Invisible wrote:
> OK, so what *is* the correct mains voltage in the UK?

A related question. why do some countries use 110 and others use 220?

BTW, here in the US I've seen AC voltage expressed as 110, 115 and 120. 
  Generally it's actually somewhere between 105 and 125, devices get 
around this by having voltage regulators. e.g. if the input voltage 
sags, it'll draw more current to keep the output voltage where it wants 
it to be, and vice-versa.. Of course there are limitations, and 
eventually the regulator will output too little voltage, because the 
supply cannot keep up with the current draw, or it'll burn out because 
the input voltage is outside it's operating limits.

This is why battery operated electronics can run until the battery is 
almost flat, even though the battery's voltage continually drops as it's 
drained.

-- 
~Mike

Post a reply to this message

Mike Raiford wrote:

> This is why battery operated electronics can run until the battery is 
> almost flat, even though the battery's voltage continually drops as it's 
> drained.

Really? That's interesting. I always thought it was the battery outputs 
full voltage until just before it actually dies (which is why battery 
meters are never, ever, under any circumstances, actually accurate).

Post a reply to this message

Invisible <voi### [at] devnull> wrote:
> Mike Raiford wrote:
>
> > This is why battery operated electronics can run until the battery is
> > almost flat, even though the battery's voltage continually drops as it's
> > drained.
>
> Really? That's interesting. I always thought it was the battery outputs
> full voltage until just before it actually dies (which is why battery
> meters are never, ever, under any circumstances, actually accurate).

Actually, the battery's resistance increases with depletion, which means the
more current you draw and the lower charge of the battery the lower the voltage
you get. The idle voltage will almost always be the same, regardless of the
charge state of the battery.

Post a reply to this message

>> Really? That's interesting. I always thought it was the battery outputs
>> full voltage until just before it actually dies (which is why battery
>> meters are never, ever, under any circumstances, actually accurate).
> 
> Actually, the battery's resistance increases with depletion, which means the
> more current you draw and the lower charge of the battery the lower the voltage
> you get. The idle voltage will almost always be the same, regardless of the
> charge state of the battery.

I have no idea what internal resistance is. All I know is that a battery 
is a chemical reaction that charges two electrodes until they reach a 
potential high enough to halt the reaction. You draw current and the 
reaction starts up again, replacing the charge just as fast as you can 
remove it - up to a point. I presume as the reactants become more 
dilute, that rate of reaction decreases so the maximum current you can 
draw decreases.

But yes, as I understand it, the potential difference of an unloaded 
battery is almost constant until it's completely dead. This (I presume) 
is why my phone tells me it's on maximum charge for months on end, until 
the exact moment when I try to make a phonecall...

Post a reply to this message

Mike Raiford wrote:
> A related question. why do some countries use 110 and others use 220?

History, most likely. :-) Whoever does something widespread first tends to 
be at odds with everyone else later, when the flaws in the methods are 
discovered but not worth fixing in an already-widespread system.

> BTW, here in the US I've seen AC voltage expressed as 110, 115 and 120. 

117v RMS, I think is the actual standard. :-)

-- 
   Darren New, San Diego CA, USA (PST)
   There's no CD like OCD, there's no CD I knoooow!

Post a reply to this message

Darren New wrote:
> Mike Raiford wrote:
>> A related question. why do some countries use 110 and others use 220?
> 
> History, most likely. :-) Whoever does something widespread first tends 
> to be at odds with everyone else later, when the flaws in the methods 
> are discovered but not worth fixing in an already-widespread system.
> 

Heh. switching our voltage overnight from 110 to 220 would have 
disastrous consequences.

And why 50hz v.s. 60hz?

Interestingly, PAL's refresh rate is 50hz, vs NTSC's 60hz refresh simply 
because of the difference in AC frequency..

>> BTW, here in the US I've seen AC voltage expressed as 110, 115 and 120. 
> 
> 117v RMS, I think is the actual standard. :-)
> 


-- 
~Mike

Post a reply to this message

Mike Raiford wrote:
>
> Heh. switching our voltage overnight from 110 to 220 would have
> disastrous consequences.

Dunno why US uses 110V, but 220V means less current, which means less
powerloss in cables etc.

> And why 50hz v.s. 60hz?

Again, dunno why US uses 60Hz, but 50Hz is less lethal (ie. you need
more current to distract your heart) than 60Hz. This has something to do
with 60Hz almost syncing to some multiply of normal, health heartbeat.
Don't remember the details (I was told this at school something like 12
years ago).

-Aero

Post a reply to this message

Eero Ahonen wrote:
> Again, dunno why US uses 60Hz,

Damn metric weenies.


-- 
   Darren New, San Diego CA, USA (PST)
   There's no CD like OCD, there's no CD I knoooow!

Post a reply to this message

> Again, dunno why US uses 60Hz, but 50Hz is less lethal (ie. you need
> more current to distract your heart) than 60Hz. This has something to do
> with 60Hz almost syncing to some multiply of normal, health heartbeat.
> Don't remember the details (I was told this at school something like 12
> years ago).

