I have felt this (?) often. Dragging fingers over a (metal) Mac Mini, some random laptop, a metal keyboard. Presumably this feeling triggers already at low power levels, otherwise I'd probably be dead.
This is because the device is using an isolated, ungrounded switched-mode power supply to convert mains voltage efficiently and using small, cheap and light components. There is a small capacitance (and extremely high resistance), between the primary and secondary windings of the transformer in the SMPS. An SMPS switches at 100s of kHz, and with this virtual capacitor inside the transformer you couple the high frequency switching into a 100kHz+ voltage noise and this would radiate out along the power conductors and your product fails its electromagnetic emissions certification. Now you can't sell it, and this is usually a problem. It might also interfere with other devices like pacemakers or fire alarms or phone signals or whatever, but it's the not being able to sell it that will really upset your boss. Bloody interfering big-state regulators, can't even blast EM radiation in your own customers' homes. So you have to fix it.
To counteract the noise, you attach a (special, you'll see "Y") capacitor between the primary and secondary sides, right next to the transformer. Now that high frequency noise sees an "easy" short-circuit and it is contained in a small loop within the power supply, not leaking out into the outside world. This needs to be much larger than the (very small) transformer capacitance so it dominates that system.
The downside of this is that now you have an appreciable capacitance between the primary and secondary side. In a grounded system, you connect the primary side of that big capacitor to ground and everything is fine. In an ungrounded system, you can only connect it to something referenced to one of the two wires you have: line or neutral. This means that a small current will flow into the capacitor from each side at line frequency (50 or 60Hz) and the secondary side, connected to the case, will float at some appreciable fraction of mains voltage. You, a damp meatsack, prodding the secondary side with your fleshy protruberances, have capacitance as well. So when you touch this case, that's floating at AC voltage around 100-200V (depends where you are in the world) there's a small current that flows between you and the case as well as the charge flows back and forth. This is what you feel via your nerves, which can detect miniscule currents in this range. Because the capacitors linking you to the mains are, in an absolute sense, small (though far larger than the transformer capacitance), the current is limited to a small amount, far under a milliamp. Actually this would still happen without the bigger capacitor, as there's already some capacitance there, but it could only provide an imperceptible current even to the bio-miracle of the human nervous system. So you have to balance EMI reduction (more capacitance better) against this sensation (more capacitance makes it more noticeable and eventually with a really huge capacitance it would be its own safety hazard).
Why a special capacitor? Well, if were to fail to a short between primary and secondary, now your isolated case is not isolated, it's connected directly to the mains supply and there's no earth wire to dump current to and blow the RCD. So if it didn't immediately blow the fuse (and even then, it needs many 10s of amps to do that to the whole circuit if you don't have fused sockets or plugs in your country, and even then it can also set itself and things nearby on fire without blowing a fuse using only an amp or two if you're unlucky) because it's sitting on, say, metal earthed desk, it's now still floating at mains voltage. However, now it won't deliver microamps, it can deliver a lot more though that carbonised previously-a-capacitor-now-a-small-resistor. If you touch it and have a decent connection to ground, instead of the interference with pacemakers, it can become a turbocharged pacemaker, but your heart doesn't work so well at 50/60Hz and may object to this by going on strike. Regulators for some reason think this is a problem, so you use a special safety capacitor called a "Y" or "X" type, depending on exactly how you use it, which is designed to satisfy them that you probably won't discontinue anyone's sinus rhythm or burn down a building (even if they've actually consented by buying your product!)
So, while is this not an intentional effect, it's also an calculated aspect of the power supply design in the absence of an earth wire and is not a sign of a defect or danger. Other than reminding you that the only thing between you and mains voltage is insulation that's carefully specified, regulated and tested to be safe, for some standardised definition of "safe", of course.
You can also see a much stronger version of this effect with a plasma globe (remember them?). The frequency and the voltage is much higher than 50Hz mains, but the principle is the same: the sphere surface is isolated plastic, there's a very small capacitance to the coil. The current flowing in the low pressure gas in the void is what makes the tendrils. With your with your hand on it, you float at very high voltage and can even produce a tiny sub-millimetre, continuous spark to other objects as the small current flows back and forth into this small capacitance. In retrospect, that plasma globe probably has questionable EMI behaviour when doing that!
Interesting! Thanks for the cool info dump. Lack of earthing seemed like the obvious source of problems, but I knew nothing about the details.
Given that one of the devices in question was a Mac Mini (which did indeed have a two-prong plug without earthing), I wonder why Apple didn't put in a little more effort to give consumers an earthed plug. There's more than enough margin on their devices, and computer-like appliances normally have grounded plugs here anyway (Netherlands).
Unearthed two-prong plugs are slightly more convenient here because they fit in more sockets. But I'd prefer earthing over that small convenience.
If you can't rely on your customers always having an earthed outlet to hand (as you can't in countries that permit unearthed sockets, or often have dodgy electrics), you have to design it to work without the earth line in case a customer uses that socket.
