equals the output power when you neglect the circuit losses? (5 marks)

  4. As you can see from your results the power factor decreases quickly when you increase the firing angle. What factors contribute to this decrease in power factor? Also explain in your own words why a high power factor is desirable? (5 marks)

  5. Explain in your own words how the firing angle is used to change the power in the light bulb? (5 marks)

  6. Using your results to support your answer. What are some of the drawbacks of using a triac with firing angle control to decrease the power to a load? (5 marks)