Well, given that a normal heartbeat ranges from 50 to 150 BPM depending 
on how much exercise you're doing...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

Post a reply to this message

On 6-6-2009 19:44, Orchid XP v8 wrote:
>> Again, dunno why US uses 60Hz, but 50Hz is less lethal (ie. you need
>> more current to distract your heart) than 60Hz. This has something to do
>> with 60Hz almost syncing to some multiply of normal, health heartbeat.
>> Don't remember the details (I was told this at school something like 12
>> years ago).
> 
> Well, given that a normal heartbeat ranges from 50 to 150 BPM depending 
> on how much exercise you're doing...
> 
It is not about the rate of the sinus node, which is indeed in that 
range, but about what happens to a muscle if you rapidly stimulate it. 
At 50 and 60 Hz (does not really differ that much, both are lethal) the 
impulses come so rapidly that the tension in the muscle build up to a 
maximum. So all your muscles including your heart are fully contracted.

If you want non lethal AC current you should significantly increase the 
frequency, then by the skin effect the current stay at the surface and 
never reaches the muscles. Standard experiment goes like this: generate 
a very high voltage with a very high frequency so you get arcs of a few 
centimeter. Connect one pole to a volunteer/victim and let the arcs jump 
off his fingers to the other pole. It won't give more tha a prickling 
sensation in the fingers but it looks cool.

It probably would not surprise you that there are good reasons for not 
having high frequency mains power supplies.

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> It might need to be more regulated on three-phase wiring, and I don't know
> if the frequency is different between the generator and the wall-socket
> (altho I doubt it), so I'm not sure just where the accuracy is enforced.

The frequency is all the same, from the high-voltage power grid down to the
light bulb, or the three-phase-driven gadget.

Which is logical given that the voltage is transformed up and down using
straightforward electromagnetic transformators (which is what the AC is for in
the first place).

It also explains why power companies are so eager to get the frequency as
precise as possible: Every phase shift between different power plants feeding
into the grid is a source of power loss. A power plant operating at a full 180°
off-phase would effectively *drain* from the grid whatever power it tried to
feed into it. Which btw wouldn't be healthy for the infrastructure, I guess,
because the annihilated power must go *somewhere*. And that somewhere will heat
up dramatically. Or produce dramatic magnetic fields. Or whatever.

So they rather pop some ultra-heavy-duty breaker at the slightest sign of an
off-phase feed.

Post a reply to this message

Eero Ahonen <aer### [at] removethiszbxtnetinvalid> wrote:
> Again, dunno why US uses 60Hz, but 50Hz is less lethal (ie. you need
> more current to distract your heart) than 60Hz. This has something to do
> with 60Hz almost syncing to some multiply of normal, health heartbeat.
> Don't remember the details (I was told this at school something like 12
> years ago).

Doesn't make much sense to me. Normal healthy heartbeat is roughly 1 Hz, with a
wide variation depending on circumstances.

Both 50 and 60 Hz are of course exact multiples of 1.

I guess that if there is a significant difference in lethality, it's probably
because at higher frequencies an AC supply provides a bit more actual power (if
I'm not mistaken, that is).

Post a reply to this message

clipka wrote:
> 
> Doesn't make much sense to me. Normal healthy heartbeat is roughly 1 Hz, with a
> wide variation depending on circumstances.

True. That's what they told us at school (at electric line), but as I
mentioned, it was 12 years ago, so the image in my memory ain't the
clearest one. And, of course, the information might still be false
(wouldn't be the first false information collected from school). Maybe
the bit harder overstimulation might make the effect?

> Both 50 and 60 Hz are of course exact multiples of 1.
> 
> I guess that if there is a significant difference in lethality, it's probably
> because at higher frequencies an AC supply provides a bit more actual power (if
> I'm not mistaken, that is).
> 

The difference was something like 30mA vs. either 25 or 28mA, so it's
not big (and as mentioned, both are lethal even with even low currencies).

Why would there be more actual power with greater frequency? The
phase-angle won't change, the current won't change and RMS voltage won't
change, so U*I*cos(fi) won't change.

-Aero

Post a reply to this message

Eero Ahonen <aer### [at] removethiszbxtnetinvalid> wrote:
> Why would there be more actual power with greater frequency? The
> phase-angle won't change, the current won't change and RMS voltage won't
> change, so U*I*cos(fi) won't change.

You need to invest extra power to get the higher frequency at the same AC
voltage and amps, right?

So where does that extra power end up if you have the otherwise same AC running
through your body?

EM field I'd guess - which may be just the extra "kick" you'd rather do without.

Post a reply to this message

clipka wrote:
> 
> You need to invest extra power to get the higher frequency at the same AC
> voltage and amps, right?

Hmm, possibly.

> So where does that extra power end up if you have the otherwise same AC running
> through your body?

