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'[OT] Current kills'
2009\07\05@015012 by

People say current kills. So what happens if you push 1 mA through an
LED @ 1000 V? Will the LED blow?

-- [ solarwind ] -- http://solar-blogg.blogspot.com/

:: People say current kills. So what happens if you push 1 mA through
:: an
:: LED @ 1000 V? Will the LED blow?

How many femto seconds worth of current are we talking about?
--
cdb, colinbtech-online.co.uk on 5/07/2009

Web presence: http://www.btech-online.co.uk

Hosted by:  http://www.1and1.co.uk/?k_id=7988359

If you use the conventional series resistor to adjust the current, it
won't matter.  998 volts will be dropped across the resistor to get the
proper current through the LED.  The voltage rating of the resistor will
need to be considered.

If you use a constant current source, with a maximum voltage of 1000
volts...it still won't matter!  The constant current source will adjust
its voltage until 1 mA flows....which will be the forward voltage of the
LED.

Jon

> People say current kills. So what happens if you push 1 mA through an
> LED @ 1000 V? Will the LED blow?
>
> -- [ solarwind ] -- http://solar-blogg.blogspot.com/
> --

solarwind wrote:
> People say current kills. So what happens if you push 1 mA through an
> LED @ 1000 V? Will the LED blow?

Assuming the current is limited to 1 mA:

Forward no, the voltage is clamped. Reverse yes/maybe, if it zeners at a
low enough voltage it will injure it slowly, faster if it zeners at a
higher voltage.

-Bob
You can't push current, the load impedance draws it.
----- Original Message -----
From: "cdb" <colinbtech-online.co.uk>
To: "Microcontroller discussion list - Public." <piclistmit.edu>
Sent: Sunday, July 05, 2009 2:10 AM
Subject: Re: [OT] Current kills

{Quote hidden}

> --
solarwind wrote:

> People say current kills. So what happens if you push 1 mA through an
> LED @ 1000 V? Will the LED blow?

I don't know who said that in what context, but if it's about people
being killed, it has not much to do with LEDs. Both scenarios have to do
with the fact that voltage and current are tied together by the
impedance of the device you're looking at. (It's called "Ohm's Law",
which I think is kind of a misnomer. It's not a law; there aren't any
laws in science (which is a good thing -- there are no lawyers either
:). It's more something like "Ohm's Definition", as it defines the
relationship between voltage, current and impedance.)

As far as LEDs go, if you put 1000V directly across a LED, there will
flow much more current than 1mA (for a short time). If you push 1mA
through it, there will be much less voltage across the LED than 1000V.
Can't have it both; they are tied together by the LED's impedance.

As far as people go, their impedance varies wildly, especially the
contact and ground resistances. Given that you are touching some source,
one thing that is important for the danger you're exposed to is the
current that is flowing through you. (Another thing is where the current
is flowing through.) If you're well isolated from ground, you can touch
a higher voltage source, because the higher ground resistance reduces
the current that is flowing. But, of course, in the end it also comes
down to current and voltage being tied together by the impedance. As
approximation, talking about the "current that kills" makes sense in
certain contexts.

<http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html>

Gerhard
On Sun, Jul 5, 2009 at 10:00 AM, Gerhard
Fiedler<listsconnectionbrazil.com> wrote:
{Quote hidden}

So what happens if you have a 1000 V potential across the LED, but
some sort of limiter that only allows a max of 1 mA to flow?

And why is it that if you touch a 120 V source, you die - yet a 15000
V static shock merely stings?
> So what happens if you have a 1000 V potential across the LED, but
> some sort of limiter that only allows a max of 1 mA to flow?

The limiter has to cope with 1W  (and 1kV).

> And why is it that if you touch a 120 V source, you die - yet a 15000
> V static shock merely stings?

What is the important difference between the 120V source and the 15kV
static?

--

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

:: And why is it that if you touch a 120 V source, you die - yet a
:: 15000
:: V static shock merely stings?

Amount of current through the the body and the duration of that
current, as well as the impedance that the current source has.

