Power Consumption vs Power Dissipation - Page 1
Forum section: electronics / beginners

author: tarun172
author_rank: Regular Contributor
timestamp: April 06, 2022, 03:25:38 pm
content: Hi All, I am trying to look for a standard definition of power consumption and power dissipation of circuit. I had several discussions with my colleagues. We were not able to reach an agreement on definition and scope of two terms. IMO for a complex design consisting of ASIC, Microcontrollers, analog and digital circuits; power dissipation is not equal to power consumption. Thinking from physics point of view, where does energy go? Does it ultimately convert into heat? Thoughts everyone?- tarun172

author: IanB
author_rank: Super Contributor
timestamp: April 06, 2022, 03:32:16 pm
content: A basic energy balance says "power in = power out + rate of accumulation of energy".For a system in steady state, this becomes "power in = power out".The main way power can leave a system is by thermal dissipation.In special cases, power can leave by EM radiation (radio transmitters, lasers, lamps, etc.), or as mechanical power (motors), or as output power (amplifiers, power supplies).For something like an electronic circuit (logic, CPU, etc.), then ALL input power leaves by thermal dissipation. There is nowhere else for it to go.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 06, 2022, 03:50:36 pm
content: We use term 'power dissipation' when we talk about temperature model of a device.We use term 'power consumption' when we talk about electrical properties of a device.

author: Benta
author_rank: Super Contributor
timestamp: April 06, 2022, 03:53:31 pm
content: Quote from: IanB on April 06, 2022, 03:32:16 pmYou forgot the photons from the indicator LEDs.

author: TimFox
author_rank: Super Contributor
timestamp: April 06, 2022, 03:59:08 pm
content: In certain apparatus, such as stabilized power supplies and (audio or RF) power amplifiers, there is a substantial flow of power from the device to a load (antenna or speaker), so the distinction is made between the power dissipated in the device (which heats up the box) and the power consumption (which governs the input power fusing and includes the output power and the dissipation). This is just a question of nomenclature, since eventually all the power from the mains ends up heating something.

author: emece67
author_rank: Frequent Contributor
timestamp: April 06, 2022, 04:28:27 pm
content: .

author: eugene
author_rank: Frequent Contributor
timestamp: April 06, 2022, 04:58:00 pm
content: The power consumed is the power drawn from the outside world. The power dissipated is the portion that the device gives up as heat. The OP and their friend can now stop arguing and move on to other things.

author: CaptDon
author_rank: Super Contributor
timestamp: April 06, 2022, 05:42:45 pm
content: 100% of the power consumed will eventually become energy lost as heat, makes no difference if it is immediate heat lost as switching losses or if it is photons from the LED's striking a wall. Even the energy output by the fans moving air will be given up as heat loss in the friction of air moving through air. The energy radiated from transmitting antennae is lost as heat eventually also.

author: eugene
author_rank: Frequent Contributor
timestamp: April 06, 2022, 09:31:53 pm
content: True, but the device in question doesn't dissipate all of that heat. I'm making the leap that in the context of electrical engineering, 'power dissipated' refers only to the power dissipated by the hypothetical device under discussion. It's not meant to evoke a discussion of the second law of thermodynamics.

author: TimFox
author_rank: Super Contributor
timestamp: April 06, 2022, 09:43:20 pm
content: Usually, when one is interested in "the power dissipated" by a device, it is to calculate the cooling requirements of the device itself.The "power consumption", however, is needed to size the fuse or circuit breaker.

author: CaptDon
author_rank: Super Contributor
timestamp: April 07, 2022, 02:06:49 am
content: Makes me think of the days of the RCA BTA-5 AM transmitter. We were running 6100 watts output with nearly 20kw total input to the unit as a single 'entity' and indeed it dissipated that 14kw as heat. Big fans pushing that heat out the rooftop vents. Then we got the solid state PWM transmitter, Roughly 8kw total line draw for 6100 watts into the phaser cabinet. What a savings in operating costs, and no vacuum tubes to age and wear out!! Got to say however, the glowing humming tube transmitter with flashovers and things blowing to smithereens on occasion made an engineer feel needed and valued!!! Then one day the F.C.C. put us all out to pasture!!!

