标签: resistor

Several weeks ago, I was walking around my office building when I ran across my chums Ivan and Darryl playing with a new toy that Darryl had picked up on eBay.   This little rascal (the toy, not Darryl) turned out to be a component tester. I have to say that I was pretty amazed by what I saw; so much so, in fact, that I raced back to my office to purchase one of my very own.   First, I placed an order for the model Darryl and Ivan were playing with -- a Mega328 ESR Transistor Resistor Diode Capacitor Mosfet Tester w/ Test hook -- which is a mega-bargain at only $15.98 USD ($0 shipping and handling):   Mega328 ESR Transistor Resistor Diode Capacitor Mosfet Tester w/ Test hook (Source: Max Maxfield)   It turns out that there are a bunch of these things. For example, while I was rooting around on eBay, I also ran across this All-in-1 Component Tester Transistor Diode Capacitance ESR Meter Inductance (they really could work on the naming of these little scamps) for only $19.33 USD (again, $0 shipping and handling).   All-in-1 Component Tester Transistor Diode Capacitance ESR Meter Inductance (Source: Max Maxfield)   It does take a couple of weeks for these little ragamuffins to wend their way from China, but I really wasn't in too much of a hurry. They both arrived a few days ago and I just now found a few minutes to take them for a spin.   A few thought off the top of my head are that the $15.98 unit is incredibly reasonably priced and I do like the fact that it comes with the three flying test leads. On the down-side, it was poorly packed, the display was loose, and it doesn’t have an "Off" button, which means that after you've pressed the "Test" button and seen the results, you have to wait for it to turn itself off automatically.   By comparison, the $19.33 unit is more "rugged" and was much better packed. It also has an "Off" button, which is jolly useful if you want to test a bunch of components. This unit didn't come with any test leads, but overall I have to say that it's my favorite.   Next, I gathered a few components together -- a resistor, a couple of capacitors, a FET, and a relay (inductor) -- whatever I found lying around, really. Both of the units come with ZIF (zero insertion force) sockets. You plug the leads from your component into the ZIF socket (it doesn’t seem to matter which pins go in which holes), close the socket, press the "Test" button, and observe the results on the display. The image below shows the test of a 10µF ceramic capacitor.   Testing a 10µF ceramic capacitor (Source: Max Maxfield)   To be honest, these testers would be worth the money if all they did was test capacitors. I can’t tell you how many of these components I have lying around that I couldn’t use (until now) because I couldn’t read their markings. The fact that these testers also work with resistors and inductors and diodes and transistors is just cream in the cake, as far as I'm concerned.   Check out this video showing the $19.33 unit in action:   One thing that did impress me is the fact that, when I tested my FET, it appears (from the diagram presented on the display) that the tester correctly identified the fact that there's an internal protection diode. I know there is such a diode because that was one of my selection criteria when I purchased these transistors (I'm going to use them to control the meters in my Inamorata Prognostication Engine and the relays in my Nixie Tube Clock, so I need to protect myself from the effects of back-EMF).   On the other hand, it may be that this diode is just part of the FET diagram they used -- I need to try this with a FET I know not to have this diode to make sure.   The only area I think these testers could use some work is the way in which they number the component pins on the display and associate these numbers with the pins in the ZIF socket -- sometimes the mapping is obvious; other times less so -- but overall I feel this is a minor niggle and I think either of these units would complement anyone's workspace and/or make a perfect gift.