Colin
--
cdb, colinbtech-online.co.uk on 6/07/2009

Web presence: http://www.btech-online.co.uk

Hosted by:  http://www.1and1.co.uk/?k_id=7988359

Actually, the lower the resistance of a load at a fixed applied
voltage , the more current flows.
Voltage is pressure and resistance is ......... resistive.
Ohms Law  --->  don't break it or you are ticketed.
Gus

On Jul 5, 2002, at 6:55 AM, Rich wrote:

You can't push current, the load impedance draws it.
{Original Message removed}
To expand on what Colin said ......
It is the integrated amount of current through the heart muscles
( over a 1 second period approximately )
that disrupts the heart beat.  You can also start a non-working heart
this way ( sometimes )
Thus the more voltage applied to push current near the heart, the
actual path the current takes ( does it go through the heart
muscles  ? ),
Applying voltage from sweaty hand to the other sweaty hand does a good
job of directing current near the heart.  More voltage helps push
the current through the skin ( which is often a good insulator when
dry ).
A good plan for stopping your heart is to push needles into your chest
on either side of the heart.   Now the current can flow easily through
your heart muscles.  Static discharge ( high voltage , low amount of
total electrons ) may stop your heart in this case.  A nine volt
battery might work too ( lower voltage, more electrons available ).
The crucial issue is 1) is there a good path for the current to flow
through your heart muscles     2)  Is there enough voltage ( push ) to
ensure adequate current flow given the resistance of the path.

From the outside of the body, it is useful to have very high voltage
sources that can supply a lot of current.   Apply that from arm to arm
or from front of chest to back of chest.  Use liberal amounts of
conductive goo on the skin.   Better yet, stick conductive probes into
your chest.  Some of the current flowing through your body is bound to

By the way,  does anyone know if being electrocuted is painful ?

Gus

{Quote hidden}

> And why is it that if you touch a 120 V source, you die - yet a 15000
> V static shock merely stings?

The 120V source can deliver current for an unlimited amount of time (or until
the circuit is broken) while the static charge is just as its name implies -
static. As soon as current begins to flow, the voltage is rapidly decreased and
all the stored enery in it is used up.

You can think of static charge as a charged capacitor (with a very low
capacitance). When the capacitor is discharged it can no longer deliver a
current through the load (your body, for instance). For normal static charges,
the capacitance is very low which means that the charge will be discharged
through your body in a matter of pico seconds.

If you touch a 10nF capacitor charged to 15000V - It will do a lot more than
merly sting.

When dealing whith static charges (charged capacitors), the charged voltage in
combination with the capacitance tells you how much energy the capacitor
stores. IEC/EN 61010 states that maximum permissible limits for a capacitance
charged to a voltage above 15000V is 350mJ. Above that it is considered to be
hazardous live. The energy in a charged capacitor is C*V*V/2 so a capacitor
with a capacitance of more than around 3nF charged to 15000V is considered
unsafe for a human to touch. When you have seen the discharge spark from a 10nF
capacitor charged to around 15kV and heard the bang it makes, you make sure to
keep the fingers away.

For sources of continuous currents (as opposed to charged capacitors), IEC/EN
61010 says that potentials higher than 33VAC rms, 46.7V peak or 70VDC that can
deliver a current of 0.5mA rms for sinusoidal waveforms, 0.7mA peak for non
sinusoidal waveforms or mixed frequencies or 2mADC when measured through a
circuit representing the impedance of a human body, is concidered to be
hazardous live. So in this sence it is the current that kills and not the
voltage as long as the voltage is above the values above. And in this case it
doesn't matter if the voltage is 100V or 100000V.

/Ruben

==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
rubenpp.sbbs.se
==============================
You cannot do what you are asking.
Either the voltage source will drop to the voltage of the LED
or more current  ( greater than 1 ma ) will flow until the LED blows up.

Imagine applying 1000 psi water pressure to a water turbine.  If you
limit the
flow of water ( current ) to the WT to a trickle, the WT will not turn
and
the pressure will be across the limiter and not across the WT.  If you
supply
unlimited amounts of water to the WT at 1000 psi ( which is hard to
do ) , the WT
will spin so fast it may blow up.

Let's say someone gives you a choice.
1)  They will direct 100 liters/minute of water into your open mouth
at .01 PSI
2)  They will direct  .01 liters/minute of water into your open mouth
at a pressure high enough to cause the flow.
3)  They will direct  .01 liters/minute of water into your closed
mouth at a pressure high enough to cause the flow.

Which would you choose first and second ?

For #3 , if you could close your mouth very tightly, the water would
Even though it is a low flow, it would hurt you.
Gus

> On Jul 4, 2009, at 11:49 PM, solarwind wrote:
>
> People say current kills. So what happens if you push 1 mA through an
> LED @ 1000 V? Will the LED blow?
>
> -- [ solarwind ] -- http://solar-blogg.blogspot.com/
solarwind wrote:

{Quote hidden}

The limiter needs to be in series with the LED. So the 1000 V are not
across the LED, but across the LED and the limiter (in series).

