Difference between revisions of "Electronics notes/Electricity and humans"

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These measure the current difference between live and neutral wire, and assumes that there being ''any'' difference means it's leaking to earth, probably via a human, and cuts power.
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These measure the current difference between the current going through the live and neutral wire.
  
<!--
+
It cuts power when there is, because one of the options is that the extra current path is via a human.
Component-wise, it's basically a transformer around both live and neutral wire.  
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When current in is equal to current out, that transformer outputs nothing. -->
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{{comment|Component-wise, you can think of it as a transformer around both live and neutral wire. When current in is equal to current out, that transformer outputs nothing.)}}
  
 
They are typically part of your house's installation, not your devices.  
 
They are typically part of your house's installation, not your devices.  
  
  
For home installations they trip when it's on the order of ~30mA for ~20 milliseconds {{comment|(varies, e.g. with type and size of installation)}} because most humans will survive that, and is long enough that it isn't too sensitive to stray current.  
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For home installations, they trip when it's on the order of ~30mA for ~20 milliseconds {{comment|(varies, e.g. with type and size of installation)}} because most humans will survive that, and is long enough that it it;s less sensitive to stray current and inductive effects.  
  
 
Yes, you can get RCD outlets, and protected extension cords, which can e.g. make sense when you do electronics repair work.  
 
Yes, you can get RCD outlets, and protected extension cords, which can e.g. make sense when you do electronics repair work.  
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Note also that you cannot count on them working on floating-ground systems.
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Note also that you cannot count on RCDs working on floating-ground systems.
 +
 
 +
The RCD detection mechanism itself works,
 +
but the assumption that the leftover mismatch travels via ground would be more of a guess.
  
A floating system means one where there isn't any wired path to ground at all,
+
...because there's not any wired path to ground at all, which also means
and as long as that's true you can't become a path ''via'' ground either.
+
you can't become a path ''via'' ground either, so in theory an RCD would also not be required - you could touch a live chassis, or the phase wire in general.
  
An RCD wouldn't work, but also wouldn't be required.
 
In theory, and not so much in practice.
 
  
Also it would defeat chassis ground protection.
+
In practice it's a bad idea,
 +
because this is only safe as long as there isn't some ''stray'' connection to ground.
  
The last two are reaons why this describes basically no homes (there are more).
+
In practice it's impossible to stay away from accidentally grounding something,
 +
this just means you don't know ''where'' it's connected.
  
 +
Or, more importantly, with how much resistance,
 +
which amounts to it becomeing harder to tell where to install RCDs
 +
so that they are still guaranteed to trip.
  
In practice it's impossible to stay away from ground,
+
This is one of a few reasons pretty much all electrical installations are earted with a big ole pipe near the main distribution board (and why there are typically strict rules about extra earths)
this just means you don't know ''where'' it's connected,
+
or with how much resistance.
+
  
This basically means you ''can'' be the path for current
 
but also that it becomes harder to tell where to install RCDs
 
so that they are guaranteed to trip.
 
  
 
{{verify}}
 
{{verify}}

Revision as of 20:51, 23 May 2020

This is for beginners and very much by a beginner.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.


Some basics and reference: Volts, amps, energy, power · Ground · batteries · resistors · changing voltage · transistors · fuses · diodes · varistors · capacitors · inductors · transformers · baluns · amplifier notes · frequency generation · skin effect


And some more applied stuff:

IO: Input and output pins · wired local IO wired local-ish IO · · · · Shorter-range wireless (IR, ISM RF, RFID) · bluetooth · 802.15 (including zigbee) · 802.11 (WiFi) · cell phone


Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer ·


Audio notes: microphones · device voltage and impedance, audio and otherwise · amps and speakers · basic audio hacks · digital audio ·


Less sorted: Common terms, useful basics, soldering · Arduino and AVR notes · ESP series notes · PLL · signal reflection · pulse modulation · electricity and humans · resource metering · Microcontroller and computer platforms · SDR · Unsorted stuff

See also Category:Electronics.


Static electricity

Intentional

Sensing

ECG, electrocardiography

Measuring the heart muscle (https://en.wiktionary.org/wiki/cardio- -cardio-]), which at skin surface is seen on the scale of millivolts.(verify)

This can be amplified by a high impedance differential amplifier

preferably a instrumentation amp to lessen the effects of AC line noise, because it's common mode interference)
and you want to use a shielded wire


See

EMG, Electromyography

Measuring skeletal muscles (-myo-), which seems to be one the scale of dozens of millivolts (verify)}

See http://en.wikipedia.org/wiki/Electromyography

EEG, Electroencephalography

Measuring activity in the brain (-encephalo-) - which (outside) is on the scale of a few millivolts and smaller (verify)

See http://en.wikipedia.org/wiki/EEG


Common EEG wave categorization

delta, δ

theta, θ

alpha α

mu μ

beta, β

gamma, γ


Galvanic skin response

galvanic skin response is sold as approximating mental states through measuring the electrical resistance of skin.