author: Siwastaja
author_rank: Super Contributor
timestamp: April 07, 2022, 07:52:05 am
content: Examples:Motor controller with 98% efficiency.Pulls 1000W from the grid: consumption = 1000W.20W is dissipated as heat in the motor controller.980W goes into the motor. Maybe 80W is dissipated as heat in the motor, and another 900W finally converted to heat in the friction in whatever that motor is turning.Bitcoin mining FPGA board:Pulls 100W from the grid: consumption = 100W99.999W is dissipated as heat in that exact board.0.001W is converted into photons in your blinking status LED, later to be converted into heat in your room walls.

author: tszaboo
author_rank: Super Contributor
timestamp: April 07, 2022, 08:31:41 am
content: Quote from: Siwastaja on April 07, 2022, 07:52:05 amOh, but didn't you hear the BTC advocates that say that BTC is actually storing energy.And they are working very hard on making it more green...

author: magic
author_rank: Super Contributor
timestamp: April 07, 2022, 03:17:41 pm
content: Quote from: Siwastaja on April 07, 2022, 07:52:05 amWhy would anyone run a machine which dissipates all its energy input by friction?Seems you could as well use a resistive heater for the same effect

author: TimFox
author_rank: Super Contributor
timestamp: April 07, 2022, 04:01:18 pm
content: Quote from: magic on April 07, 2022, 03:17:41 pmA belt sander, maybe?

author: eugene
author_rank: Frequent Contributor
timestamp: April 07, 2022, 05:51:17 pm
content: I think he was making the observation that eventually all energy ends up as heat. Of course how it gets used before becoming heat is more interesting.Suppose I drive an EV to the top of a hill and park. Some of the energy from the battery becomes potential energy due to the mass of the car being lifted to the top of the hill. That's not heat yet, is it?

author: IanB
author_rank: Super Contributor
timestamp: April 07, 2022, 07:24:36 pm
content: The OP seemed to suggest that ASICs, CPUs and other kinds of circuits have some intrinsic kind of power consumption that is not dissipated outside the circuit, making power consumption different from power dissipation. But such is not the case. For a typical logic chip on a circuit board, the power consumption of that chip is exactly equal to power dissipation, if we neglect small factors such as output signals that go somewhere else in the circuit.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 08, 2022, 07:58:45 am
content: Quote from: magic on April 07, 2022, 03:17:41 pmBut it all ends up in friction, finally (at least if you count all forms of friction, for example, a vehicle hitting air molecules at high speeds).Similarly for lighting, all photons end up heating up your room, why not use resistive heater instead? Because you want to have light, even if it's only the temporary form of energy.

author: magic
author_rank: Super Contributor
timestamp: April 08, 2022, 08:06:12 am
content: Okay, there may be examples of useful machines that perform no work, but many (most?) do.If you construct a building, the energy stays in it until it collapses.If you pressurize a tire, energy stays in it.Even a ballistic missile performs work for half of its trajectory

author: TimFox
author_rank: Super Contributor
timestamp: April 08, 2022, 12:26:57 pm
content: And the belt sander does its useful work completely by friction.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 08, 2022, 01:57:58 pm
content: Quote from: magic on April 08, 2022, 08:06:12 amNo, amount of potential energy stored in the height of the materials is completely trivial, almost negligible. Maybe 0.0001% of the chemical energy in the fuel of construction equipment is stored in the mechanical potential energy. Rest goes into heat at building time; mostly in hydraulic circuits (which are highly inefficient). You could theoretize this percentage goes up if all the equipment runs off electric grid and uses ropes and pulleys instead of hydraulics, but it will still be meaninglessly small.Similarly, for a tire, if the energy stayed in like you claim, you would never have to add air. And, again, efficiency for inflating the tire is crap, because it doesn't matter. The potential energy stored in the pressure difference between tire and atmosphere never does work anyway.