How much of the 1000 V will be across the limiter, and how much will be
across the LED?

Try to apply 1) Ohm's Definition (V = I × R), 2) the fact that in a
series circuit the current is the same through all devices, and 3) a
data sheet of a LED of your choice (that gives you the voltage across
the LED when you push 1 mA through it).

For extra points, compare this to the standard circuit of a LED with a
series resistor, and suggest a suitable "limiter" circuit for this
situation.

Gerhard
Very good points, Ruben.

I once got to use an ESD simulator to test some equipment for
susceptibility. It was based on the human body model (I think 100pF
with 1K in series) and could charge that capacitance up to 15kV. I can
tell you that 8kV was definitely painful. I didn't try shocking myself
with anything higher. The typical ESD events that we experience when
shuffling around the floor are more in the 3 to 5kV range.

Also, there are varying "definitions" of what is a safe voltage to
touch. I would be suspicious of anything more than 24V for a DC
source. I know from experience that, when my hands are dry, I can
touch 50V DC and not feel a thing, but if I do the same thing when I
am sweaty, it REALLY hurts. I once had an intern who told me that she
had touched a 48V battery pack and the whole side of her face went
numb. It was a hot July in a non air-conditioned warehouse. Based on
the jolts I had gotten from the same voltage, I definitely believed
her and cautioned her not to touch the bare battery terminals again!
It's really tempting to disregard the danger because a) for most of
the year our skin is dry enough that we don't feel it and b) it is
unlikely to cause lasting harm. However, my understanding is that 10mA
is the maximum safe level (most standards are even more strict, saying
something like 2mA), and 100mA can definitely kill you if it takes the
proper path through your body (around the heart or through the brain).

Ohm's law works very well for most metals. It is not a very good model
for the human body. Sweat and grip pressure greatly change the amount
of current which will flow for a given voltage. I also greatly suspect
that, as the voltage climbs, there is a nonlinear increase in the
amount of current due to electrochemical breakdown of the skin. Also,
all bets are off once the skin is punctured because the internal body
fluids are MUCH more conductive.

Sean

2009/7/5 Ruben Jönsson <rubenpp.sbbs.se>:
{Quote hidden}

>
On Sun, Jul 5, 2009 at 9:15 PM, Sean Breheny<shb7cornell.edu> wrote:
{Quote hidden}

But it would still technically follow ohm's law since the resistance
You have a sharp eye, Solarwind :) You caught the inconsistency in my
explanation.

In the example I gave, one could say that grip pressure and sweat
change the resistance, but that the current to voltage relationship
would still be linear.

That might be true, I don't know enough about human skin.

However, there ARE examples of materials and devices which do NOT

Diodes, for example. Their voltage drop is approximately the natural
log of the current.

Another example is a salt water solution with conductive electrodes
stuck into it. I don't know what the voltage to current relation is
here but I suspect that it is very nonlinear and, therefore,
non-Ohmic.

A spark gap is another. When a spark gap is NOT arcing, you can
increase the voltage until it reaches the breakdown voltage (depends
on the gap/electrode geometry, the gas they are in, presence of
ionizing radiation, etc.). Up til this point, the current will be
zero. Then, suddenly, the arc starts and the current begins to rise.
As the current rises, the voltage actually DROPS. If you attach a
current source to the arc, and you turn up the current, you will see
the voltage go DOWN, instead of up as you would expect with an Ohmic
material.

I would suspect that skin has an Ohmic region (i.e., up to a certain
voltage it will be mostly Ohmic), but that it behaves in a non-Ohmic
fashion at higher voltages.

Sean

On Sun, Jul 5, 2009 at 9:41 PM, solarwind<x.solarwind.xgmail.com> wrote:
{Quote hidden}

> -

> >
> > Ohm's law works very well for most metals. It is not a very good model
> > for the human body. Sweat and grip pressure greatly change the amount
> > of current which will flow for a given voltage. I also greatly suspect
> > that, as the voltage climbs, there is a nonlinear increase in the
> > amount of current due to electrochemical breakdown of the skin. Also,
> > all bets are off once the skin is punctured because the internal body
> > fluids are MUCH more conductive.
> >
> > Sean
>
> But it would still technically follow ohm's law since the resistance
> of your body would change?

All currents flowing in a circuit follow ohm's law.