At least part of it goes to heating the generator, which runs faster :).

> EM field I'd guess - which may be just the extra "kick" you'd rather do without.

That actually sounds reasonable. My head is atm too empty to remember
how to calculate energies of EM fields, but it would make sense, while
the field sure is frequency-dependent.

-Aero

Post a reply to this message

Invisible wrote:
> Mike Raiford wrote:
> 
>> This is why battery operated electronics can run until the battery is 
>> almost flat, even though the battery's voltage continually drops as 
>> it's drained.
> 
> Really? That's interesting. I always thought it was the battery outputs 
> full voltage until just before it actually dies (which is why battery 
> meters are never, ever, under any circumstances, actually accurate).

Here's an example of a charge pump voltage regulator:

   http://www.ti.com/lit/gpn/reg711-33

Note the input voltage range; +1.8V to +5.5V vs. the output voltage; +3.3V
for the REG711-3.3.

-- 
Tor Olav
http://subcube.com

Post a reply to this message

Tor Olav Kristensen wrote:
> Invisible wrote:
>> Mike Raiford wrote:
>>
>>> This is why battery operated electronics can run until the battery is 
>>> almost flat, even though the battery's voltage continually drops as 
>>> it's drained.
>>
>> Really? That's interesting. I always thought it was the battery 
>> outputs full voltage until just before it actually dies (which is why 
>> battery meters are never, ever, under any circumstances, actually 
>> accurate).
> 
> Here's an example of a charge pump voltage regulator:
> 
>   http://www.ti.com/lit/gpn/reg711-33
> 
> Note the input voltage range; +1.8V to +5.5V vs. the output voltage; +3.3V
> for the REG711-3.3.

I also recommend reading this:

   http://www.maxim-ic.com/appnotes.cfm/an_pk/737/

-- 
Tor Olav
http://subcube.com

Post a reply to this message

Invisible wrote:
> This (I presume)
> is why my phone tells me it's on maximum charge for months on end, until
> the exact moment when I try to make a phonecall...

Well, for that you need to consult Murphy's Law, not physics :)

Post a reply to this message

>  Of course you can generate an out-of-phase signal. It's just an AC
> generator like everything else. What happens is that if you are out of
> sync with the other generators, there will be an energy loss because the
> out-of-sync signal dampens the others.

1) That "energy loss" will be enough to annihilate your generator
2) Even if you built your generator to withstand it, the resultant torques 
and currents would pull it into sync with the others
3) It's not even relevant in practise, because there are protection 
mechanisms to avoid exactly the above happening

Post a reply to this message

> I have no idea what internal resistance is.

A pretty accurate model of a battery is a perfect voltage source in series 
with an "internal" resistor.  This then nicely explains how the more current 
you try to draw the lower the apparent voltage will be at the terminals of 
the battery.

Post a reply to this message

scott wrote:
>> I have no idea what internal resistance is.
> 
> A pretty accurate model of a battery is a perfect voltage source in 
> series with an "internal" resistor.  This then nicely explains how the 
> more current you try to draw the lower the apparent voltage will be at 
> the terminals of the battery.

As far as I'm aware, resistors affect only current, not potential. But 
hey, I'm sure you know more about this than me...

Post a reply to this message

Eero Ahonen <aer### [at] removethiszbxtnetinvalid> wrote:
> clipka wrote:
> >
> > You need to invest extra power to get the higher frequency at the same AC
> > voltage and amps, right?
>
> Hmm, possibly.
>
> > So where does that extra power end up if you have the otherwise same AC running
> > through your body?
>
> At least part of it goes to heating the generator, which runs faster :).
>
> > EM field I'd guess - which may be just the extra "kick" you'd rather do without.
>
> That actually sounds reasonable. My head is atm too empty to remember
> how to calculate energies of EM fields, but it would make sense, while
> the field sure is frequency-dependent.
>
> -Aero

It’s not really the power it is the frequency. What happens is that the
electrical impulses to the heart are interrupted and fibrillation occurs. IIRC
there are parts of the heart that receive the electrical impulses from the
brain then transmit secondary impulses to the muscle tissue to contract and
relax in an organised way. The electrical shock interrupts this process and you
get a disorganised high frequency oscillation in different parts of the heart. A
defibrillator is used to stop this effectively stopping the heart and the
natural sinus rhythm can be resumed. Before defibrillators became commonplace
the manual method was to strike the victims chest above the heart with a
controlled blow using the pinkie side of your fist. This was supposed to have
the same effect. I don’t know if it ever worked but that was what I was taught.

Stephen

Post a reply to this message

Invisible wrote:
> scott wrote:
>>> I have no idea what internal resistance is.
>>
>> A pretty accurate model of a battery is a perfect voltage source in
>> series with an "internal" resistor.  This then nicely explains how the
>> more current you try to draw the lower the apparent voltage will be at
>> the terminals of the battery.
> 
> As far as I'm aware, resistors affect only current, not potential. But
> hey, I'm sure you know more about this than me...