The idea is that skin conductivity changes based on hormones the brain produces when in a state of emotional arousal.


They are the main basis of classic lie detectors (alongside things like pulse and respiration) - the ones that are no longer admissible in court.

One problem is that it cannot distinguish between different forms of arousal ie. anger, fear, startle response, and sexual arousal.

Or distinguish from someone doing this intentionally.

Or consider that people can distract themselves intentionally. Which can be trained.

Or distinguish between people getting nervous not because of guilt but thinking of social implications or even just anger because of strong personal ethics.

Or consider that sociopaths won't have these responses.

retinal electrooculography

Stimulating

Conductive

TENS, Transcutaneous Electrical Nerve Stimulation

Aims to desensitive nerves, so is used for short-term pain relief.

Higher frequency (and less strength?) than EMS, only enough to feel.

Electric massage is closer to TENS than EMS(verify)

See also:


EMS, Electrical Muscle Stimulation

Also known as NMES (neuromuscular electrical stimulation) and electromyostimulation.

Used to help muscle training, such as that to avoid atrophy.

It uses pulses that are strong and long enough to trigger muscle contraction, and helps engage more of an muscle that you consciously engage. It's not that effective without your help, though.


See also:


Massage

Purple wand

On (not) causing tingling

Laptop tingle / zap

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)



Symptoms:

  • continuous tingling on exposed metal (sockets, screws, metal unibodies, e.g. in your hands, on your lap)
  • felt only when the adapter is connected
  • Felt more clearly when
the contact are is small (similar current focused through fewer of your neurons)
you have good contact with the device (e.g. sweaty hands)
you have good contact with the floor (e.g. no shoes or socks, moist concrete)
When measured, this is AC, and up to a few dozen volts.
also, some electrical codes that "everything delivering more than some amount of Watts of power must be grounded"
also ground may be helpful to shield the brick's EM emission, for EMI compliance
also, companies that design for many countries may comply with many at once, meaning they're sometimes redundant beyond necessity


Adapters can also choose comply with Class II, which roughly says "insulated with enough layers that you can never really touch anything". Which is not very hard to meet. In some devices it's even more convenient than Class I, in others less.


Whether adapters are Class I or Class II is not directly relevant, as that applies mainly to the black block's dealing with wall voltage, rather than its output. However...


Connecting AC ground to DC output

As components, transformers and switch-mode units typically do not tie the AC-side ground to their output-side, because device builders want this as an option.

(There are specific categories of products where it is required, but in general it's not, and there are upsides to not doing so, e.g. avoidance of ground loops)

Wallwart adapters often don't do this either. And, since most don't use grounded plugs, can't. They are implicitly floating/isolated outputs. If they meet relevant safety specs while doing so, there's no direct added point to adding safety ground (...yet this is a more complex discussion).

It turns out that laptop adapters regularly do connect the two. If it's there on the main-side in the first place, of course.

a 2-pin on the mains side means there's no earth to be connected.
a 3-pin on the mains side, it is relatively likely (but not guaranteed) that AC ground is tied to DC output ground.


Sooo... Is grounding of the laptop (via the DC ground) a good idea or not?

In both cases it's safe, because any serious product will be meeting other certification that ensures this.

Beyond that, the best choice depends on which problem you are most interested in solving.

Not grounding the laptop (2-wallplug-pin adapters, and a few 3-pin that don't) will in theory give completely floating output. In practice capacitances in and around the power supply means it's still pulled to at most roughly half-line-voltage away (often with some line-frequency waveform on top). Often specifically due to decoupling capacitors in the design (which you need). The amount will varying with capacitor size, and (if compliant to e.g. UL) will necessarily be at a current capacity that cannot be harmful, but will often be enough to feel.

upside: avoids ground loop when you connect with common-mode interconnects
downside: can have this tingle issue

Grounding the laptop (most 3-wallplug-pin) avoids the tingle effect, by being able to siphon it off. However, you can more easily create a conductive ground loop - but only when you connect to another grounded device with a cable that connects ground to another ground-referenced device (which e.g. includes a lot of cases involving audio).

upside: avoids the tingle
downside: makes it easier to get (ground loop) interference issues on baseband interconnects - like consumer audio cables between laptop and amplifier
In more pathological cases (powered, similarly-grounded devices, including some DIY) it could even mean damage


In laptops, it is additionally possible that voltage conversion within the laptop may be inadequately filtered (often due to size constraints) and also help introduce some tingle(verify), but this would only be relevant when grounded via something else.(verify)


There are sometimes specific workarounds, e.g. if you have exactly one common mode audio connector you could isolate it (to avoid a conductive ground loop).


