Electronics notes/Electricity and humans

From Helpful
(Redirected from Double insulated)
Jump to: navigation, search
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 · ·  Various wireless · 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 ·

Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions

Audio notes: See avnotes

Microcontroller and computer platforms Arduino and AVR notes · ESP series notes · STM32 series notes · · · ·

Less sorted: Ground · device voltage and impedance, audio and otherwise · electricity and humans · power supply considerations · Common terms, useful basics, soldering · PLL · pulse modulation · signal reflection · resource metering · SDR · Project boxes · Unsorted stuff

See also Category:Electronics.


ECG, electrocardiography

Measuring the heart muscle (-cardio-), which at skin surface is seen on the scale of a millivolt or so, which is quite weak.

Also, there is also typically coupling of electrical fields (and easily AC line noise), so you want to be using a differential amplifier to mostly ignore that.

And probably a shielded or twisted wire to minimize induction.

This can be amplified by a high impedance differential amplifier, preferably a instrumentation amp.

You may also want a passband filter, to avoid some disturbances.


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



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:


Purple wand

Static electricity, ESD, and humans

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)

ESD and electronics

Avoiding ESD


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 bodies, e.g. in your hands, on your lap)
  • felt only when the adapter is connected
  • Felt more clearly when
the contact area is small (similar current focused through fewer of your neurons)
you have good contact with the device (e.g. sweaty hands/legs)
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.

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

Human ratings

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

On current capacity

Is 120V safer than 230V?

Circuit breakers

On extension cords

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)
  • Earth leakage circuit breaker (ELCB)
  • Appliance leakage current interrupter (ALCI)
  • Leakage Current Detection Interrupter (LCDI)
  • FI - Fehler and I indicating current (verify)
  • Differential circuit breaker

Apparently, some styles specifically measured earth leakage, and work slightly differently(verify).

But the more common variant measure the current difference between the current going through the live and neutral wire, because in regular service, those two currents should be exactly the same.

And if they are not, we can assume that the extra path is via ground (either the earth wire, or the literal ground), and we can reasonably assume that this might be going through a human, so it cuts power.

Both electrical codes and building habits vary.

Some countries put chunky ones on an entire phase, at the power board, only.

Others add them per socket, e.g. in the kitchen and shower.

Some do both.

Both are a "in relatively new buildings" thing.

Component-wise, you can think of it as a small sensing transformer coil 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 RCD extension cords, which can e.g. make sense when you do electronics repair work, both

because you can get one that responds faster and for lower current,
and because it avoids annoying other people in the same house as a faster-tripping workstation RCD one should generally avoid the house RCD from tripping.

Also, more RCDs make it a little easier to diagnose who/what tripped it.

Building sites frequently put a series of larger-and-smaller RCDs on different areas for similar reasons.

Note that they do not replace circuit breakers, in the sense that they do no react to overcurrent. In a distribution board they are often alongside the real breaker for the same leg.

Though yes, there are models that do both in one unit, see RCBO, but with the idea(verify) that replacing them separately is probably a little cheaper, this is often only done when there's space limitations.

See also:

Arc faults and its protection

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 passes enough current enough that a fuse or circuit breaker would soon disconnect the live wire.

One of the main uses is earthing a conductor that should never be live - but might become so when things fail. For example, the chassis of a toaster or washing machine or such.

If they eventually break so that wiring comes loose and touches the outside, and you want to design for that possibility.

With safety earth on that chassis, that's a short across your house's breaker, which will probably trip, meaning the dangerous voltage on that chassis is on there for a fraction of a second.

(Also it's across two chunky wires that won't have any issue with that current for factors longer than that)

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 few immediate implications, but you will notice this when you next touch this and you are even vaguely earthed.

The current through you is also much lower than any real use, so there is no reason for a breaker to trip.

(Which is why RCDs exist, and complement breakers)


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

Earth wiring is connected to that pole.

Typically this is also bonded to things like radiator pipes, water pipes, steel structures. So generally all these things 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