The BBC Micro used the extended variant of the classic 'computer in keyboard' design; like the Apple II, the case went back far enough that you were encouraged to put the monitor on top of it.

All the electronics were in the back of the case underneath the monitor. There was a metal box around that part of the machine, but a plastic case around the metal. The BBC was noted for being very expandable, so there were a lot of chips in an expanded machine. The clock speed was double that of other 6502 computers. The power supply was inside the case, and early models used a linear power supply.

There was no cooling fan, no ventilation, and no easy way for heat to escape in any direction.

Yet the BBC Micro had a reputation for reliability, so failure from overheating cannot have been a common occurrence. Admittedly it was used on an island where the ambient temperature is usually quite low, but even so:

Given all of the above, how did it avoid overheating?

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    "the ambient temperature is usually quite low" ... try telling me that when we haven't just had multiple days in a row with temperatures in excess of 32C (90F) – Jules Jul 27 '18 at 12:26
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    @RossRidge - yes, the airflow of the BBC case is probably the best designed of any early 80s micro. It has plenty of ventilation slots, and lots of headroom above all of the components. – Jules Jul 27 '18 at 13:05
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    @Jules being an ex-pat, I have learnt that the detail people don't always seem to appreciate is: air conditioning is not an expected feature of homes in the UK. Let alone 30+ years ago. I left in 2012 so warn me if I'm out of touch. – Tommy Jul 27 '18 at 13:38
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    Afaict the metal you describe was only on BBC micros that were exported to countries with stricter EMC requirements, the UK BBC micros were in a plain plastic case. – Peter Green Jul 27 '18 at 14:14
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    It may have taken my reading to the last line of your question to realize by "cool" you did not mean "trendy". :-P – Charlie Gorichanaz Jul 28 '18 at 17:27

First of all, though the vast majority of BBC Micros were supplied with a switch mode power supply, anecdotal evidence suggests that the few that did have linear power supplies are actually more reliable. The surviving switch mode power supplies have almost universally required component replacements by now. But it seems at least some of the linear supplies are working without any replacement.

The power supply is kept in a separate box inside the case. Its airflow is distinct from the main ventilation of the case, being ventilated by a series of slots on the rear of the case (as pointed out by Ross Ridge in comments below). This means that most of the temperature produced doesn't influence the rest of the environment, which is somewhat better than many other micros. For example, the linear power supply of the Sinclair Spectrum 16K/48K was attached to a heat sink extending over a large proportion of the circuit board and caused various parts of the board to get quite warm in operation.

As well as the separated power supply, there's also quite a lot of space inside the BBC case. This can be seen from the second image on this page. The case height is about 70 mm, meaning there's roughly 50 mm clearance above most components in there. And the main circuit board sits in the space behind the keyboard, not underneath it, which improves airflow substantially.

Also, it's worth noting that the 6502 runs quite cool. Even at 2 MHz, it only dissipates 450 mW. (Compare this with the Z80, which dissipates about 1W to reach comparable speeds.) Outside of the PSU box, there probably isn't much that gets warm at all.

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    The Video ULA was really the only thing that got warm enough to need a heatsink. – ConcernedOfTunbridgeWells Aug 18 '18 at 11:53
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    I remember seeing various owner added small heat sinks on this myself included. Do recall there were a couple of chip types one may been less prone to it but a small jumper wire link was needed to make all the modes work , it was otherwise mode 7 and nothing else or all modes apart from 7 . I remember posting on one Acorn forum about this years back now. – AndyF Jan 11 '20 at 23:14

The chips on the motherboard were quite widely spaced. This is still a design consideration when making boards more heat tolerant. Also, the chips didn't generate the sort of heat that more modern, faster processors do.

Air did circulate through the BBC case. Entering through the ports on the underside and leaving via cooling slots at the back. There was no need for forced cooling.

That said, it was common to add a heat sink if you had updated your BBC. Either by putting in a 65C02 CPU or additional sideways ROM / RAM - or both. Other tricks were to put spacers at the back of the case to enlarge the cooling vents there.

However, as I said at the beginning, such tricks simply weren't needed in the standard configuration.

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    Ironically, the 65C02 would actually produce considerably less heat than the NMOS version, even if run faster. A couple of the custom support chips (ULAs) benefited from a heatsink though. – Chromatix Aug 5 '19 at 23:02
  • Worthy of a mention: Although not that common occasionally you will find one up for sale where a previous owner has hacked in the case a small fan. – AndyF Jan 12 '20 at 14:42

Given all of the above [now below], how did it avoid overheating?

Short answer: Not enough heat produced in the first place.

Long read: (as usual only basic physics needed).

(Standard Disclaimer: I hope I did remember all the formulas right, didn't add (many) calculation errors and most important picked the right English words.)

