标签: dissipation

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

Heat dissipation is often tied with power usage; that's not news. But sometimes, in our focus on minimising power consumption and thus subsequent dissipation, we forget that there are situations where properly managed dissipation is the whole point of the design.   This became very clear after I read a fascinating article in a recent issue of IEEE Spectrum about the ineffectiveness of the standard home oven. The article, " Recipe for a Better Oven " by Nathan Myhrvold (yes, the former chief scientist at Microsoft) and W. Wayt Gibbs, discussed the many shortcomings of the standard home and even commercial baking oven.   It was both illuminating and somewhat discouraging for many reasons. It's not just that the oven is wasteful in terms of energy use, it's that it also does a poor job despite that inefficiency; whereas I had hoped that the inefficiency was a necessary part of the tradeoff for getting the baking done right.   I already knew that the simple on/off control of the gas line or electric power was a marginal approach. Anyone who has studied any control theory knows that a loop with on/off control, rather than true proportional control and a PID (proportional-integral-derivative) control strategy, is not good at keeping the error between setpoint and actual value small, unless you are willing to switch that on/off valve at a fairly high rate.   While it is true that thermal situations tend to have relatively long time constants and so can accept a somewhat lower cycling rate than some other situations, oven vendors like to keep the valve's cycle time to a few minutes. So it is common to see swings of ±50°F (about ±25°C) around the setpoint. That's just not good enough for many recipes or foods.   The problems of the standard oven design and subsequent performance go well beyond the basic temperature-control loop. Only after I read the article did I realize how much I hadn't known about the implications of oven design and the cooking process.   For example, the heat source doesn't cool the food directly; instead, it heats the oven walls (metal or brick), which then re-radiate the heat to the food. The article explained how this leads to very uneven heat zones in the oven for conventional metal-wall ovens, and numerous other problems. (The article did highlight some ways to improve an oven's performance, which was nice to see.)   These other problem are not trivial, either, as they have to do with how different foods are affected by the radiant heat of the oven walls versus direct heating; not surprisingly, it does make a difference, just as different types of electrical load impedances affect power supply performance. (There are times when I felt the whole discussion of oven idiosyncrasies was akin to the mysteries of black-body radiation , which were finally resolved with the development of quantum theory in the 1920s by Max Planck and others.)   The irony of the oven problem is that for most engineers, the primary concerns regarding heat are twofold: minimizing generation of it in the first place, through the use of low-power circuits and high-efficiency supplies; and maximizing its removal, using convection, conduction, and radiation via use of fans, heat sinks, cold plates, and even advanced active techniques. It takes a 90° or even 180° shift in thinking to begin to understand heat and its effective delivery as an outcome rather than as an obstacle.   Have you ever been faced with a design challenge that required that you reroute your established way of looking at something, to a very different course? Did you realize the differences right away, or did you have to really step back to understand the many inherent assumptions you were making and how they had to be changed as well?

Heat dissipation is typically linked with power usage; that's not news. But sometimes, in our focus on minimising power consumption and thus subsequent dissipation, we forget that there are situations where properly managed dissipation is the whole point of the design.   This became very clear after I read a fascinating article in a recent issue of IEEE Spectrum about the ineffectiveness of the standard home oven. The article, " Recipe for a Better Oven " by Nathan Myhrvold (yes, the former chief scientist at Microsoft) and W. Wayt Gibbs, discussed the many shortcomings of the standard home and even commercial baking oven.   It was both illuminating and somewhat discouraging for many reasons. It's not just that the oven is wasteful in terms of energy use, it's that it also does a poor job despite that inefficiency; whereas I had hoped that the inefficiency was a necessary part of the tradeoff for getting the baking done right.   I already knew that the simple on/off control of the gas line or electric power was a marginal approach. Anyone who has studied any control theory knows that a loop with on/off control, rather than true proportional control and a PID (proportional-integral-derivative) control strategy, is not good at keeping the error between setpoint and actual value small, unless you are willing to switch that on/off valve at a fairly high rate.   While it is true that thermal situations tend to have relatively long time constants and so can accept a somewhat lower cycling rate than some other situations, oven vendors like to keep the valve's cycle time to a few minutes. So it is common to see swings of ±50°F (about ±25°C) around the setpoint. That's just not good enough for many recipes or foods.   The problems of the standard oven design and subsequent performance go well beyond the basic temperature-control loop. Only after I read the article did I realize how much I hadn't known about the implications of oven design and the cooking process.   For example, the heat source doesn't cool the food directly; instead, it heats the oven walls (metal or brick), which then re-radiate the heat to the food. The article explained how this leads to very uneven heat zones in the oven for conventional metal-wall ovens, and numerous other problems. (The article did highlight some ways to improve an oven's performance, which was nice to see.)   These other problem are not trivial, either, as they have to do with how different foods are affected by the radiant heat of the oven walls versus direct heating; not surprisingly, it does make a difference, just as different types of electrical load impedances affect power supply performance. (There are times when I felt the whole discussion of oven idiosyncrasies was akin to the mysteries of black-body radiation , which were finally resolved with the development of quantum theory in the 1920s by Max Planck and others.)   The irony of the oven problem is that for most engineers, the primary concerns regarding heat are twofold: minimizing generation of it in the first place, through the use of low-power circuits and high-efficiency supplies; and maximizing its removal, using convection, conduction, and radiation via use of fans, heat sinks, cold plates, and even advanced active techniques. It takes a 90° or even 180° shift in thinking to begin to understand heat and its effective delivery as an outcome rather than as an obstacle.   Have you ever been faced with a design challenge that required that you reroute your established way of looking at something, to a very different course? Did you realize the differences right away, or did you have to really step back to understand the many inherent assumptions you were making and how they had to be changed as well?