Adding an earth conductor would require a thicker, less flexible 3-wire cable, a bigger plug (in the countries that have the option of a 2-pronger) and a three-pin power connector if it's not a captive cable, which is bigger and heavier. To ground that Y capacitor node means taking the earth conductor from the socket to near the SMPS transformer, and you have to maintain strict clearance and creepage distances between that conductor and the live/neutral regions at all points. Modern power supplies are very compact, so this is actually more of an ask than you might expect.
As the device is already designed to meet or exceed safety requirements without the earth wire, all it does is avoid the tingle and add weight, volume, design effort and component cost. As mass market devices produced by the hundreds of millions, this is not deemed a good tradeoff.
You can avoid this on your own, however, by grounding the case yourself, by connecting any exposed metal to a local earth point. Remember that Macs are anodised, so the surface isn't really conductive, (which actually amplifies the effect at the point of contact as a thin insulator is also a capacitor!).
I see! Apple had the option to supply a fat three-prong plug here, which wouldn't fit in any unearthed socket, except for fake ones (which do exist but are generally older extension cords over here). But as you say, they likely didn't think this was worth the trouble + customer inconvenience if they couldn't rely on earthing elsewhere.
At least in the UK, it's infamously only Earthed if you use the long extension cable (instead of plugging 'wall wart' in directly) that they no longer supply.
> Unearthed two-prong plugs are slightly more convenient here because they fit in more sockets.
That argument doesn't work here, all are 3 pin. It's a conducting third pin too, even though it's NC, which isn't even compliant.
My suspicion is that it's way more noticeable on European voltages (frequency could also be a factor?) so they don't notice in the US and don't care. I have a few products that do it; they're all US companies except one Japanese (slightly lower than US even at 100v, and product primarily sold to US I suspect).
My Framework laptop will shut down if it touches a charging but un-Earthed MacBook.
I absolutely *loved* reading your comment! It was the right mixture of knowledgeable and humorous, and kept me gripped, not unlike a fine wodehouse novel. :)
I don't think it's quite the same thing. Laptops in particular often have machined (?) surfaces that feel very weird to the touch, regardless of electricity. HP in particular is a regular offender, though I haven't touched many apple computers.
I've experienced it and it's markedly different how a laptop feels when plugged into the wall Vs not: it's because of a relatively high voltage (but also high impedance, so not dangerous, though sometimes painful if it focuses through a small point like a USB cable) hum you tend to see in the ground of most modern power adaptors due to noise reduction capacitors.
No, this is a different thing. It's leakage current from ungrounded power supplies (the two-prong apple power brick) and it's extremely noticeable. Couple hundred microamps, feels like licking a 9 volt battery.
Can you provide an example of an object where this can be felt? I’ve touched hundreds of painted electrical enclosures on energized transformers, panels, switchgear, etc and have never noticed this.
There are many examples of humans being able to observe a difference, yet deny their capability to discern on the basis of abstract rational knowledge.
Another example is frame rates: while its well known that the bandwidth of light sensitive cones on the retina is around 30Hz cycle frequency (and thus ~60Hz framerate), many gamers felt the difference of higher framerates, even though many of the same gamers denied observability of this difference on the basis of knowledge of this basic biological fact.
The biological fact is not wrong: if screen pixel projects are at rest on the retina. But when the eye is rotating to follow a depicted object on the screen, then speeds higher than ~60 pixels per second result in motion blur that would be absent when observing a real life object moving at the same angular speeds. The biological fact is still true, but simply not naively applicable in the context of motion blur due to eyeballs rotating while observing a display.
It's more complicated than that, brain cuts out the blurry "in between" frames when you move your eyes. Which means it has to extrapolate the images from before the movement to fill the gaps. There's many weird effects of this hack - for example when you look at something moving very fast (like wheels of passing vehicles) but don't move your eyes - it looks blurry. But when you move your eyes - the frame in your brain "freezes" and you can notice non-blurry details on the fast moving things. There are even ways to force eyes to see stuff from the past (like seconds' hand on a clock moving at different rates depending if you move your eyes).
You can't actually say that human eye has a strict fps, so the higher screen fps the less weird interactions with the ugly hacks in our brains.
when your eyes are motion tracking an object there is no cutting-out going on, that only happens when the brain intentionally decides to track a new object
the phenomenon I describe is much simpler, and is remediated by having the backlight of a 60 Hz display have a very short single on strobe. If they backlight remains on or flashes multiple times during a single frame then the pixel will draw a long streak on the retina (because the eye was rotating to track a depicted moving object)
I'm not sure if you've seen them, but there are LED glow sticks that look normal when stationary but leave sort of a trail of distinct after images rather than a smooth blur when waved. I've seen LED Christmas lights that have a similar effect when you scan your eyes past them quickly. It's called the phantom array effect, and it's caused by a flickering light (LEDs lit with less than 100% duty cycle) moving rapidly across the visual field.
