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Hot or Not? Find Out How to Calculate Component Heatand Why You ShouldMarch 3, 2014 By Mike Szczys 44 Comments

How hot are your key components getting? There’s a good chance you’ve built a project and thought: “Well I guess I betterslap a heat sink in there to be safe”. But when working on a more refined build you really need to calculate heat dissipationto ensure reliability. This is actually not tough at all. The numbers are right there in the datasheet. Yes, that datasheet packedwith number, figures, tables, graphs, slogans, marketing statements, order numbers… you know right where to look, don’tyou?

Hackaday has you covered on this one. In under 10 minutes [Bil Herd] will not only show how easy these calculations are,he’ll tell you where to look in the datasheets to get the info you need quickly.

Above, [Bil] used his bench as a whiteboard to illustrate the thermal resistance equation. In this case each resistor symbolrepresents part of the heat dissipation. You must consider all places where heat can be transferred: (from left to right) thecomponent die (junction) to the component case, the component case to a heat sink, and the heat sink to ambient air. Heillustrates each of these dissipation points in the video.

Calculate thermal resistance – Theta Junction toCase Tjc

An example of the junctiontocase is shown to the right. This is a TO3 case which has had the lid cut off. It’s a muchsimpler way to look at a chip die than trying to decap a component with a plastic case.

Make with the Math Already!

Okay, okay, we’re getting there. The math is not hard… just multiplication and addition, so hang on a minute more.

Gather the following values: maximum power you plan to use with this component, maximum heat rating of the part,maximum ambient air temperature in which this component will be used, and the theta values from the datasheets. Theta,which is a measure of degrees per watt, is often listed as a symbol: Θ Multiply theta by the max wattage and you will knowhow much temperature to add to your equation

Datasheets: Finding Θ and Temperature

Because [Bil] does such a great job in the video we’re giving you the quick version here. Temperature generatingcomponents will include a maximum operating temperature like the one shown below (click through for full datasheet) whichis for a linear regulator:

The theta for “JuntiontoCase” is found a bit further down the same datasheet in the Electrical Characteristicstable. Datasheets will also provide a “JunctiontoAmbient” value (also shown below but not used in our calculations) usedto calculate how much power you can use without any type of active or passive cooling. This answers the question of: “do Ineed a heat sink?”.

Finally, you want to look at values from the heat sink being used. [Bil] looks at the datasheet of a heat sink which lists athermal resistance of 25.8Θ with the chart below on the left showing how that number may be altered with moving air (afan). The chart to the right covers the use of interface agents like thermal grease, and a mica pad (for electrical insulation)with thermal grease. Both of those values are circled but only one will be used in the calculation.

Putting It All Together

If we assume an ambient air temperature of 38 C (100 F) and a maximum power of 2 W all of the numbers we need havebeen collected.

Max Temp = Junction + Mica/Grease + Heat Sink + Ambient

Max Temp = (4Θ * 2W)º + (0.4Θ * 2W)º + (25Θ * 2W)º + 38º

Max Temp = 8º + 0.8º + 50º + 38º

Max Temp = 96.8º

The maximum temperature rating for this part is 125 C, which means that this part is being properly cooled. [Bil] goes onestep further in the video, showing how to calculate how much more reliable the properly cooled part will be.

Resources

Texas Instruments LM317N Datasheet (PDF)Texas Instruments App Bulletin for mounting TO3 packages (PDF)AAVID THERMALLOY 7173DG heat sink example

Generating electricity from alcohol This SMD Reflow Hot Air Gun Hangs Around Your Workbench Hackaday Links: Sunday, June 9th, 2013 Propane forge built from a soup can Water cooled Raspberry Pi

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Mike Szczys

Filed Under: Featured, howto Tagged With: bil herd, electronics, heat, herdhacks, theta

Comments

1. fartface says:March 3, 2014 at 11:23 am

If in doubt, double the heatsink area.

Reply Report commentpcf11 says:March 3, 2014 at 11:25 am

Sounds like a wise policy to me.

Reply Report commentF says:March 3, 2014 at 12:48 pm

Wasting money on extra heatsink capacity is a wise policy? Making your project bigger than it needs to

Seguir 260

be is a wise policy? i think doing the math and getting the heat sink right is the wise policy. Heat is notvoodoo magic.