Try putting 2 resistors in series, connect the power (you'll have eg.
+12V on R1+, R1- on R2+ and R2- on ground) and measure voltages between
R1+/R2-, R1+/R1-, R2+/R2-, you might be surprised

+
|
R1
|
R2
|
-

-Aero

Post a reply to this message

On 15-6-2009 14:02, Stephen wrote:
> It’s not really the power it is the frequency. What happens is that the
> electrical impulses to the heart are interrupted and fibrillation occurs. IIRC
> there are parts of the heart that receive the electrical impulses from the
> brain then transmit secondary impulses to the muscle tissue to contract and
> relax in an organised way. 

No the heart is autonomous, will even beat outside the body for some 
time (hours) if oxygenated. It's frequency can be modulated by stuff in 
the blood and by nerve tissue that is close to the sinus node when they 
release special molecules.

> The electrical shock interrupts this process and you
> get a disorganised high frequency oscillation in different parts of the heart. 

No, but what happens is much more complicated (i.e. you need a 
complicated model to describe it and even then you see it happening 
without getting much more understanding, but it works).
Standard procedure to induce VF in an open chest heart: hold a 9V 
battery against it. And, no, I don't know how it actually works, but it 
does.

> A defibrillator is used to stop this effectively stopping the heart and the
> natural sinus rhythm can be resumed. Before defibrillators became commonplace
> the manual method was to strike the victims chest above the heart with a
> controlled blow using the pinkie side of your fist. This was supposed to have
> the same effect. I don’t know if it ever worked but that was what I was taught.

I does work sometimes. http://en.wikipedia.org/wiki/Precordial_thump Do 
not apply to people that are not in VF: 
http://en.wikipedia.org/wiki/Commotio_cordis

Post a reply to this message

andrel wrote:

> No the heart is autonomous, will even beat outside the body for some 
> time (hours) if oxygenated.

Does anybody else find it slightly disturbing that andrel knows such things?

I mean, I know there's a good reason, but even so... kinda grusome.

Post a reply to this message

andrel <a_l### [at] hotmailcom> wrote:
> On 15-6-2009 14:02, Stephen wrote:
> > It’s not really the power it is the frequency. What happens is that the
> > electrical impulses to the heart are interrupted and fibrillation occurs. IIRC
> > there are parts of the heart that receive the electrical impulses from the
> > brain then transmit secondary impulses to the muscle tissue to contract and
> > relax in an organised way.
>
> No the heart is autonomous, will even beat outside the body for some
> time (hours) if oxygenated. It's frequency can be modulated by stuff in
> the blood and by nerve tissue that is close to the sinus node when they
> release special molecules.
>

I thought that you might respond :)
It was “lies for children” :P


> > The electrical shock interrupts this process and you
> > get a disorganised high frequency oscillation in different parts of the heart.
>
> No, but what happens is much more complicated (i.e. you need a
> complicated model to describe it and even then you see it happening
> without getting much more understanding, but it works).
> Standard procedure to induce VF in an open chest heart: hold a 9V
> battery against it. And, no, I don't know how it actually works, but it
> does.
>

You are the expert I'm only a first aider


> > A defibrillator is used to stop this effectively stopping the heart and the
> > natural sinus rhythm can be resumed. Before defibrillators became commonplace
> > the manual method was to strike the victims chest above the heart with a
> > controlled blow using the pinkie side of your fist. This was supposed to have
> > the same effect. I don’t know if it ever worked but that was what I was taught.
>
> I does work sometimes. http://en.wikipedia.org/wiki/Precordial_thump Do
> not apply to people that are not in VF:

Unless you want to kill them ;)

Stephen

Post a reply to this message

On 15-6-2009 17:30, Invisible wrote:
> andrel wrote:
> 
>> No the heart is autonomous, will even beat outside the body for some 
>> time (hours) if oxygenated.
> 
> Does anybody else find it slightly disturbing that andrel knows such 
> things?
> 
> I mean, I know there's a good reason, but even so... kinda grusome.

As you may be aware, it is my job. It comes with being in a experimental 
cardiology lab. BTW a beating heart is absolutely not gruesome, I would 
rather say beautiful. I'd be happy to show you if you happen to be in 
Amsterdam some time and an experiment is running. ;)

Post a reply to this message

Invisible wrote:
> Does anybody else find it slightly disturbing that andrel knows such 
> things?

You don't watch enough TV.

-- 
   Darren New, San Diego CA, USA (PST)
   Insanity is a small city on the western
   border of the State of Mind.

Post a reply to this message

>>> No the heart is autonomous, will even beat outside the body for some 
>>> time (hours) if oxygenated.
>>
>> Does anybody else find it slightly disturbing that andrel knows such 
>> things?
>>
>> I mean, I know there's a good reason, but even so... kinda grusome.
> 
> As you may be aware, it is my job. It comes with being in a experimental 
> cardiology lab.