However, as soon as the current is not a DC current and/or the load is not a
pure resistive value or changes with voltage, time, temperature or whatever it
becomes a bit more complex. To complicate matters even more, the load can have
inductive or capacitive components which makes the current, voltage and
resistance (or rather resitance + reactance) to change when the frequency is
changed. The current actually also becomes out of phase with the voltage.

/Ruben

==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
rubenpp.sbbs.se
==============================
On Mon, Jul 6, 2009 at 1:27 AM, Sean Breheny<shb7cornell.edu> wrote:
{Quote hidden}

This is definitely something I should read up on. I thought Ohm's law
was universal.
Somewhat relevant to this discussion:

>From the Australian Standards (with some editing):
*

APPLIED PART
*

A part of the EQUIPMENT which in NORMAL USE:

— necessarily comes into physical contact with the PATIENT for the EQUIPMENT
to perform its function; or

— can be brought into contact with the PATIENT; or

— needs to be touched by the PATIENT.

*PATIENT LEAKAGE CURRENT*

Current flowing from the APPLIED PART via the PATIENT to earth or flowing
from the PATIENT to earth originating from the unintended appearance
of a voltage
from an external source on the PATIENT.

*ALLOWABLE VALUES OF PATIENT LEAKAGE CURRENT*

— DC: 0.01mA (normal conditions), 0.05 mA (fault conditions)

— AC: 0.1 mA (normal conditions), 0.5 mA (fault Conditions)

*Transient currents occurring during the first 50 ms following a fault shall
be **disregarded.*

2009/7/6 Ruben Jönsson <rubenpp.sbbs.se>

{Quote hidden}

>
I would disagree, Ruben. Ohm's Law is not a fundamental circuit law
(like Kirchoff's Voltage Law). It is an empirical law which says that
for most conductors, V=IR where R is a constant. If R is not a
constant (i.e., if it is a function of V or I), then what you are
doing is linearizing the non-linear V to I relationship about one
operating point. In that case, you really need to add more constants
to make Ohm's law useful: V=(I-Io)*R+Vo where Vo is the voltage drop
when I=Io. This is what small-signal analysis is all about. In a
general sense, the "resistance" of an LED, for example, has little
meaning. If anything, it is usually taken to mean the actual Ohmic
resistance of the wires and contacts which make up the LED, not the
ratio of the entire voltage drop to the current.

Sean

2009/7/6 Ruben Jönsson <rubenpp.sbbs.se>:
{Quote hidden}

>
Medical standards is mostly (always?) more stringent than industrial standards
regarding allowed currents through a human body. One reason for this is that a
patient may have an open wound and not have the protection normally offered by
the skin.

/Ruben

{Quote hidden}

> > --
I think this is a fairly good for seeing how a diode behaves (as describing
how the diode changes it's resistance over supply voltage hence is not
conforming to ohm's law):

Take a look at the graphs below - the breakdown is where zener works and the
foward where 'normal' diodes are.
For thumb of rule you may can think diode as infinite resistance before the
forward voltage region and infinite conductance after that (on that graph
you can see that it is not exactly the case but for the sake of simplicity
forget the small details for now).

LED is not much different, however, you have a bigger forward voltage drop
-- depending on your LED it can vary from 1.8V to 5V or maybe even more --
you need to check it on the datasheet of your LED to figure this out. It
also describes the maximum continuous current it can take. All you need is
to subtract the voltage drop from the input voltage and using ohm's law set
the resistor to get the appropriate current.

R = (Vsupp - Vdrop) / I

You could use a constant current source as well but then it makes your
circuit far too expensive and is not much better than a 1 cent worth of
resistor calculated to a specific voltage supply.

Tamas

On Mon, Jul 6, 2009 at 7:08 AM, solarwind <x.solarwind.xgmail.com> wrote:

{Quote hidden}

> -
Hello PIC.ers,

----- Original Message -----
From: "solarwind" <x.solarwind.xgmail.com>
To: "Microcontroller discussion list - Public." <piclistmit.edu>
Sent: Monday, July 06, 2009 3:41 AM
Subject: Re: [OT] Current kills

{Quote hidden}

FWIW, I've found that you can't even trust a 12v car battery to be a
completely `safe' emf.

Try standing up to your chest in fresh water (lake, dam) leaning over the
gunwhale of a ski-boat to connect the +ve terminal of the system battery.
Your hands aren't exactly bone dry either.
If the outboard motor is already trimmed down (into the water) then its
aluminium body earths the -ve terminal to the water very effectively.
Ask me how I know all this....

Heaven knows how much better the loop conductivity becomes in seawater.

best regards,   John

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