author: IanB
author_rank: Super Contributor
timestamp: April 08, 2022, 02:29:50 pm
content: The tire case is interesting. When you inflate a tire you notice the air gets very hot when it leaves the pump (feel the outlet hose when pumping a tire). This hot air soon cools down to ambient temperature, but it means in effect that most of the input power to the pump is dissipated as heat and very little remains in the air inside the tire.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 08, 2022, 04:23:33 pm
content: Quote from: IanB on April 08, 2022, 02:29:50 pmYes, the process is very dissipative, already due to the poor efficiency of the typical compressor motor, but also the compressor block heats up like no tomorrow. A simple 1kW input air compressor requires a massive fan. As you note, the losses continue in the tire itself.Probably less than 5% of the electrical input ends up being "stored" in the potential energy of the pressure. And even this just slowly dissipates, it is never let to do any work (that's not the purpose).You totally can deliberately store energy in compressed air, but that requires something to convert the energy back to some useful form of energy. Tire is not example of such energy storage, though.

author: IanB
author_rank: Super Contributor
timestamp: April 08, 2022, 04:41:17 pm
content: Quote from: Siwastaja on April 08, 2022, 04:23:33 pmNot to derail the thread, but there is an interesting paradox here.The pressure energy stored in volume of gas is given by pressure x volume (e.g. N/m2 x m3 => Nm => J).But compressed air more or less follows the ideal gas law, and the ideal gas law (Boyle's law) says that at a given temperature, pressure x volume remains constant (double the pressure, halve the volume).The conclusion is that compressed air contains no more pressure energy than uncompressed air.How to explain this entirely unintuitive conclusion? Why do pressure vessels explode with great force?The answer is that the energy of expanding/exploding compressed air comes from the temperature of the air, not the pressure as such. When the air is compressed its free energy increases, which means the ability to access the thermal energy contained within it increases. For a given temperature the thermal energy is the same in the compressed and uncompressed state, but in the compressed state the stored internal (thermal) energy is more available to do work.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 09, 2022, 04:17:03 am
content: Energy can not be "consumed".It's a misnomer, just like calling beans "vegetables". (It's fruit, because it's something that plants make after the flowers and to procreate).Energy can be converted to some other kind of energy electricity, heat, sunlight, or chemical energy in a battery, but it can not be consumed.So when someone tells you that some gadget consumes energy, they are either lying, mislead, or they use some kind of made-up definition.If it's some redneck sticking to that point, just ignore him and walk away. If it's your wife, just smile and love her anyway.

author: Nominal Animal
author_rank: Super Contributor
timestamp: April 09, 2022, 06:41:53 am
content: Quote from: Doctorandus_P on April 09, 2022, 04:17:03 amOr just using stupid, illogical language – just like people who say "twice more" when they actually mean "twice as much" (which includes basically all reporters writing in Finnish, even though the language actually supports the logically correct expressions).In this particular case, that stupid language stems from the simplistic view that the device "eats" energy to produce an useful effect. Which is not true, because devices just convert energy in some form to some other different forms, one of the resulting forms always being heat. If the purpose is not to produce heat, then that heat is usually called "waste heat".To translate from this stupid language, "power consumption" is the amount of energy one needs to supply to the device, and "power dissipation" is the amount of waste heat it produces.In laptop, desktop and server processors, TDP (thermal design power) is a related term. The instantaneous heat produces varies a lot, because the same processor package contains also connectivity logic, cache RAM, and so on. TDP indicates the amount of waste heat any cooling system should be able to dissipate, so that the semiconductor junction temperatures in the package do not exceed their operating limits. It is nowhere near exact, as it depends on what the processor and its subsystems are doing at any point in time, but it is a widely accepted term to approximate the "power dissipation" in a laptop/desktop/server processor.Almost all of the input power in computers is "dissipated", turned into waste heat. As discussed above, a little bit of it goes to producing light, sound, or electromagnetic waves (radio, WiFi, LTE). When driving external buses most of the energy is turned into waste heat in the transceivers and transmission lines. The majority of it becomes waste heat.(If you live in cold climate, and often use electricity to heat your house, placement and configuration of the computer matters a lot. Indeed, I've helped build one machine that had inverted airflow –– intake at the top, exhaust at the bottom –– just because it happened to make sense for its use case.)If the circuit OP discussed does anything useful other than heat up, then indeed not all of the "consumed power" is "dissipated" as waste heat. Sometimes, the difference may be tiny, but of vast importance to humans. Think of things like LCDs: changing the crystal orientation with an electric field takes very little energy, but to do so in an useful way requires quite a bit of processing, maybe even a radio receiver (for a radio-synchronized clock). Or better yet, an e-ink reader! At steady state, it really requires no energy; it only takes energy to change its state. To make the new state meaningful, the energy conversions need to occur in rather interesting patterns; thus, the change can take quite a lot of energy. In such cases, you would be technically correct to say that basically all energy "consumed" by the device is "dissipated" as waste heat; but, inhumanterms, the fact that the tiny bit that was not, produces the desired and useful effects (the new picture we can read or look at), means that that tiny bit is worth all the waste heat!With that in mind, I do suspect that the underlying cause of the disagreement between tarun172 and their colleagues is that they did not specify clearly enough thedomainin which they consider the circuit or system. Very little energy is needed to produce many human-useful and desired effects, compared to the amount of energy converted to waste heat. However, humans definitely see the waste heat as worth it, to get the desired effect. So, do you and your colleagues consider that tiny bit of non-heat energy as significant or not? One answer equates e-readers and big resistors connected to a Li-ion battery, the other distinguishes clearly between them. Both answers/viewpoints are correct, just in different domains.