Heat in a device only builds up to a point where the energy put in (*1) put into than the device can is in equilibrium to the temperature needed to transfer exactly this amount of energy to the outside. It does not matter what kind of transport mechanism is used. Avection, convection, conduction or radiation, active or passive cooling is all the same and differ only by the resistance they provide - or not as we want to get it out without heating up much.

It's all about energy, and energy is counted in joule. Energy is power over time. Since it's sufficient to look at the BBC as static system (energy fluctuations are rather minor and equaled out over time), we can look at a fixed time interval and cancel out all time dependent calculation. Since one joule is one watt over one second, using one second is a good idea (*3), which now allows us to base further examination on power measured in watt (and equal watt with joule in all calculations). And number of watt is a value given to us by the manufacturer. Convenient, isn't it?

The BBC master's power supply was rated at (just) 35 watt (*4). This is what the PS was is supposed to deliver to the computer, its output. A PS is converting, and any conversion in this universe got losses. In case of a linear PS it's usually somewhere between 40 and 60%, while a switched delivers 80+%.

Assuming 50% efficiency (for the linear), we get about 70 watt to be dissipated as a first number. But let's get that number up to 100 watt. That not only simplifies calculations, but it also takes into account the many modified and all pimped up units.

The task is now to get rid of 100 watt of power.

Heat transfer is based like any transfer on a flow, the heat flow Q (*2) again measured in watt. So we just let 100 watt flow out, right, and temperature inside will stay the same as outside? Nope. Any flow needs a difference in height (or pressure) for water and in temperature difference here. Further this difference defines how much energy can be transferred through a given barrier within a specific area. In theory this allows to transfer any amount we need. Except, to keep our BBC working we can define a maximum temperature we do not want to exceed. Above ~150 °C solder joints may start to fail. Even more chips are usually only specified to work up to 70-85 °C. Assuming that the operating environment is usually below 35 °C we get a maximum usable difference of 40 °C.

The other important value is the surface the energy needs to pass. A BBC Master is roughly 40x35x6 cm^3 For the sake of simplicity we just use the top surface size, 40x35 or 0.14 m^2.

Bottom line, we want to get 100 watt through a surface of 1/7th of a square meter with not more than 40 degree temperature difference. Right?

The underlaying formula here is Q = A times delta-T times k. With A as size in square meter, T in Celsius and k as a material specific value. Flipping the formula toward k gives k = Q / (A x dT) with or 100 / (0,14 * 40) which equals ~18. Thus the material(s) used should have a thermal resistance of 18 watt per square meter per Kelvin or less. For real material this is described by a material specific constant called lambda and defined as W/m*°C. By dividing that constant by our value we will get therefore the maximum thickness.

So let's have a look at some materials and how thick a case made out of this can be to keep within the borders set above:

Material / lambda / thickness in cm

Silver   / 429    / ~24,000
Steel    /  80    / ~ 5,500
Concrete /   2.1  / ~    12
Glass    /   0.75 / ~     4
ABS(*)   /   0.18 / ~     1

(*) Using ABS not only as middle of the road plastic, but also as it's one of the less thermal conductive ones (which makes it great to be used in 3D printers;). most others are better conductor - like PET with 0.24.

With this number our case may have steel walls of like 55 meter, 12 cm concrete or 1 cm of ABS.

There was a metal box around that part of the machine, but a plastic case around the metal.

The relation above already shows that the steel cover can be (almost) ignored as its conductivity is about 400 times higher than plastic. And unless the plastic is thicker than 1 cm it's all is good.

The BBC's case is less than 3 mm of plastic which again can be easy turned into the temperature difference due to its proportional nature. With 40 degree temperature difference (to transfer 100 W) through 10 mm (1 cm) plastic, using only 3 mm thereof will result in about 12 degree temperature difference (40/10*3)

Or the other way around, with that 3 mm cover even a power consumption of 333 watt (100*10/3) would not damage the machine ... only typing might become a hot experience :))

Result: Not enough heat produced to drive the internal temperature anywhere critical

I love physics (and SI units).

Now, with using exact measurements and so on we will come to a difference of way less than 10 degree Celsius between machine and environment, making the BBC good for environmental temperatures of 60-70 degree. Keep in mind all calculations used where always rounded toward the 'bad' side using a rather large margin. Also, only conductivity between fluids (air) and a solid material was used. No special cases like radiation and so on.

Admittedly it was used on an island where the ambient temperature is usually quite low

Now considering the last few weeks I say living on an island might not make much difference - and more serious, while the BBC was a big player in the UK, it sold as well ok-ish in France and Germany - despite the almost non-existent support.