(Side note: a third pin on the brick's laptop-side output output will rarely/never be a separate ground. That's typically communication, e.g. identifying the power capacity of the adapter, and sometimes verification to make knockoffs harder to make)




http://www.unitechelectronics.com/sparks.htm

https://www.google.com/search?q=tingle+unearthed+devices

http://www.thailandguru.com/grounding-earthing-electrical.html

https://www.aptsources.com/wp-content/uploads/pdfs/Floating-Output.pdf

-->



Phone zap / tingle

Mostly similar to the laptop example above.

Many of these chargers are ungrounded wallwart-style, making this a likely thing.


One difference to the laptop case is that there are a lot more cheap-and-particularly-crappy phone chargers out there (than there are low-grade laptop adapters).

Some of them just low quality output, a few actually unsafe.

See e.g. [1] [2]


It the touchscreen doesn't work as well, or at all (interferes with the way capacitive touch works) whenever it's on a charge cable, it's probably just a low-quality charger and you can fix that by buying a better one. It can be hard to tell quality from looks. Reputable brands are a decent though not sure-fire way, and there are some decent cheaper-brand ones.


Less intentional / On (not) killing people

Where do things get risky?

"It isn't the volts that kill you, it's the amps"

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Amps kill, yes.

But I=V/R, so amps are tied to volts.

And to resistance, but in most circumstances resistance is relatively fixed.

So given the same voltage source, and roughly the same resistance, the current will also be relatively fixed.


For example, A 12V car battery is usually capable of at least 50A, for the starter motor. The only reason that motor draws that much is because it is designed with less than an Ohm of resistance. That motor will always have roughly that resistance, so always draw dozens of amps.


The resistance across a person's skin is on the scale of dozens to hundreds of kiloOhms. So that car battery across your skin -- and let's be pessimistic and say you've worked yourself to be drenched with salty sweat and resistance is as low as 1kOhm across your skin, means I=12V/1000kOhm, or ~12 milliamps. Across your skin you will probably not even feel that. And if you're dry the resistance is multiples higher and the current multiples lower.


Human resistance is also part of choices of everyday voltages:

Up to the order of 24V won't affect you beyond a tingle
a 110 or 230V (household) jolt will surely hurt, but in most won't kill you
it can be nough current to force your muscles to hold the source, which is potentially deadly
much higher than this and bets are off


Note that there are a few cases of high volts without high amps, though they are relatively rare. That is, some things have tiny current capacity, meaning their ability to deliver power, once they start, drops so quickly

Perhaps the best example is a Van der Graaff generator[3], which can built thousands of volts and still safely zap your students on the nose with (no hard feelings).


Human resistance

  • our skin is at least least dozens of kilo-ohms
...for most paths relevant to electrifying us. It also depend on distance, a very short distance will hurt more.
  • if you're extremely sweaty, so have a water-salt mixture on you (electrolytic), it may be as little as 1 kOhm
  • if you somehow get a larger contact area, it's effectively a bit lower


The other thing that matters is the path the electricity takes

  • skin-to-skin means most electricity flowing will be through the skin, and not far below
this will hurt because you've got nerves there, but not really interfere with organs


  • if you bypass the skin, e.g. stab yourself with electrodes or stick them into wounds, then you'll often stick to what are effectively electrolytes
Two issues:
electrolyte means lower resistance, meaning a dozen volts is now already dangerous.
the path will more easily go through more much more of you -- most relevantly the heart. The issue here is that a dozen mA is enough to risk fibrilation.