author: Nominal Animal
author_rank: Super Contributor
timestamp: April 09, 2022, 06:47:32 am
content: TL;DR: tarun172, ask your colleagues whether they are talking in quantitative or qualitative terms.The tiny fraction of input energy that is not turned into waste heat in electronic circuits, matters a lot to us humans (and is thus very important in qualitative terms), because our devices are horribly energy-inefficient. In quantitative terms, indeed basically all input energy is turned into waste heat in electronic circuits, unless the circuit itself provides energy to other devices or circuits.

author: magic
author_rank: Super Contributor
timestamp: April 09, 2022, 07:21:05 am
content: By that token, talking about food consumption also makes no sense. You just transform it into... some other things

author: Nominal Animal
author_rank: Super Contributor
timestamp: April 09, 2022, 07:29:45 am
content: Quote from: magic on April 09, 2022, 07:21:05 amIf we look at Wiktionary forconsume,To use upTo eatTo destroy completelyTo waste away slowlyTo trade money for good or services as an individualTo absorb information, especially through the mass mediathen I have to disagree. The definition works for food and fuels (tangible matter), but not so much for electrical energy.Then again, itisused for electrical energy in common speech as well, so I must admit my opinion is the minority one.Languages can be a pain in the butt, so to speak.

author: Zero999
author_rank: Super Contributor
timestamp: April 09, 2022, 11:36:28 am
content: Quote from: IanB on April 08, 2022, 04:41:17 pmIt's also why decompressing a gas causes its temperature to drop: the heat within it becomes distributed over a much larger volume. If the same heat energy in a larger vs smaller volume, the larger volume will be cooler.

author: IanB
author_rank: Super Contributor
timestamp: April 09, 2022, 05:16:16 pm
content: Quote from: Zero999 on April 09, 2022, 11:36:28 amThe temperature does drop when decompressing a gas, but not for that reason. The internal (heat) energy of an ideal gas is a function only of temperature. Therefore the heat energy contained within the gas is independent of the volume it occupies, and for the same heat energy in a larger vs smaller volume, both volumes will have the same temperature.The reason a gas cools down when it expands is that the gas has to do work against the surroundings as it expands (the gas is pushing out and the surroundings are pushing back). The energy to do this work comes from the internal heat energy of the gas: this energy is depleted and the gas cools down.This process is neatly illustrated in a heat engine like an internal combustion engine. The heat energy from the burning fuel enables the combustion gases to do work pushing down the piston in the cylinder, which through the transmission is transferred to the wheels of the vehicle. Thus the heat energy released by burning gasoline is transferred to kinetic energy in forward motion of the car.