Power Dissipation in Circuits plus an EXPERIMENT | Basic Electronics


Power dissipation typically refers to how much power Volts times amps, is converted to heat in a circuit. So with power dissipation, You’re mostly thinking of resistors But it can refer to other things that are acting as resistors. And now, this is something that happened a lot in my first semester of Circuits I Is that people would say, “Oh, okay. How much power is being dissipated in this capacitor? this inductor?” And in reality, capacitors and inductors In their ideal form, don’t dissipate any power. Their energy storage devices. So in reality, there’s a little bit of serious resistance that can dissipate power But it is typically negligible and when you are doing that sort of stuff in class, you can just ignore it. So, if you ever think “Okay. Well, I got the current and the voltage across this current through it.” And trying to do that with a capacitor and inductor, Stop. Because you are not approaching it the right way. Almost always, when we’re talking about power dissipation, It is in resistance, objects Like basically, space heaters that sort of thing and obviously resistors themselves but not with anything else. We can talk more, We’ll talk more about what other things act like resistors in a bit But first I want to talk about How do we figure out how much power is being dissipated? So, Power equals voltage times current or current times voltage And if you actually remember Ohm’s law where, V equals IR or I equals V over R You can take that power equation and put it into two other forms And that’s basically by replacing the V and replacing the I, respectively. So you can actually take P where you replace the V So you have P equals I times I times R or P equals I squared R or you can replace the I and then you have P equals V times V over R or P equals V squared over R So in the end, there’s three ways to describe the power equation. That’s, P equals I V P equals I squared R or P equals V squared over R. So that gives us some flexibility and solving for power depending on what we have the information we have. If we don’t have the current or if we don’t have the voltage But we do have the resistance, We can switch it around and we can figure it out from there. So I would actually like to do a quick sample. I have right here a 100 ohm resistor. And I chose 100 ohms because that makes the math a lot easier for me. And with this 100 ohm resistor, I have the power supply, something checking the current running through it, and then a voltage meter. Just to show to make sure because this power supply Sometimes, it’s not extremely precise in the voltage that it’s showing. So with that, I want to discuss how much power this can take and how it changes over time. This is a quarter watt resistor. So that means it can take 250 milliwatts of power before it burns out and starts to have any issues. If we think about that with 100 ohms, that means we could do 5 volts over 100. Ohms gives us 50 milliamps. 50 milliamps times 5 volts gives us 250 milliwatts. So from that again, We know that for this 100 ohm resistor, we can only do up to 5 volts across it before we are going to be exceeding the amount of power that it is able to safely dissipate. Because again, 5 volts over 100 amps gives you 50 milliamps 50 milliamps times 5 volts, 250 milliwatts Ok. With that, let’s go up here and put 5 volts across this to make sure that I did my math right. And then I would also be curious to see how hot this gets. Because again, Power dissipation is dissipated in the form of heat. So getting up to 2ish volts, 20 milliamps, I mean this makes sense. Again, I chose 100 milliamps or 100 ohms Because I don’t really like doing math in my head that I’m not good at. So, we are now approaching the maximum that this is rated for. So I’m showing 4.8 volts. Let’s try and get that up to five. 4.8, come on. This power supply is a bit wonky sometimes. Okay. So we’re at five volts 50 milliamps. So right now, can’t see anything. But shortly, it’s going to be getting hotter and hotter. I have a temperature sensor but I do want to wait. I’m just gonna actually let this run for maybe 30 to 45 seconds. And then we will check the temperature to see how much warmer it is And I’ll touch it with my fingers to see how hot it gets. So I’ve been taking measurements for about the last minute. And it seems to have settled at a fairly steady state of about 85 degrees fahrenheit which the room is about 75 degrees fahrenheit. So, a 10 degree increase. Now, it’s a bit difficult because it’s such a small target So I’m having to take multiple measurements like that one just got up to 88. But honestly, even if I touch it, It’s definitely warm, but I can keep my finger on it without getting burned. So, we’re not even close to getting to the point Where we’re gonna damage this And I think that’s because when they design these, They give you a pretty conservative point at which they say, “Yeah, you go beyond this. We’re no longer gonna guarantee it and you’re likely” “gonna cause damage in the long run” But it’s probably fine in the short run. Again, If you’re designing something, especially if it’s gonna go into a consumer product or any sort of product, You definitely want to have your own conservative measurements as well. But I think we could go quite a bit higher on the voltage here And quite a bit more power before we see anything. Now, I’ve actually never melted one of these And I don’t know if it’s possible, But I would be curious at what point And how much power you will be dissipating before it starts to have some serious concerns. You know, I’m gonna grab my safety goggles. Again, I like my eyeballs. If you like your eyeballs, It doesn’t matter if you don’t look cool, If you can see. So I’m going to take some more measurements. I just bumped it up. So now we’re in the 95 degrees fahrenheit range. So I’m not sure if I can do this math in my head anymore 6 volts times 60. Oh boy. Let’s get up to 7 volts 70 milliamps Because I think I could do that in