Phy 3311 Electronics Lab #1: Soldering, and Ohm’s Law You will need: Fluke 115 multimeter (on the “electronics cart”) Extech multimeter (also on the “electronics cart”) BK Precision Power Supply (stored below the instructor’s table) Three resistors with R between 1kΩ and 10kΩ (try to pick three unique values!) One PASCO “RLC Circuit” (on the cart; you will only use the light bulb for this experiment) “Quick start” guides for multimeter and power supply, and “resistor color code” cheat sheet Part 1: Soldering In this part you’ll learn about a very useful technique known as soldering. Soldering is a method whereby metal conductors can be joined together using molten metal. Soldering provides a good electrical contact between the conductors, and when done correctly, a strong mechanical bond as well, so it’s vastly preferable to taping or twisting wires together, or any jury-rigged fix like that. For the type of soldering we’ll do, the solder used is an alloy of about 60% tin and 40% lead (so-called “60/40” solder). This alloy has a very low melting point, lower than lead or tin themselves (it’s very close to what metallurgists call a “eutectic” mixture, the mixture at which the melting point is lowest). The heating is provided by a metal iron, which is heated up by an electrical current to a temperature on the order of 175-200 °C (350-400 °F). This is above the melting point of 60/40 solder. (Due to the nasty properties of lead, 60/40 solder is gradually disappearing in favor of “lead-free” solder, which has a somewhat higher melting point, although it works in just the same way.) Soldering can also be performed with higher-melting metals such as copper and silver (which is known as “brazing,” or in the plumbing industry as “sweating,” as pipes are often joined in this way), but this generally requires the use of a torch with an open flame. And welding is a process whereby the metals to be joined are themselves melted; this requires, in most cases, even higher temperatures yet! The idea behind soldering is simple. First, the components to be joined, known as the “work,” are mechanically placed together; if possible they should be twisted or tied together to make the bond even more secure. Otherwise, just press them together. Then, the hot iron is pressed to the work, and used to heat it up. Once the work is sufficiently hot, solder is applied to the work (not to the iron!) until it melts and flows freely onto the work, coating the joint thoroughly. Most solder includes some quantity of rosin flux, which helps reduce oxidation of metals, which improves the quality of the contacts made. For large jobs, you can also apply flux directly to the work surface. Once the solder has been allowed to freely flow over the work, covering all surfaces and filling all interstitial spaces, the iron is withdrawn and the solder cools and solidifies. Finally, you can test the bond for electrical and mechanical quality. Tips: 1. Note that you apply the iron to the work, and the solder to the work – the solder should not touch the iron! The idea is that the work should be hot enough that the liquid solder flows freely over the surface; if you let hot solder pour onto the cold work it does not form an adequate bond, resulting in a cold solder joint which is electrically and mechanically inferior. 2. That said, before applying the iron to the work, you should melt some solder onto the iron’s tip; this improves the thermal contact between the iron and the work, so it heats up more quickly. But you still must wait for the work to heat up enough to melt the solder. 3. Soldering works best if all surfaces are clean. Therefore, it’s a good idea (especially if surfaces are dirty) to scour them a bit, so you have nice metal surfaces onto which the solder can adhere. You can also wipe off the tip of the iron (when it’s cold, or when it’s hot by using a damp sponge after tinning the tip). 4. Very often, it feels like you don’t have enough hands. There are devices that are sold which you can use to hold the work in place while you hold the iron with one hand, and the solder with the other. 5. Solder gets very “drippy.” Protect all surfaces against dripping solder, and take care not to splash it on yourself (it’s unpleasant, as you may imagine). 6. Heat conduction – remember, even parts of the work distant from the iron can get hot… 7. It’s important to select an iron with an appropriate power range. For very small work, you want a low-power iron that won’t burn through everything (this is especially true on printed circuit boards!). For large hunks of metal, you need a powerful iron that will heat the work faster than heat can be conducted away. Now, try to solder some wires together. You don’t need to write about this in your lab report, just have fun and try to practice your technique! Part 2: Resistance and the Ohmmeter Pick out one of your resistors. Measure and record its resistance using the ohmmeter setting on the Fluke (yellow) multimeter. How does this value compare to the nominal value? What is the percentage difference? Is it within the tolerance specified by the fourth color band? For reference, the percentage difference between the numbers x and y (assuming x and y are rather close in value) is: Percentage Difference = 100 * |x–y| /x Do this for the other two resistors. Do these fall within the tolerance range as well? Part 3: Ohm’s Law Set up the circuit shown below. You will need the BK Precision power supply (using the “A” variable output), two multimeters (use one of the yellow Fluke meters for the voltmeter, and one of the gray Extech meters, in “mA” setting, as the ammeter), one of your three resistors (your choice!), and enough wires to connect the circuit. Start with the voltage and current control knobs turned fully counterclockwise. Notes: Pay careful attention to how the multimeters are connected! The ammeter needs to be in series with the resistor R, with