Indeed. But still... kinda freaky.

> BTW a beating heart is absolutely not gruesome, I would 
> rather say beautiful. I'd be happy to show you if you happen to be in 
> Amsterdam some time and an experiment is running. ;)

I once saw an operation on TV. They cut open a guy's chest, ripped his 
ripcage apart, and inside was his heart. I swear, it honestly looked 
like it was trying to CLIMB THE HELL OUT OF HIS BODY AND RUN AWAY!! O_O

*shivers*

Post a reply to this message

>> Does anybody else find it slightly disturbing that andrel knows such 
>> things?
> 
> You don't watch enough TV.

I always thought I watch far too much TV. :-P

Post a reply to this message

On 15-6-2009 17:33, Stephen wrote:
> andrel <a_l### [at] hotmailcom> wrote:
>> On 15-6-2009 14:02, Stephen wrote:
>>> It’s not really the power it is the frequency. What happens is that the
>>> electrical impulses to the heart are interrupted and fibrillation occurs. IIRC
>>> there are parts of the heart that receive the electrical impulses from the
>>> brain then transmit secondary impulses to the muscle tissue to contract and
>>> relax in an organised way.
>> No the heart is autonomous, will even beat outside the body for some
>> time (hours) if oxygenated. It's frequency can be modulated by stuff in
>> the blood and by nerve tissue that is close to the sinus node when they
>> release special molecules.
>>
> 
> I thought that you might respond :)
> It was “lies for children” :P

Thanks for assuming that there are only kids in this newsgroup. ;)

I disagree, a lie for children is when you explain something highly 
simplified and/or deliberately wrong because the actual facts are to 
difficult. Suggesting that the heart is driven by the nervous system is 
simply wrong and what actually happens is just as easy to understand. 
Involving the brain will in a bright child immediately lead to questions 
what happens when you sleep or are unconscious. The child might even 
fear to go to sleep. It does not explain coma or how a paralysed person 
can live. So, please, if you have children, tell them a simplification 
of the truth, but never that your brain dictates when the heart has to 
beat.

Post a reply to this message

On 15-6-2009 17:56, Invisible wrote:
>>>> No the heart is autonomous, will even beat outside the body for some 
>>>> time (hours) if oxygenated.
>>>
>>> Does anybody else find it slightly disturbing that andrel knows such 
>>> things?
>>>
>>> I mean, I know there's a good reason, but even so... kinda grusome.
>>
>> As you may be aware, it is my job. It comes with being in a 
>> experimental cardiology lab.
> 
> Indeed. But still... kinda freaky.
> 
>> BTW a beating heart is absolutely not gruesome, I would rather say 
>> beautiful. I'd be happy to show you if you happen to be in Amsterdam 
>> some time and an experiment is running. ;)
> 
> I once saw an operation on TV. They cut open a guy's chest, ripped his 
> ripcage apart, and inside was his heart. I swear, it honestly looked 
> like it was trying to CLIMB THE HELL OUT OF HIS BODY AND RUN AWAY!! O_O
> 
> *shivers*

Was that a dramatazation or an actual procedure?

Post a reply to this message

>>> BTW a beating heart is absolutely not gruesome, I would rather say 
>>> beautiful. I'd be happy to show you if you happen to be in Amsterdam 
>>> some time and an experiment is running. ;)
>>
>> I once saw an operation on TV. They cut open a guy's chest, ripped his 
>> ripcage apart, and inside was his heart. I swear, it honestly looked 
>> like it was trying to CLIMB THE HELL OUT OF HIS BODY AND RUN AWAY!! O_O
>>
>> *shivers*
> 
> Was that a dramatazation or an actual procedure?

It was meant to be the real thing. Apparently there was something or 
other wrong with this guy's heart, so they were going to rip it out and 
stick a new one in. (I wasn't really paying attention.) The new heart 
looked rather limp and pale. But the still-beating one looked evil! (And 
several sizes too big for the chest cavity, BTW...)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

Post a reply to this message

On 15-6-2009 19:26, Orchid XP v8 wrote:
>>>> BTW a beating heart is absolutely not gruesome, I would rather say 
>>>> beautiful. I'd be happy to show you if you happen to be in Amsterdam 
>>>> some time and an experiment is running. ;)
>>>
>>> I once saw an operation on TV. They cut open a guy's chest, ripped 
>>> his ripcage apart, and inside was his heart. I swear, it honestly 
>>> looked like it was trying to CLIMB THE HELL OUT OF HIS BODY AND RUN 
>>> AWAY!! O_O
>>>
>>> *shivers*
>>
>> Was that a dramatazation or an actual procedure?
> 
> It was meant to be the real thing. Apparently there was something or 
> other wrong with this guy's heart, so they were going to rip it out and 
> stick a new one in. (I wasn't really paying attention.) The new heart 
> looked rather limp and pale. But the still-beating one looked evil! (And 
> several sizes too big for the chest cavity, BTW...)