Mitigating sample and hold motion blur by blacking out the display for some of the frame time turns your display into a flickering light source, so it can potentially produce the same effect for small bright objects on dark backgrounds when either the object moves rapidly on screen or you scan your eyes across the screen. It's fairly niche in that it'd only affect some rather specific scenes and even then can depend on how the scene is viewed (it won't occur if you track the only fast-moving bright object with your eyes, and it's much reduced in higher ambient light), but the flicker fusion rate is generally several kilohertz, so you'd need to boost the hell out of the frame rate to guarantee the effect is eliminated altogether.
More generally, an object that is moving rapidly enough for its screen position to change by many pixels per frame can have its motion look jerky even without motion blur. And, in fact, motion blur can help disguise that.
About turning the screen on and off, it’s very slow if I want to change it in real time. It would be nice to have a little nob on the side and just change the phase one way or the other in correspondingly :)
I think you're the one denying rational knowledge here. People are different, and there are in fact people who cannot tell the difference between higher frame rates, as shown in studies.
Just because you can see it doesn't mean others can.
the problem is not rational knowledge, its misapplying rational knowledge in circumstances where it does not apply.
its extremely easy to reproduce, and I have not encountered anyone who fails to see it once told how to reproduce it:
suppose your screen is 1920 pixels wide (Full HD). suppose a patter contains high spatial bandwidths, i.e. high spatial resolutions like text.
obviously if a monitor is able to display say at a 60 Hz frame rate, then motion blur would be essentially absent if the font bitmap is moved 1 pixel per frame, or at least the motion blur would be constrained to 1 pixel of horizontal blur.
to traverse about 1920 pixels from the left side of the screen to the right at 1 pixel per frame (or 60 pixels per second) would take 32 SECONDS. Half a minute!
consider much faster velocities, say 3 seconds for a word to move from the left side of your screen to the right, thats motion blur such that a pixel is blurred the length of 10 pixels! Most people would not consider this very fast at all.
now take note of the DPI of the screen and print the same text at the same dimensions on paper, and hold it up close to your screen, and move it from left to right in about 3 seconds, while following it with your eye. You wont see motion blur on the real piece of paper, but you will on the digitally rendered text of same dimensions and velocity...
once you understand this phenomenon, you will understand in what sense the flicker bandwidth of human retina at rest with respect to a visual stimulus should not be confused for a sufficient framerate to accurately reproduce the perception of visual stimuli representing motion, unless the backlight is strobed at a very low duty cycle. but then flicker becomes visible again unless we bump the framerate up a notch again.
I’d say lots of behavioural cues, for instance related to dominance. Many people feel like they’re behaving freely because they’ve completely internalized the constraints their social position imposes upon them.
The McGurk effect https://www.youtube.com/watch?v=2k8fHR9jKVM is another example, which might be described as people learning to lip read without them being aware that they have done so.
Yeah, I don't understand what I was supposed to be seeing or hearing in that clip, and they never bothered to explain it.
The guy says "Baa" and we see his lips move accordingly. Then we hear him say "Baa" again, but this time he's obviously pronouncing something else, judging by his lip movements. We don't know what he's saying (or at least those of us without lip-reading skills don't), but nothing unusual or tricky seems to be happening.
From making the same motions with my mouth I was able to figure out that the other thing he's saying is "faa", so I'm guessing some people literally hear "faa" even though the audio is "baa"?
> how many other things there are that people subconsciously learns, without it ever becoming obviously noticeable for them?
95%+? Stuff like how to jiggle keys on a keyring to get the right one for the door you're about to unlock? This only works for your keyring, it's highly kinetic, tactile, initial condition dependent etc, yet yet can usually be done unconsciously IME with the 6 keys on my keyring.
I _think_ I can tell whether a car is an ICE / hybrid / electric just by the way their weight shifts around corners. My friends maintain they cannot, but under test conditions,
I'm sure they'd be fine.
Are you sure it's weight distribution and not conducted engine vibration and gear shift patterns? Seems like it'd be difficult to control for (assuming we're already controlling for engine noise).
I'm pretty sure it's weight distribution. Like I can tell the difference between a hybrid Camry and a pure ICE Camry based on how it bobs over speed bumps, not just in cornering / acceleration.
I remember the first time I noticed that I could guess correctly which fingertip a friend shining a tiny red pen laser was shining it on, just from minuscule temperature differences.
That you can feel artificial lighting as heat (the actual light, demonstrated as it was a laser some distance away) at such a low power level (<5mW) was pretty surprising to me.
I noticed if I stare into the forest without focusing on any particular thing my vision starts to shimmer a bit and sometimes there are distinctive little perturbations in my peripheral vision.
My mind wants to immediately look directly at the perturbations at an almost instinctive level. If I do look I frequently see a leaf moving in the wind or a bird or squirrel moving around in the tree.
By intentionally using this effect rather than actively scanning the forest I find I am much better at finding birds when bird watching or seeing larger animals such as foxes, deer, or snakes moving in the grass.
It's amazing how many things our bodies are capable of that we don't know about until we step away from electronics and machinery and just let the body do its thing.
That just makes me wonder how many other things there are that people subconsciously learns, without it ever becoming obviously noticeable for them?