When you need to measure current through a load such as a motor, there are various obvious options: Use a Hall-effect device, a transformer (for AC only, of course), or an in-line series sense resistor (often called a "shunt resistor," but this is misleading, as it is not bypassing the load). Conceptually, the sense resistor is an attractive option, because it is inexpensive, can be placed anywhere in the load line in principle, and produces a voltage output based on Ohm's law: V = I × R. What could be simpler?   As with so many other engineering topics, the reality is that what looks simple at first actually involves tradeoffs and conflicts. That's true for the sense resistor, as well, with issues such as the resistor's value, location, and physical installation. Two application notes (found in the References section) made this quite clear.   Using a low-value sense resistor is a common technique for measuring current through a load, but applying even this simple component for such a basic, straightforward application has its subtleties and tradeoffs.   Consider the most obvious parameter: the resistor value. A higher-value resistor develops a larger voltage drop for a given current, and that larger voltage is easier to use as a feedback signal. Whether you digitize it or use it in analog form, the higher voltage provides greater noise immunity and better resolution.   However, that larger value also means there is increased voltage drop between the rail and common (often referred to as "ground," even if it is not a true Earth ground) and less voltage for the load, reducing system performance and efficiency. Further, the resistor itself dissipates power. This wastes available power and means there is more heat to be removed from the system. Dissipation in tens of watts is fairly common. Finally, the sense resistor is within the load-control loop, and this will affect the loop dynamics, stability, and performance, since it is in that loop but not part of the "real load" that the system is driving.   Balancing these factors, most sense resistors are chosen with sub-ohm, milliohm, and even sub-milliohm values to minimize IR drop, I 2 R self-heating dissipation, and load disruptions. The corresponding voltage across the resistor is usually about 1 V full scale, meaning that the sensing circuit needs to be designed for good analog response and resolution at relatively low levels.   The low resistance value also has a ripple effect on design, layout, and physical configuration in a way that many engineers may not be used to considering, especially if most of their experience is with the resistance in the more familiar kilohm range. At the milliohm and lower values for the sense resistor itself, the associated resistances of the PC board, solder connections (if any), and the placement of the voltage sense-lead pickoffs become significant.   Even a few centimeters of PC-board track between the sense resistor and the sense-circuit input may be a significant fraction of the sense-resistor value. There's also the temperature coefficient of resistance α of the PC board's copper to consider: ΔR/R 0 = αΔT; for copper, α = .00386.   I also wonder: If you are interviewing a candidate for a power-related or analog-circuit role, would a good place to start be to ask about an apparently simple subject such as current-sensing options, especially the use of current-sensing resistors? Then you could dive deeper into topics such as the pros and cons of high-side versus low-side sensing or techniques for isolated or differential sensing (often needed, especially with high-side sensing). Perhaps we'll look at those issues in future columns.   What's been your experience with current sensing and sense resistors? Are there other topics which you assumed at first would be simple, only to surprise you when you dug deeper, did the math, and looked at topologies?   References Choosing the right sense resistor layout, Texas Instruments Optimize High-Current Sensing Accuracy by Improving Pad Layout of Low-Value Shunt Resistors, Analog Devices

When developing precision electronics or working on a detailed worst-case analysis, one quickly learns to consider parameters that may not be so important in other applications. One of the more interesting things to learn is that the tolerance of a resistor is just the starting point. It does not actually define the maximum or minimum value the resistor could be within your circuit.   The key parameters associated with a resistor are as follows. * Tolerance: This defines how close to the nominal value is allowable for the resistor when it is manufactured. A nominal 1,000Ω resistor with a tolerance of ±5% will have a value ranging between 950 and 1,050Ω. This value will be fixed; the value of the resistor will not vary during its life due to the tolerance. However, the engineer has to consider the tolerance in design calculations and ensure the circuit will function across the entire potential value range. * Temperature coefficient: This describes how the value of the resistor changes as a function of temperature. It is defined as parts per million/Kelvin; common values are 5, 10, 20, and 100 PPM/K. Actually, the best way to think of this is parts per million per ohm/Kelvin. A 1,000Ω resistor with a temperature coefficient of 100 PPM experiencing a ±60K temperature change over the operating temperature range (240-360K, based on an ambient room temperature of 300K) will experience a resistance change of ±6Ω based on its nominal value.     Obviously, the lower the temperature coefficient, the more expensive the resistor will be. (This is the same for low-tolerance resistors.) * Resistor self-heating: For really high-precision circuits, it is sometimes necessary to consider the power dissipation within the resistor. The resistor will have a specified thermal resistance from the case to ambient, and this will be specified in°C/W. The engineer will know the power dissipation within the resistor; this can be used to determine the temperature rise and hence the effect on the resistance. To determine the maximum and minimum resistance applicable to your resistor, you must consider the tolerance, the temperature coefficient, and the self-heating effect. As you perform your analysis, you may notice some of the parameters are negligible and can be discounted, but you have to consider them first to know whether or not you can discount them. For some precision circuits (gain stages in amplifiers, for example) it may be necessary to match resistors to ensure their values are within a specified tolerance of each other and have the same temperature coefficients. In certain circuits, it is also important to make sure that critical resistors are positioned correctly to ensure both terminal ends of the resistor are subjected to the same heating or cooling effects. Otherwise, the Seebeck effect may need to be considered. When using forced airflow, for example, it may be necessary to ensure that both resistor terminals are perpendicular to the airflow, so the component is of uniform temperature. To what level do you consider these effects in your own designs? Also, are there any other factors you take into consideration when selecting a resistor? Adam Taylor Head of Engineering Systems E2V  