I think I would not use 'rip' and 'stick' to describe the procedure. 
That the heart to be explanted is big and ugly is to be expected. The 
ones I see are all failing hearts and these are huge, and if the patient 
is older they are yellow with fat. Yes, I do sometimes see them. Guess 
what we are doing with them...

Post a reply to this message

>> A pretty accurate model of a battery is a perfect voltage source in 
>> series with an "internal" resistor.  This then nicely explains how the 
>> more current you try to draw the lower the apparent voltage will be at 
>> the terminals of the battery.
>
> As far as I'm aware, resistors affect only current, not potential.

That statement doesn't even make sense.  The voltage across the terminals of 
a resistor is equal to the current going through it times the resistance, 
that's all you need to know and then you can figure out any resistor circuit 
with a bit of algebra.

By the way, notice how the lights dim on your car (and maybe your radio also 
shuts off?) when you start the engine?  That's because the starter motor is 
drawing a huge current and causing the battery voltage to drop to way below 
12V.

Your lights are connected directly to the battery supply, so they dim, and 
the radio might only be specced to work down to 8V or something, in which 
case it shuts itself off until the voltage comes back up again.  Some radios 
can survive very low voltages for short periods of time, so they usually 
stay on while the engine is being started, but not on all cars.

Post a reply to this message

>> As far as I'm aware, resistors affect only current, not potential.
> 
> That statement doesn't even make sense.

As the resistence of a circuit increases, the current flowing through it 
decreases. But that doesn't affect the actual potential across the circuit.

Or rather, that's what the books I've read claim, anyway...

> By the way, notice how the lights dim on your car (and maybe your radio 
> also shuts off?) when you start the engine?

Yeah. That's because there's a limit to how much current the battery can 
supply.

Indeed, I'm told the only reason cars use lead acid batteries (which are 
very heavy yet don't actually hold that much charge) is so that they can 
briefly deliver the astronomical current required to turn the starter 
motor. From the sources I've read, if it weren't for that, you'd only 
need a battery a fraction of the size to run the spark plugs, headlights 
and other electrical bits.

Post a reply to this message

Invisible wrote:
>
> As the resistence of a circuit increases, the current flowing through it
> decreases. But that doesn't affect the actual potential across the circuit.

That's as long as the resistance on the feeding wires is small enough to
be uncompareable to the resistance of the actual device. Actually, even
then the change of the potential is just so small that you can't measure it.

> Or rather, that's what the books I've read claim, anyway...

I bet they are "for teh n00bz" -level books :).

> Yeah. That's because there's a limit to how much current the battery can
> supply.

And that's because of the internal resistance. If there were no such
thing, what would limit the current?

> Indeed, I'm told the only reason cars use lead acid batteries (which are
> very heavy yet don't actually hold that much charge) is so that they can
> briefly deliver the astronomical current required to turn the starter
> motor. 

Actually cars use lead acid batteries because they still are just the
best way to deliver that current reliably. NiCD -batteries can't stand
uneven charging, NiMH -batteries can't deliver enough current IIRC,
Litium batteries (both LiPo and Li-ion) can't stand water-freezing
temperatures. It all actually makes sense :).

> From the sources I've read, if it weren't for that, you'd only
> need a battery a fraction of the size to run the spark plugs, headlights
> and other electrical bits.

Actually you wouldn't need a battery to run those things, you'd just
need a battery/capacitor to regulate the current coming from the alternator.

-Aero

Post a reply to this message

>> Or rather, that's what the books I've read claim, anyway...
> 
> I bet they are "for teh n00bz" -level books :).

Well, it does say "ages 5 and above"...

>> Yeah. That's because there's a limit to how much current the battery can
>> supply.
> 
> And that's because of the internal resistance. If there were no such
> thing, what would limit the current?

I'm sure there's a logic there somewhere - it just doesn't make any 
sense to *me*. ;-)

>> Indeed, I'm told the only reason cars use lead acid batteries (which are
>> very heavy yet don't actually hold that much charge) is so that they can
>> briefly deliver the astronomical current required to turn the starter
>> motor. 
> 
> Actually cars use lead acid batteries because they still are just the
> best way to deliver that current reliably. NiCD -batteries can't stand
> uneven charging, NiMH -batteries can't deliver enough current IIRC,
> Litium batteries (both LiPo and Li-ion) can't stand water-freezing
> temperatures. It all actually makes sense :).

...as I said, you need the wattage to turn the starter motor. ;-) If you 
only needed electricity for the sparks and lights and stuff, you 
wouldn't need nearly as much power. (Freezing would still be an issue 
tho...)

>> From the sources I've read, if it weren't for that, you'd only
>> need a battery a fraction of the size to run the spark plugs, headlights
>> and other electrical bits.
> 
> Actually you wouldn't need a battery to run those things, you'd just
> need a battery/capacitor to regulate the current coming from the alternator.

