Electronics notes/Electricity and humans

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This is for beginners and very much by a beginner. It's meant to try to cover hobbyist needs, and as a starting point to find out which may be the relevant details for you, not for definitive information.

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: IO and wired communication · localish communication · wireless (ISM RF, GSM, RFID, more) · 802.11 (WiFi) · 802.15 (including zigbee)

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 · Bluetooth serial · JY-MCU · DMX · ESC/POS notes

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

Less sorted: Common terms, useful basics, soldering · Microcontroller and computer platforms · Arduino and AVR notes · ESP series notes · Electronics notes/Phase Locked Loop notes · mounts, chip carriers, packages, connectors · signal reflection · pulse modulation · electricity and humans · Unsorted stuff

See also Category:Electronics.



ECG, electrocardiography

Measuring the heart muscle, which at the surface of the skin 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).


EMG, Electromyography

Measuring skeletal muscles, 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 - 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


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 engae.

See also:

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)


  • 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)






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, which in most circumstances is relatively fixed. If we're talking about people, R, your resistance, across your skin is on the scale of kiloOhms.

And because that's relatively stable, you can approximately relate the current that will be on you to the voltage it comes from.

In which case it's only only the higher volts that lead to the higher amps.

Case in point: 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.

You - let's say you just came out of the rain and are just 1kOhm across your skin, meaning I=12/1000, i.e. ~12 milliamps. You will probably not even feel that.

These orders of magnitude are 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 cases you don't risk dying unless you keep holding it (which in some cases you have no control over because muscles, which is one of a few reasons you should still not be careless about this)
much higher than this and bets are off, though

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.


  • 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)

Part of your house's installation, not your devices.

Apparently known as:

  • Residual-current devices (RCD)
  • ground fault circuit interrupter (GFCI)
  • ground fault interrupter (GFI)
  • residual-current circuit breaker (RCCB)
  • earth leakage circuit breaker (ELCB)
  • appliance leakage current interrupter (ALCI)
  • FI - Fehler and I indicating current (verify)

These measure the current difference between live and neutral wire, and assumes that there being any means it's leaking to earth via a human.

For home installations they trip when it's on the order of over ~30mA for over ~20 milliseconds (varies, more with type of installation) because most humans will survive that, and that sort of figures isn't overly sensitive to stray current.

Note that they are not circuit breakers in the sense that they don't care about plain overcurrent between wire and neutral (/ground wire). So they complement but do not replace plain circuit breakers.

...though yes, there are models that do both ground protection and overcurrent protection - see RCBO. It seems GFCI sometimes can indicate either(verify)

Note also that you cannot count on them working on floating-ground systems (verify) which is part of why that describes basically no homes.

Safety earth


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)


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