So:

  • Say you touch your music player's or laptop's adapter (most are between 5 and 20 volts), or car battery
Say it's approx 10V DC
For numerical convenience's sake, say your resistance is 5kOhm
Via I=V/R, the current is on the order of 2 mA.
This may be just enough to feel, but possibly not. Up to 5mA is considered harmless.
  • Say you manage to stick your fingers into wall-plug voltage (120 or 230 volts AC), and you get 50 mA.
You'll certainly feel that, and sustain this sort of thing and you will be in trouble
it's also enough to affect muscles, which is a problem when those muscles grab what you're touching
(side note: DC would feel less like a sting than AC)
  • say you do the latter when sweating profusely
you are covered with a decent conductor, which means there will be more current flowing
...mostly over your skin, so mostly it'll just hurt more.
...but also a little more into you, so if well placed, and sustained, your heart will notice

For a fast death you need at least hundreds of volts if sweaty, thousands if not, or stick electrodes through your skin into your inner electrolytes. This is why electricity isn't all that dangerous, assuming you take basic care.


On a related note, amp ratings on adapters barely relate to danger. Because this is not the current it will output, this is its current capacity, describing how much current it could deliver if asked by a load with low enough resistance. How much it will -- is still determined by the voltage and your resistance.

(Actually, the current capacity says something about its minimum effective resistance, which does matter a tiny bit, but it's still as relevant as the second bit:)

Residual-current devices

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Known under varied terms, including:

  • Residual-current devices (RCD)
  • Ground fault circuit interrupter (GFCI)
  • Ground fault interrupter (GFI)
  • residual-current circuit breaker (RCCB)
(Note: an RCBO has both residual-current and overcurrent protection)
  • Rarth leakage circuit breaker (ELCB)
  • Appliance leakage current interrupter (ALCI)
  • Leakage Current Detection Interrupter (LCDI)
  • FI - Fehler and I indicating current (verify)
  • Differential circuit breaker


These measure the current difference between the current going through the live and neutral wire.

It cuts power when there is, because one of the options is that the extra current path is via a human.

Component-wise, you can think of it as a transformer around both live and neutral wire. When current in is equal to current out, that transformer outputs nothing.)

They are typically part of your house's installation, not your devices.


For home installations, they trip when it's on the order of ~30mA for ~20 milliseconds (varies, e.g. with type and size of installation) because most humans will survive that, and is long enough that it it;s less sensitive to stray current and inductive effects.

Yes, you can get RCD outlets, and protected extension cords, which can e.g. make sense when you do electronics repair work. They do not replace overall GCFI, but can be practical in that they'll act a little faster than your central one, good for you and less annoying for the other people in your house. Plus it's a little easier to diagnose who/what tripped it. Building sites may well put smaller GFCIs on different areas for similar reasons.


Note that they are not circuit breakers, in the sense that they will not trip when there's overcurrent between wire and neutral (/ground wire). So they complement, and do not replace plain circuit breakers.

...though yes, there are models that do both in one unit - see RCBO.



See also:

Safety earth

Why?

Earth wiring within a house is there for safety.

Short version: it is a return connection that is much lower impedance than you are - and low enough that a fuse or circuit breaker would soon disconnect the live wire.


The main example is earthing a conductor that should never be live - but might become so when things fail. Consider most chassis, like the outside of your toaster or washing machine. These devices should be sturdy, but could be abused enough that something comes loose and now touches that outside, and you want to design for that possibility.


With safety earth on that chassis, that's a short across your house's breaker - also quite possibly a fuse within the device, on its live wire. Both of those will disconnect live quite quickly, meaning you'll not see the dangerous voltage for very long - and even even if you touch it within this time, the earth wire is much lower resistance to earth and (since this is a voltage divider) you shouldn't feel much.


Without safety earth, the live wire touching the chassis mean nothing other than that it is now at mains potential. This has no immediate implications, but you will notice this when you next touch this and are even vaguely earthed.

And, significantly, that current path through you might be high enough to make you very unhappy or dead, long before it's enough to trip the breaker for high-current reasons.

(Which is why RCDs exist, and complement breakers)



What?

Earth (or earth ground) refers to a physical pole hammered into the earth somewhere nearby.

Typically this is also bonded to things like radiator pipes, water pipes, steel structures, so often are at most a few ohms to this earth.


Devices like think ovens, washing machine, etc. are typically designed with safety earth, and using them safely will require that connection to earth - which, due to electrical code, usually just means 'socket with earth pin'.

Some (big) devices will even test the grounding and refuse to work if not present. Yet most will assume that your electrical wiring is not violating electric code. Just at all, or because they have an earthed plug and it's technically a code violation to have an earthed socket that isn't actually earthed.


(Note that powering things on generators is a little more interesting, ground-wise)


Earth versus ground

Double insulation

Galvanic isolation, floating outputs, floating ground

Isolation transformers

Certifications