Not if you expect the headlights to work while the engine isn't running. ;-)

Post a reply to this message

andrel <a_l### [at] hotmailcom> wrote:

> Thanks for assuming that there are only kids in this newsgroup. ;)
>

Only?


> I disagree, a lie for children is when you explain something highly
> simplified and/or deliberately wrong because the actual facts are to
> difficult. Suggesting that the heart is driven by the nervous system is
> simply wrong and what actually happens is just as easy to understand.
> Involving the brain will in a bright child immediately lead to questions
> what happens when you sleep or are unconscious. The child might even
> fear to go to sleep. It does not explain coma or how a paralysed person
> can live. So, please, if you have children, tell them a simplification
> of the truth, but never that your brain dictates when the heart has to
> beat.

OK I stand (or sit) corrected. You’ve taken the heart out of my interest :)

Stephen

Post a reply to this message

Invisible wrote:
> 
> Well, it does say "ages 5 and above"...

As assumed :).

>>> Yeah. That's because there's a limit to how much current the battery can
>>> supply.
>>
>> And that's because of the internal resistance. If there were no such
>> thing, what would limit the current?
> 
> I'm sure there's a logic there somewhere - it just doesn't make any
> sense to *me*. ;-)

That logic IS the internal resistance. You'd just need to put some mind
to it (and maybe read a book for ages 15 and above), and you'd certainly
figure it out.

>> Actually cars use lead acid batteries because they still are just the
>> best way to deliver that current reliably. NiCD -batteries can't stand
>> uneven charging, NiMH -batteries can't deliver enough current IIRC,
>> Litium batteries (both LiPo and Li-ion) can't stand water-freezing
>> temperatures. It all actually makes sense :).
> 
> ...as I said, you need the wattage to turn the starter motor. ;-) 

Yes. You can have the wattage from numerous different battery types. The
problem is that you can't have the wattage from any other battery type
reliably enough, or in a package that actually fits in the car.

> If you
> only needed electricity for the sparks and lights and stuff, you
> wouldn't need nearly as much power. (Freezing would still be an issue
> tho...)

Yep, indeed it would. Lead acid handles freezing temperatures incredibly
well and in small package.

BTW, when there's *really* cold outside, the battery won't give as much
current out, thus not starting the car. To beat this you can use the
internal resistance to heat up the battery a bit (eg. by putting your
park lights on) and then start the car. Viva la resistance!

> Not if you expect the headlights to work while the engine isn't running.
> ;-)

True. The headlights are traditionalle 55W halogens and tail light
(which are lit when headlights are lit) 5W halogens (and there's usually
4 of them). Parks are also usually 5W halogens, that makes
55*2+5*6=140W, which makes ~12A of current, which makes under 6 hours to
drain the normal ~60Ah battery empty or less to drain it to the point
the current won't be enough for starting the engine. Therefore it's a
good idea not to use the headlights too much while the engine ain't
running ;-).

-Aero

Post a reply to this message

> That logic IS the internal resistance. You'd just need to put some mind
> to it (and maybe read a book for ages 15 and above), and you'd certainly
> figure it out.

LOL! Now I have the mental image of electronics textbooks having age 
certificates.

Stuff like "Certificate 18; Contains non-conventional current flow 
direction and nonlinear diode behaviour. May be shocking to certain 
readers." ;-)

Obligatory XKCD quote: http://www.xkcd.com/567/

Post a reply to this message

> NiMH -batteries can't deliver enough current IIRC,

NiMH should be fine for current, they are used in electric vehicles after 
all.

I suspect the main drawback of NiMH is that it self-discharges pretty 
quickly compared to lead acid.  Your car might not start after leaving it 
for a few months, which would be unacceptable to most car makers.

Post a reply to this message

scott wrote:
>> NiMH -batteries can't deliver enough current IIRC,
> 
> NiMH should be fine for current, they are used in electric vehicles
> after all.

That's the one I wasn't sure about, therefore IIRC :).

> I suspect the main drawback of NiMH is that it self-discharges pretty
> quickly compared to lead acid.  Your car might not start after leaving
> it for a few months, which would be unacceptable to most car makers.

Might be. Anyway, it has such great disadvantage compared to lead acid
on this matter, it's not used :). We can be sure that car manufacturers
and battery manufacturers have calculated it.