*1 - Energy put into a computer, gets almost completely turned into heat as other forms like EM radiation/light are quite low, and no mechanical output produced.

*2 - Usually it's noted as uppercase Phi or Q with a dot above - here Q will do it. Similar for all other symbols.

*3 - A second is always handy when calculating in SI units as being one of the fundamental units in MKSA :))

*4 - Here is, BTW, a real nice writeup about how to give more power to a fully loaded system - and stay within the original thermal specs as 60 W with 80% efficiency is about the same as 35 W with 50%. Nice, isn't it?

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    5500 cm is 55 meters, not 5.5. – Vikki - formerly Sean Jul 29 '18 at 0:17
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    What this answer ignores is the air. Air has terrible thermal conductivity and if the air stayed still the machine would almost certainly overheat, but the air doesn't stay still it convects. So the real question which is much harder to answer is how efficient that convection process is and hence what is the effective thermal resistance from the heat-sensitive components to the inside of the case and/or the outside air. – Peter Green Jul 30 '18 at 14:19
  • @PeterGreen Yes, your're right, but heat isn't just transported in a fluid via convection, but also due equalisation. For all practical purpose, especially with an enclosure that small, it can be assumed that the air has the same temperature over all. Convection only becomes realy relevant at larger domains. Here effects will be within less than 10% - irrelevant when we have a 10 times margin as shown. Similar one could ask about the heat ransfer away from the case, as that also includes many factors. still, for practical purpose in an average environment this can be ignored. – Raffzahn Jul 30 '18 at 14:24

In general, computers of the day, even into the Pentium era (mid 1990s) and beyond, did not produce enough heat for it to be an issue - as long as the chips had basic ventilation. Simply the power draw was not enough. Even the first, second and third generation Pentium processors often did not need a cooling fan and instead relied on passive cooling via a heatsink.

I personally remember being astonished that we needed to put a cooling fan on our 486DX 33 MHz processor when we overclocked it from 33 to 100 MHz (try getting a stable 3x overclock with air-cooling today!). Similarly, I remember having a Pentium 3 450 MHz processor (released in 1999) with only passive cooling, although it did have a somewhat sizeable heatsink. It was only really (in my recollection, for home computers) from around 2000 onwards when Intel chips started to need a heatsink and fan as a standard thing. Commercial computers (servers etc.) would have had forced cooling significantly before this, though.

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    The first Pentium 90 my office got would have been a lot more stable with a cooling fan. It crashed every day or so - which was a nuisance for a machine that was supposed to run a modelling programming which took a week (which is why we wanted the faster Pentium). – Martin Bonner supports Monica Jul 29 '18 at 14:24
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    Looking at my PC construction history (fadden.com/tech/my-pcs.html), my 90MHz Pentium 90 from 1995 had a CPU heat sink + fan. The 266MHz Pentium-II that went into a machine in 1997 came on a card with the heat sink and fan built in. I don't know that it was necessary, but IIRC it was pretty typical for 90MHz Pentium and later to have a fan. – fadden Jul 29 '18 at 17:50
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    @fadden - I ran a 100MHz Pentium 24hrs a day for approximately 5 years without any form of heat sink, and it never suffered from it. Having adequate airflow in the case is enough; a fan was only necessary if that wasn't possible. The Pentium 100MHz had a typical dissipation of about 4W; without a heat sink and with moderate airflow its thermal resistance is about 12C/W, which means that as long as the case temperature remains under about 35C it'll stay at a reasonable temperature that's unlikely to cause harm. I should have used a heat sink for it, but practically it was fine without. – Jules Jul 29 '18 at 21:58
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    Adding a fan would have been completely unnecessary, however. – Jules Jul 29 '18 at 21:59

Almost all these old machine did have SOME ventilation. The BBC seems to have less than normal, but there are some ventilation slots underneath, and on the back on some of them.

That said, it does seem to have less than similar systems of the era.

The fact is - those old 70s/80s home computers just didn't run as hot as modern Intel/AMD systems! Your typical 8 bit CPU didn't even require a heat sink on top, let alone a fan. Even most 16 bit CPUs were fine just running without special cooling.

The modern Intel-derived desktop CPU evolved in such a way that performance was gained at the expense of thermal efficiency, and it needs a dedicated cooling system. Laptops are generally using a different (but compatible) range that uses less power and needs less cooling.

On the other hand, your mobile phone or tablet is almost certainly running an ARM CPU, which despite its relatively high performance, can be put into a small device with no large heatsinks, let alone fans or liquid cooling. Those old machines like the BBC micro should be though of as more like your mobile phone in terms of cooling rather than some liquid cooled gaming PC. In fact, Acorn, who made the BBC, later designed a new CPU of their own, a revolutionary new design, known as the Acorn Risc Machine, or ARM...

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