my head. Ok. We’re now at almost a half watt and it’s hitting a hundred degrees. What happens if we hit a watt which would be four times as much? Also, I’m wondering, Taylor, do we have a fire extinguisher? Just in case something goes wrong here. We have a sink in the next room. Okay, that’s never something you want to do on electronics. But we have one in the kitchen area. So if we have to we can do that, okay. So we are now basically doing one watt through here and it’s at 112, 130. Wow, it just jumped 120. Okay. It’s getting some pretty strange readings here, But it’s obviously getting significantly hotter. I don’t want to touch it anymore. But it is doing one watt. I’m anticipating getting some weird smells here fairly soon. 140, 130, 140 I have to admit, again I’m impressed how well this is doing. Of course, if you have a 140 degree fahrenheit part in your device, And it’s not designed for that, You are not gonna like that. 165, 170 I honestly am not sure if I want to see this thing go up in flames. Oh. I’m smelling it now. Hundred and seventy. I’m starting to hear some weird noises too. A bit of a strange ticking noise. I don’t know if that’s things coming apart in there. Do you hear that ticking? Yeah. Okay, so we are at 13.5 volts. I don’t know if my insurance is gonna cover burning the house down doing this. Things are starting to move and it’s starting to smoke. Okay, there we go. Oh, I know it’s happening. I’m melting my leads hot. Dang it. Okay, so I just turned it off. That is one reason why you don’t want to do that. Is because that power the heat being dissipated by that ran right up Those metal leads and melted my leads. So now I’m gonna have to get new leads. That’s not going to be a good one to justify to the boss. But, you can see how hot it got that. It melted these things. And that was the smell again this. You can see there’s a little bit of blackness over there. So it’s starting to melt the resistor but totally melted through all four of my leads. Man, that is a serious bummer. But now you can see what happens. You put too much current Have too much voltage across, get too much power. You’re gonna melt things. And you’re gonna have problems. So this is a great example of power dissipation that you don’t typically want. You don’t really get a benefit from your resistor getting really hot and melting all of your stuff is not really ideal. But there are other times when you do want that power dissipation. And that’s really the only time I can think of it as like a space heater. And even old incandescent lights are used sometimes as heaters. I know my father, he has an old refrigerator in his shop That he keeps his welding rods in and he puts an incandescent light in there. Just to keep it warm in the cold Idahoan winters. So there are times when you want that but most of the time you don’t want power dissipation. So a lot of the power dissipation you have in your small electronics is actually due to the switches in there and not due to the resistors. So when a switch is completely closed, there’s no resistance, no voltage drop. So even though you get a huge amount of current Since your voltage drop is basically zero, V times I, V being zero, You don’t have any power. When the switch is completely open, Resistance is infinity. But there’s no current. So again, no powers burned. In reality, that’s not exactly true because When it’s closed, there’s gonna be a little bit of resistance. And since it’s not a mechanical switch in the semiconductor material. When it’s open, there’s still going to be a tiny bit of leakage current. The biggest issue though is when it’s opening. So when it’s opening and when it’s closing, there’s a brief time there that resistance is finite Current is finite and voltage is finite. And it’s that switching time where you get most of your heat. So that’s why the faster you switch, the cooler your stuff is. Because when it’s You get a little bit of current and a little bit of voltage across it That’s when you get your power. So there’s a couple of ways to deal with power dissipation on a larger scale. First, make sure your resistors are rated to the correct wattage. This was not rated for over a watt. Which is what we ended up putting it into and that’s why it got hot and melted and destroyed everything. You also want to make sure that your ICs ratings include or don’t include heat sinks and plan accordingly. Some resistors actually have places to put heat sinks on them. Some ICs do the same. Others don’t . Know that when you’re reading the datasheet. What it’s talking about. Because it says, “oh, yeah, you can do 10 watts through this.” You think, “sweet, it’s great!” But you don’t put a heatsink on it and it’s only rated to 2 watts without the heatsink You’re gonna have issues. Finally, when designing PCBs make sure your traces are big enough that you have a low enough resistance that they don’t get too hot. We actually have a resistance calculator on our website that will show you exactly how to figure that out by giving you the trace width, the amount of current, whether it’s internal or external, we can do all those calculations for you. So jump on to circuitbread.com if you need to figure out how, why do you need to make your PCB traces. And as I mentioned earlier, if you decrease your switching time on the semiconductor level Then that is going to decrease your power dissipation as well. And so that’s power dissipation in a nutshell. That’s an example of the heat and the problems that you can have with too much power dissipation. Also, that’s what causes your phones to get hot. Is that switching in there? It’s generally not something you want. But it’s also something that you can control to a certain extent. If you have any questions or comments, please leave them below. If you like this video, please like and subscribe. And also check out circuitbread.com, my website. So if you have any other questions or want to check out our tutorials or equations, it’s all right there.

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