Reply Report commentnelsontb says:March 3, 2014 at 1:30 pm

It’s also the reason most old things lasted almost forever, while the new stuff almost doesn’t survivewarranty, if a screw needs to be at least 1 mm thick to hold a part, that’s what will be used, if thescrew rusts a bit or was 0.1 mm too thin from a manufacture defect it will break, but hey you savedthe company $0.01 per screw, that will look good on the report, the warranty returns can be blamedon the crappy chinese assambler, that did you asked them to do, so at least you are safe, thecompany can take the bad rep if the bean counters are happy, no loss here

Reply Report commentMichael says:March 3, 2014 at 3:30 pm

And in the case of laptop computers, it’s EXACTLY this reason: They only last for a fewyears because they save on cooling capacity by using less heatsink metal, which is of coursebulky, expensive and heavy.

nelsontb says:March 3, 2014 at 4:38 pm

@Michael:

Computers used to be able to encode movies all day long, now it’s normal for your (very)expensive ‘gaming’ laptop to overheat without external fans, even while doing mundanetasks, blame a fruit company for starting a heavy fad of form over function and the generalpublic for going along with it

Eric says:March 3, 2014 at 5:19 pm

I was totally with you until you brought up Apple. What do they have to do with this? Myprevious laptop was a Lenovo Ideapad that literally melted from the heat. I am now using aMacBook Pro and it is extremely welldesigned. I would argue that the thermal system onApple products is consistently better than the alternatives.

In fact, I would say the same is true for audio hardware, displays, and most other hardwarecomponents.

John Smith says:March 3, 2014 at 6:58 pm

There’s a lot of reasons stuff “doesn’t last as long.” Some of it nostalgia – tubes burn outsuper fast, you know. Some of it is that old transistors were physically larger. More silicone,more insulating layer, more metals… Overvoltage and transient conditions are nowhere asdamaging. They have more mass to dissipate instantaneous heat (from short bursts of activity)and keep their temperature down.

And of course, some of it is the reduction in weight of supporting material. The heatsinks, thefasteners, the frame, it’s all pared down. THAT is because we like being able to carry ourelectronics with our hands, and not a bloody forklift. The idea that laptop companies usesuper tiny fasteners ‘to save money’ is laughable to me. They do it because it lets them buildthe bloody things smaller and lighter.

nelsontb says:March 3, 2014 at 8:05 pm

@Eric:

Sorry did retail tech support for a while some years ago, still have a sore image of thecompany over the reasons for denial of warranty and general assembly quality. screens goinggray or orange and they blame the manufacturer while doing nothing for over a year, hddswith only 1 year warranty even when the law here is 2 years, aging white plastic macbooksthat melted, deformed or discolored for lack of proper cooling, where they replaced the fanbut left the plastics has they where only cosmetic even if the part of the keyboard isdeformed, supplied a batch of defective i7 macbook pros and requested that our company doa firmware ‘update’ that lowered the i7 to i5 specs to avoid overheating. it was prettypathetic. But its true it’s not only them, saw a lot of companies that i respected lowering theirquality and having shady practices, some brands (toshiba and asus if memory serves me well)would take a returned laptop and for a small fee would repackage it to look new.

You mentioned audio and displays, and i have to agree with you and tekkieneet (bellow),looks like nobody makes a decent psu anymore, crappy chinese capacitors, sometimes eventouching heatsinks, way underrated diodes, lack of distance between primary and secondarysides of main transformer, passive cooling in completely closed spaces with ICs running >90C, it’s nuts how these get approved.

tl;dr Never buy a laptop/anything from retail from a store where you don’t have a friend tocheck the system for unusual return levels (bad batches) and run the serial number in thesupport system to make sure you are actually receiving a new machine

Nicholas says:March 3, 2014 at 9:16 pm

I think another thing that needs to be touched on is the fact that electronics seem to beproduced as being disposable these days. So I’m sure it would be reasonable to assume thatcompanies these days don’t manufacture their products to last because they believe peoplewill already by lining up to buy the newest greatest thing. It seems like a vicious circle to me.They decrease the retail of the product so people can “afford” something new. But bydecreasing the retail they are essentially are decreasing the quality because there is no waythey would ever want to shrink their profit margin.

tekkieneet says:March 3, 2014 at 2:01 pm

Not so sure if crappy components are the only reason why things fail a lot sooner these days.

A lot of things fail because of they are considered to be “mundane” stuff for a typical designer’s orparts purchaser’s these days. e.g. power supply related issues or bad caps or bad mechanical part.The rest of the product that don’t have crappy alternatives tend not to be the cause of failures(although they could be victims).