the red lead attached to the “A” terminal on the meter. The voltmeter needs to be in parallel with (“across”) R, with the red lead attached to the “V Ω” terminal on the meter. Try to get the polarities correct; the meters will autocorrect if you get it wrong but your measured values will have minus signs (see if you can get them right!) Set the voltage output on the power supply to approximately 2 V (as read on the supply’s frontpanel voltmeter) by turning the current-control knob fully clockwise, and slowly increasing the voltagecontrol knob. Now, use the ammeter and the voltmeter to record the exact voltage and current values. Next, set the voltage output on the power supply to approximately 4 V, and repeat your voltage and current measurements. Then, turn the voltage up in 2 V increments and repeat. Once you reach 12V, scan back down, filling in the “odd” values down to 1 V. Now, an interlude with two very valuable data-taking tips, which will save you time and avoid future embarrassment: 1. Note how I had you “skip steps,” going up and then down in alternating steps. This is a valuable technique to “cover a lot of ground” as quickly as possible so you can see if something funky is happening, and since you’re interpolating “early” and “late” data points, also will tip you off if there is some systematic shift (i.e. your resistance value gradually changes, or your power supply is deviating from what it’s supposed to do). 2. You don’t have to set the voltage to exactly 2.000 V, 4.000 V, etc. – any values will do! So don’t waste huge amounts of time setting exact voltages (even if it feels fulfilling to do so) – just get it “close enough,” wait for it to settle down to a constant value, then make your measurement. According to Ohm’s Law, if you plot V (on the y-axis) versus I (on the x-axis) the data points will fall on a straight line whose slope will be equal to the resistance (V = IR), and whose intercept will be zero (zero current drawn at zero voltage drop). Use Excel (or another program) to plot the data (voltages on the y-axis and currents on the x-axis) and perform a linear fit. Does the data fit well to a line? Try fitting with the y-intercept allowed to vary freely; is the value of the y-intercept consistent with zero? How does the slope compare to the nominal and measured (with the ohmmeter) resistance values? Finally, compute the power (P = VI) that the resistor dissipates at each voltage level. How rapidly does the power dissipated increase with the applied voltage? If the maximum power the resistor can dissipate is 0.25W, at what level of applied voltage would that be reached? (You can do this more easily by plugging Ohm’s Law into the power law to get P = V2/R.) Part 4: Non-linear resistances (non-Ohmic devices) Using the multimeter, record the resistance of the light bulb on the RLC circuit board (it should be a few ohms; if it is much larger or smaller than that you probably have a bad connection, so try again; if it is still out of whack, the bulb is probably blown so let me know and get another board!). Connect the ‘A’ output of the power supply to the light bulb terminals on the RLC circuit board. Note: you should not allow the voltage to exceed 7 V! Scan from 0.5 V up to 7 V in 0.5 V steps (you may want to try the hints from before – scan up in quick steps, then come down and fill in, and don’t worry about getting exact 0.500 V steps!). You may need to adjust the current knob if the supply goes into “current controlled” mode as indicated by the red light near the terminals. At each step, measure both V and I using your multimeters. What happens to the bulb as the voltage increases? When you get home, plot V versus I that you measured. Do these points fall along a straight line? Why or why not? Compute the power delivered to the light as a function of voltage. How does this amount of power compare to a typical light bulb in a household lamp? What does the resistance measurement you got with the ohmmeter correspond to on this plot? Suggestions for your lab report For this lab you will write up a brief summary of the experiment and your observations, to be turned in at next week’s lab. Introduction: Ohm’s Law – What is it? Why does it seem to be true? How to measure voltage, resistance and current (similarities, differences between how you use the multimeter for these measurements). Methods: A brief summary of the measurements you made, as described in this writeup. Maybe include a circuit diagram or two (it’s good to start practicing, since you’ll have to do it later!) Results: The three resistance values you measured in Part I. How do their values compare with the nominal values from the colored bands? What are the percentage differences? Are they reasonable given the tolerance values from the fourth color band? From Part 2, plot V versus I for the resistor you chose. Do the values fit to a line? What is the slope? What is the intercept? From Part 3, what is the resistance for the light bulb you determined with the ohmmeter? Also plot V versus I for the light bulb. Do the values fit to a line? Discussion: Ohm’s Law predicts that there should be a linear relationship between the voltage across, and the current through, a resistor. Did you find this to be the case? If so, does the slope of your V vs. I graph correspond to the value of resistance you found with the ohmmeter? What is the value of the intercept from your fit? What should the value of the intercept be? Do your results make sense? What happened with the light bulb? Does the light bulb filament obey Ohm’s Law? If it does not, why do you suppose that is? Also, if it does not, your ohmmeter seemed perfectly happy to spit out a resistance value when you measured it: what does this value correspond to in your V versus I plot? Can you think of issues that impacted your ability to perform the experiment? Any ways to improve the accuracy or reliability of your measurements?