-Aero

Post a reply to this message

Eero Ahonen <aer### [at] removethiszbxtnetinvalid> wrote:
> scott wrote:
> >> NiMH -batteries can't deliver enough current IIRC,
> >
> > NiMH should be fine for current, they are used in electric vehicles
> > after all.
>
> That's the one I wasn't sure about, therefore IIRC :).
>
> > I suspect the main drawback of NiMH is that it self-discharges pretty
> > quickly compared to lead acid.  Your car might not start after leaving
> > it for a few months, which would be unacceptable to most car makers.
>
> Might be. Anyway, it has such great disadvantage compared to lead acid
> on this matter, it's not used :). We can be sure that car manufacturers
> and battery manufacturers have calculated it.
>
> -Aero

The main advantage of lead acid batteries is they can be charged with constant
voltage where other batteries like NiCd, NiMh, LiIon or LiPo need specialized
charge electronics to limit the charge current and monitor the cell voltage.
Not so for lead acid. The only thing you need in the car is a voltage regulator
which regulates the output voltage of the alternator to roundabout 14V and the
battery is happy.
Also: Lead Acid Batteries like it if they are fully charged against for example
NiCd thad lose capacity very quickly if you have them laying around fully
charged.

Post a reply to this message

Eero Ahonen wrote:

> 
> That logic IS the internal resistance. You'd just need to put some mind
> to it (and maybe read a book for ages 15 and above), and you'd certainly
> figure it out.
> 

I was going to say something like the reaction rate inside the battery 
caps the amount of current the battery can supply, but I guess that 
would be under internal resistance.

> 
> Yes. You can have the wattage from numerous different battery types. The
> problem is that you can't have the wattage from any other battery type
> reliably enough, or in a package that actually fits in the car.
> 

Lead-acid works well in this application because it can tolerate only 
brief cycles. If you drain a lead acid battery completely, it won't 
recover well. But the advantage of that chemistry is the fast reaction 
rate, which allows the battery to supply a huge amount of amperage in a 
very short time. Other chemistries can tolerate deep cycling (near full 
discharge), but cannot tolerate the very small discharge/recharge cycles 
that are required in a car, e.g. NiMH and NiCd both tend to develop a 
"memory" (NiMH significantly less so, but it does happen) Other 
chemistries don't like being cycled frequently, and will wear down rapidly.

>> If you
>> only needed electricity for the sparks and lights and stuff, you
>> wouldn't need nearly as much power. (Freezing would still be an issue
>> tho...)
> 
> Yep, indeed it would. Lead acid handles freezing temperatures incredibly
> well and in small package.
> 
>

The alternator usually provides all of the current needed to run the 
accessories when the engine is on. This can be proven if you've ever had 
to jump an entirely dead battery and drive to the local auto shop to buy 
a replacement...

-- 
~Mike

Post a reply to this message

scott wrote:
> which case it shuts itself off until the voltage comes back up again.  

I always thought they were just wired up to work in the "on" position but 
not the "start" position.

-- 
   Darren New, San Diego CA, USA (PST)
   Insanity is a small city on the western
   border of the State of Mind.

Post a reply to this message

> The alternator usually provides all of the current needed to run the 
> accessories when the engine is on. This can be proven if you've ever had 
> to jump an entirely dead battery and drive to the local auto shop to buy a 
> replacement...

A lot modern cars already disconnect (electronically) a partially-charged 
battery while cruising to reduce fuel consumption.  Only when you lift off 
the throttle or brake will it connect it up to be charged, thus avoiding 
wasting fuel to charge the battery.

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> scott wrote:
> > which case it shuts itself off until the voltage comes back up again.
>
> I always thought they were just wired up to work in the "on" position but
> not the "start" position.

Depends how the car is wired.
Same with the headlights. Some cars shut them off while starting, others don't.

Post a reply to this message

>> which case it shuts itself off until the voltage comes back up again.
>
> I always thought they were just wired up to work in the "on" position but 
> not the "start" position.

Before cars turned into computers, the radio system typically was just 
connected directly to the battery through a fuse, and has an "ignition" 
signal line to indicate whether it should be on or not (this signal is on in 
both the "on" and "start" position, obviously).

Car manufacturers give all sorts of detailed specs for radio systems, 
detailing exact input voltage patterns and what the system must or must not 
do.  For older radio systems they only had to work down to 10 V or 
something, and when the voltage dropped below that they had to behave 
sensibly (ie not blurt out massive clicking noises or static).  This was 
purely because it is cheap and simple to make a radio work with a supply 
between 10-16V compared to 0-16V (yes, newer radios do have to theoretically 
work with a 0 volt supply for a fraction of a second).

Recently though, especially with these systems where the engine is switched 
off every time you stop, the radio system is required to keep working during 
engine start events.  There are quite detailed specs about what the radio 
must "survive" in terms of supply voltage fluctuation, because the supply 
voltage is anything but smooth in a car.

Post a reply to this message

scott wrote:

> Recently though, especially with these systems where the engine is 
> switched off every time you stop, the radio system is required to keep 
> working during engine start events.  There are quite detailed specs 
> about what the radio must "survive" in terms of supply voltage 
> fluctuation, because the supply voltage is anything but smooth in a car.

Did you see the one on Fifth Gear where they decided to test the theory 
that a car can survive a lightning strike by... driving a car into a 
giant laboratory and hitting it with lightning, with the presenter still 
sat inside?

IIRC, afterwards the car worked great. But the radio was behaving very 
strangely...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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