Most designers don’t even know about (nor seem to care about) Temperature rating, ESR, Ripplecurrent rating, Lifetime etc nor they are aware to not put the capacitors right next to or down streamof hot objects like heat sinks inside a power supply. etc Experience is irreplaceable in that area, butcompanies opt to have new grads as they are “cheaper”.

In the long run, making crappy products damages the “Brand” that (only) the marketing peoplecares about. That costs a lot more than the $0.01 you are trying to save on crappy capacitors. Theyshould at least try to make they last beyond the attention span of the consumers.

Reply Report commentpcf11 says:March 3, 2014 at 8:38 pm

Waste money? Who buys heatsinks? I got a whole box of them I’ve salvaged. I’ve got acres ofspace too so volume isn’t really an issue for me either. Quit projecting your inadequacies onto

others and life might improve.

Reply Report commentnelsontb says:March 4, 2014 at 8:11 am

even just building 10 units that need reliability we buy heatsinks instead of using the junkparts bin, unless we can find 10 same model units that can do the job, it’s simply not practicalto make a pcb that can fit all the different heatsinks and then test all of them seperatly

pcf11 says:March 4, 2014 at 11:56 am

@nelsontb I do not usually make multiples of projects but when I do I make customheatsinks out of larger ones. All it took was sawing one

http://www.instructables.com/files/orig/FBK/OPKS/GT19NE3T/FBKOPKSGT19NE3T.jpg

What can I say? I have a really nice band saw.

2. tekkieneet says:March 3, 2014 at 12:10 pm

The units in OP are inconsistent. Some uses “38 C” and some uses “38º”. 38º is for angle measurement or is itreferring to C or even F?

My personal preference to use ºC/W (and not Θ) in my calculation (for dimensional analysis).

your example: (4Θ * 2W)º – Not even sure what that means at all???on the other hand: (4ºC/W * 2W) gives 8ºC

The W units cancels out in the calculation, the temperature shows up as ºC which makes a lot more sense.

Reply Report commentrj says:March 3, 2014 at 12:25 pm

Dimensional analysis really is the best thing.

Reply Report commenttechnolomaniac says:March 3, 2014 at 1:39 pm

Excellent point, the units are always always always more explicit this way. Too often I’ve hosed something upemploying shorthand / abstractions. The voice of my 3rd grade math teacher comes to mind “Don’t skip stepsand always show your work”. :)

Reply Report commenttekkieneet says:March 3, 2014 at 2:29 pm

BTW (4Θ * 2W)º is numerically correct, but it make a mess of units.> degrees per watt, is often listed as a symbol: Θ

So that expression have a unit of ºCº (an extra º) which make absolute no sense for the equation. Either youleave out all the units inside the bracket (which I don’t recommend especially when you are explaining things topeople) or you leave everything in and *not* add units to the whole thing.

There are only so many Greek alphabets and some of them are overloaded with different meanings. Looking hipcan make a mess of things.

I sort of learn some of that in grade school doing factions. I went into it again when the professor in a Physicclass was very much into it. Kind of disappointed to see them dropping the units as soon as they plug in anequation in my engineering classes.

Just being able to pick out nonsense from a equation *without* even knowing enough about it is very useful inreal life. I found a few of those in datasheet!

Reply Report comment3. Shawn Swift says:

March 3, 2014 at 2:37 pm

Can anyone verify that the calculations in this video are correct? I question it, because if accurate it would mean thateven if you had a perfect heat sink on the chip, the chip temperature would still rise to ambient + Jc.

Also, what if the ambient temperature is below Jc? The temperature wouldn’t actually decrease until the ambienttemperature was negative. But what’s special about a negative temperature versus a temperature that’s simply belowthat of the chip?

Reply Report commenttekkieneet says:March 3, 2014 at 3:25 pm

I didn’t bother to watch the video.

From your context, You are talking about a delta T(JC) for first paragraph.

A “perfect” (or infinite sized) heatsink would have a Theta Heatsink to Air = 0ºC/W. So basically temperature atheatsink is the same as that of the air surround it.

The junction temperature by definition be the ambient temperature + Junction to ambient difference. Thetemperature difference is caused by thermal resistance and the amount of power between transfer between thetwo. There are still the thermal resistance between the die (junction), the case, heat sink compound that is notaccounted for.

For the second paragraph, you’ll need to clarify what you are talking about. JC by itself only say Junction toCase and nothing about whether it is resistance or temperature difference. I can only guess here.

