As a diversion for the weekend: Have you ever wondered why computers run so hot? No? Okay, I’ll tell you. It’s actually kind of a hoot. (We’ll get back to the more serious topic of algorithms and AI, and wrap up that series, next week.)
You kind of have to wonder. Humankind has gone from oil and gas lamps, to incandescent copper filaments, to fluorescent lights, and now to LEDs. The trend here seems towards cooler more efficient light sources. But computers seem to need bigger and bigger fans!
The short answer: It’s all those short circuits!
Millions (if not billions) of short circuits happening several billions of times per second!
It’s like an army of tiny imps running around inside your computer dropping crowbars across the power rails (but just for a nano-jiffy; they pick them right back up).
If you work much with computers you probably know that the heat is correlated with the computer’s (clock) speed. The faster a computer runs, the hotter it runs.
When you know what’s going on under the hood, the heat makes perfect sense. It’s due to how the transistors in modern computer chips work. (And has nothing to do with imps.)
To make sense of it, let’s back up and talk about what a transistor is, especially with regard to computers.
Don’t worry, we’ll keep it really simple; it’s Saturday!
What’s more, talking about them in the context of computers turns out to be a much easier discussion for the same reason a lot of “computer stuff” is actually very simple when you come down to it: It’s all just ones and zeros.
I mentioned transistors when we talked about creating a computer model of a computer (in The Computer Connectome).
At the time, we treated them as tiny black boxes with three connections (leads). Now we need to look inside the box.
The transistor symbol shows the three leads as lines leading away from the circle symbolizing the transistor.
(For the record, clockwise from left side: base, collector, emitter)
Basically, think of them as light dimmers. Current enters from the lower right (the emitter) and exits from the upper right (the collector).
A much smaller amount of current gets siphoned off and exits stage left (through the base). The actual amount exiting the base proportionally controls how much can leave the “main” exit.
By varying the amount of base current, you control the collector current. Just like turning a light dimmer up and down.
There’s a lot more to the picture when we use them as dimmers, but that’s the analog world. The digital world is much simpler (just ones and zeros).
Computers rip out the light dimmers and install simple switches. You can have any light setting you want so long as it’s either on or off.
Now, either some current leaves the base or not, which switches the transistor fully on or fully off, respectively. That’s binary logic in action. And it makes designing circuits way easier!
Your average computer these days has a CPU chip with billions of transistors. Other chips inside the box also have high transistor counts.
For a number of engineering reasons, these chips generally use a certain kind of transistor, called a field-effect transistor — FET (“phet” or “eff-ee-tee”). [One of the key reasons is that FETs are voltage-based devices, whereas the transistors just discussed are current-based.]
In particular, again for reasons, computer logic chips use a certain kind of FET, called a metal-oxide semiconductor FET — MOSFET (“moss-phet”). [These are the ones that have to be handled carefully. A static zap can destroy them.]
But none of that matters. What matters to us is that they use a certain MOSFET configuration, called complementary MOSFET — which fortunately has the mercifully short nickname, CMOS (“sea-moss”).
It matters because the CMOS configuration has a funny thing about it.
As you can see from the diagram, it consists of two MOSFET transistors connected in a totem-pole fashion.
The little plus-in-the-box is the positive power supply, and the triangle of horizontal lines is ground (zero volts), so the two transistors bridge the power rails.
If they were both turned on, that would be a dead short!
As it stands, if one is on while the other is off, the output is connected to either ground or plus voltage. Those are the binary “one” and “zero” values.
The input to these is, likewise, a connection to ground or plus voltage.
If you look closely, the two transistor symbols aren’t the same. The little arrows are in different places and point different directions. That’s because the transistors are different; they have reversed polarity!
It’s like one is the anti-matter version of the other. Or the Bizarro version. They operate the opposite of each other, and this is critical!
The input signal connects to both, but since they are opposites, an input that turns on one turns off the other. This ensures that both transistors aren’t on at the same time.
Normally. And that’s the rub.
For a very brief instant — a nano-jiffy — when they switch, both transistors are on. There is a dead short through them.
Doing it that way allows them to switch much faster (because reasons), but it means they generate heat during the switch.
How much heat depends on the number of transistors (of course) and also how often they switch, which is where clock speed comes in.
The more often they’re switching, the more often they’re shorting. Computers today switch billions of times per second.
And they have billions of transistors (but not every transistor necessarily switches at each clock tick).
But millions and millions do each billionth of a second, and all those shorts generate significant amounts of heat. That’s why modern computers need heat fins and big fans.
(Once, just the computer box itself had a fan, but over time the CPU chips needed their own little — or not so little —fans.)
Sit and watch your computer do nothing for 60 seconds. If only half your transistors switch every clock tick, assuming a billion transistors and a 2 gHz clock, that’s 60 quintillion dead shorts.
No wonder the poor darlings need fans to cool off!