My grandparents' TV went dead. My dad, a radio and TV repair person, took a quick look and thought the damage was caused by a line spike, maybe from lightning. The tuner didn't appear to be damaged. He didn't have the time to take on this repair, although more likely he thought it was a lost cause. He issued me a challenge to fix it. Wait a minute, pop! You want me to rebuild a TV that was possibly hit by lightning? Cool! I knew it wouldn't be pretty, but I accepted. My dad was correct. Only the IF section was toast. Really, fried remains of resistors, coils, and caps burnt to a crisp. It looked like someone had taken a torch to the IF section. I got out the SAMS service manual. I can't remember the exact make. The TV may have been a Philco, RCA, or Magnavox. I started the rebuild with the easy stuff first, replacing the tubes, resistors, and caps. I had to match up the resistor values from the schematic, since the colour codes were different shades of burnt umber... pun intended. The IF coils were my biggest worry. Luckily the coils had only minor smoke damage with a couple of open windings. I carefully cleaned up the coil forms and re-wound the open ones with the same size copper wire. Six weeks had elapsed since I started, and finally the work was done. It was time for the smoke test. It was the most nerve-wracking moment: Would my Frankenstein TV come to life or... just smoke? I carefully flipped on the switch, standing well back, just in case. It was ALIVE! NO smoke! It really worked. The picture and sound were both functional. Actually, very functional indeed considering what I had done to the IF section. On Thanksgiving day my grandparents had their TV back. Note: To protect the innocent the exact details and names have been changed. Plus this happened about 40 years ago, when I was 15 years old and just starting 10th grade! Dave Grindel submitted this article as part of Frankenstein's Fix, a design contest hosted by EE Times (US).  

Electronic-circuit design and hands-on engineering do not not appeal that much to younger people. That's because active and passive components are too small to handle, probe, swap, and generally do much with—unless you are a pick-and-place machine or bed-of-nails prober. Yes, we have seen articles explaining how a skillful amateur can load and solder those high-density PC boards with ICs whose leads are under the package and passives almost too small to see. But reality is that it is fairly hard and unforgiving, and even if you are successful, it's pretty much impossible to poke and probe. And if you decide you want to try a different resistor in that circuit. . . well, good luck on removing and replacing the grain-of-pepper one that is soldered down. But there's still hope out there. The other day, I received a printed—yes, printed—catalog from a company that provides all sorts of electronic and electromechanical components for hobbyists and professional prototypes. Some of these components were new, while others (such as motors and gearboxes) were likely removed from equipment or a supplier's overstock; almost all were interesting. This particular catalog was from All Electronics Corp. (click here ) but there are many other supply houses out there with similar offerings. And what joy it was, to thumb through the pages. There were small and fractional-horsepower motors (AC and DC), gearbox assemblies, rocker and toggle switches, multipole relays, LEDs in various configurations, specialty items such as a switch which trips a tiny amount of air pressure, and much more. The joy wasn't just nostalgia, which is a non-starter in our industry. All the items displayed and detailed on the pages combined to stimulate ideas and remind me of the possibilities of what you could actually build, touch, adjust, poke around in, reconfigure, and more. Almost everything could actually be handled, soldered or wired to, resoldered and rewired, reconfigured, and in general, be real and tangible. So go ahead, go to the web site of this company or others like it, or even better, get your hands on a paper catalog. It will confirm that there is still an opportunity for the joy of discovery. You can plan do-it-yourself construction of circuits and systems with human-sized parts, with motors and switches and lights than do and mean something, and which provide a hard-to-explain—but very real—sense of satisfaction. Just looking at those motors and switches, I could see some potential projects might want to try. Is my point valid? Or am I just longing for a time and type of hands-on project that is gone, and no longer realistic? Do you have a similar supply house/source you use?