If your chip is colder than the ambient (e.g. the cold side of a peltier cooler). then there will be heat flow into thechip. The power would therefore have a negative sign and the equation should work out.

Not too sure about Theta heatsink to ambient whether it would be the same as air current (convection) is going adifferent direction.

Reply Report commentMichael says:March 3, 2014 at 3:37 pm

Indeed, even with a perfect heatsink, the junction would still be hotter than ambient, due to the junctiontocasethermal resistance.

Reply Report commentShawn Swift says:March 3, 2014 at 6:33 pm

You’re right. I guess I didn’t really have a clear picture in my head before about what R0jc really meant.

Reply Report comment4. Bubba Gump says:

March 3, 2014 at 3:21 pm

Or…

You could…

Read the Flir thermal imaging camera hacks on this website, buy a camera for a grand, and just point and shoot.

We bought one for our lab at work and it more than paid for itself already. There is theory and paperwork and thenthere is actual world. I’ve been doing this for almost 30 years and I hate to tell you this but sometimes (GASP!)Datasheets lie, or if the components come from China, they just cut and paste and the data does NOT line up.

Reply Report commentMike Szczys says:March 3, 2014 at 6:01 pm

I would recommend doing both math and real temperature measurement. If the two are not even close you knowsomething is wrong with the system.

Reply Report comment5. phase2682 says:

March 3, 2014 at 3:32 pm

Nicely done Bil, explanation was accurate. Glad you threw in the reliability discussion though in my experience mosttemperature accelerated mechanisms are 0.7eV activation energy rather than 0.99eV. 0.99eV will give you worst casedegradation in reliability for operation at elevated temperatures versus a lower temperature while 0.7eV provides mostconservative improvement of reliability for operation at lower temperatures versus a higher temperature.

Also like the 2112 tshirt!

Reply Report commentBil Herd says:March 4, 2014 at 9:40 am

Thanks phase. Basically what you said about ,7 was going through my head when I mentioned not going intoBoltzman’s. In this case I scarfed the example directly from page 3 ofhttp://www.ti.com/lit/an/snva509a/snva509a.pdf as a clean example which is probably why they chose .9 also.This way I am quoting TI rather than proving I need new reading glasses when extracting from the graph. >:)

Reply Report comment6. tekkieneet says:

March 3, 2014 at 3:41 pm

Or you apply the heatsink compound the wrong way (*) and the numbers would be very different.

Still having the basic knowledge and telling people to read more section of the datasheet instead of blindly using partsor getting technical info from 3rd party like youtube video or a blog is bad.

One of the places, I have access to a FLIR camera and I get very similar results on my board as my paper calculations.(There were people who thought my calculations was pessimistic.)

Reply Report commentpcf11 says:March 3, 2014 at 8:40 pm

I have access to my fingertip. I can usually tell with it if something is really hot, or not.

Reply Report commenttekkieneet says:March 3, 2014 at 9:02 pm

Can I borrow your fingers next time when I am trying to find out the temperature of a transistors runningon the primary side with 170V DC *while* the power supply is on or when one that is under faultconditions?

There is also the documentation side of things that I can put the picture of whether your finger is burnt in adesign and verification report. BTW I put an outline of board components overlay on top in the FLIRpicture. It took about same amount of time for a full set of data under various load vs just one datum doingthe old way of measuring individual critical components with taping thermal couples over various placewithout shorting them out.

Reply Report commentpcf11 says:March 4, 2014 at 12:00 pm

Shut it off then touch it if you’re that big of a pussy.

Reply Report commenttekkieneet says:March 4, 2014 at 12:42 pm

That doesn’t sound like a professional approach way of writing a verification report. Measurementswith uncalibrated methods and nonreproducible results are useless and waste of time.

There is nothing gain for chest pounding and you are being a liability for people you work for. You“recommendation” is of no use.

Reply Report commenttekkieneet says:March 3, 2014 at 9:05 pm

BTW I got an even better trick for the HaD crowd.

Stick a piece of thermal printer paper onto each of the parts on a board to tell if the chips are getting hot.That doesn’t tell you how hot the actual temperatures are.

Reply Report commentBil Herd says:March 4, 2014 at 5:37 am

True on the finger tip probe but one thing the math shows us it’s possible to burn your finger onsomething that is working correctly. (been there, done that. Had an imprint on one finger tip for a coupleof years)

Reply Report commenttekkieneet says:March 4, 2014 at 10:23 am

There are some cases where you intentionally run a component hot.e.g. Schottky diodes in DC application like OR’ing or reverse polarity protection. The forward voltagecan drop a few tens of millivolts if you keep the junction temperature hot. If the power feed drops, thediode junction would cool off so the leakage current would drop as well.

60C case temperature isn’t “hot” for some embedded applications that use parts with automotive/milspecs. I would expect that your finger probes would start top feel pain above that as you can get a 3rddegree burn at 60C with 5 second contact. I don’t know how well the burn chart translate to fingerprobes.

http://www.antiscald.com/prevention/general_info/table.php

Reply Report commentpcf11 says:March 4, 2014 at 12:01 pm

I don’t do jackass shit like that.

Reply Report comment7. Agent24 says:

March 4, 2014 at 12:09 am

What if you have a heatsink that doesn’t have a datasheet? Can you calculate some usable specification from itsdimensions and material?

Reply Report commentBil Herd says:March 4, 2014 at 5:35 am

Yes you can, I couldn’t find the TI Appnote that I used to use that was fairly straight forward approximation bytaking the size and finish of the metal.

With that said, the reason I showed a “standard TO220 heatsink” was to help get an intuitive idea of what theeffectiveness of a common heatsink is and when you might need one. Next time you see that something may runhot it may be useful to remember that you can probably deal with 2cWatts and that you need a little extra roomfor your heatsink.

Also if you go to Thermalloy’s website (or other heat sink mfgs) you may be able to pick a heatsink thatmatches your mystery heatsink and get an idea. And then multiply time .75 in case your off a bit.

Reply Report commentDax says:March 4, 2014 at 8:13 am

Theoretically yes, but all the easy ways to calculate it are quite inaccurate. The best way is to bolt a powerresistor on it and measure the temperature rise.

In principle, you take the mean path lenght through the bulk material of the heatsink to all points along thesurface to calculate the thermal resistance of the heatsink, or some approximation of it. Then you add to it themetalair interface thermal resistance according to the total surface area exposed to moving air as follows:

http://www.engineeringtoolbox.com/convectiveheattransferd_430.html

Reply Report commentBil Herd says:March 4, 2014 at 9:50 am

I should have also included that by all means you can get a temperature probe out and measure it as asystem/environment to take into account other variables such as airflow, etc. If you know the power beingdissipated you can then just solve from case to junction.

I once had a boss/president who decided that we would put a fan inside an airtight Hoffman enclosure. We triedto explain to him that all he was doing was stirring the same air and heating it further in the process. In thatexample I would have had to take a measurement as tangible.

Reply Report commentDax says:March 4, 2014 at 5:35 pm

Although a fan inside a closed box can mean the difference between overheating and working, becausestationary air is an insulator and moving air carries heat between the components and the inner surface ofthe box.

Reply Report commenttekkieneet says:March 4, 2014 at 10:31 am

If it is a DIY one off project, measure it. I bought a thermometer with type K thermal couple probe for $3

something recently. The cheap probe is decent and can easily do 175C or so for quick measurement. I tested itagainst melting ice/boiling water and it seems accurate enough for DIY project.

For anything else, it is worth while to spend the $$$ and buy something with a datasheet.

Reply Report comment8. lmn says:

March 4, 2014 at 4:15 pm

instead of “maximum power you plan to use with this component” should be “maximum power _dissipated_ with thiscomponent”. It’s a HUGE difference.

E. g. Sharp S116S02 optorelay. The datasheet says “Thermal resistance between junction and ambient 40ºC/W”. Loadit with 100W (e.g. halogen lamp). Room temp. 20ºC. Calculations would be: 40ºC/W * 100W + 20ºC = 4020ºC (!)

Reply Report comment9. bitknitting says:

March 8, 2014 at 6:54 am

um..(please pardon a potentially dumb question and tell me why? why? why?). When I look at this datasheet:http://www.nxp.com/documents/data_sheet/74HC_HCT125.pdfit does not have the thermal resistance value stated. It gives max ambient temp (125C) as well as total powerdissipation (500mA for the chip I’m looking at). So does that mean as long as the current is < 500 mA, no heat sink isneeded? (and thus I don't have anything to calculate…) or is there additional info I'm not considering?

Reply Report commentAgent24 says:March 9, 2014 at 3:10 pm

I think they don’t have it in there because there is no situation where that IC (or AFAIK any of the 7400 or 4000series) would need a heatsink. If you are doing something to it that makes it get that hot, you’re using it wrong.

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