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>> OK, now I'm confused. A graph of current against voltage? But current
>> is completely *determined by* voltage! o_O
>
> Of course it is determined, but it's only a linear relationship (V=IR)
> for resistors. LEDs are diodes and are certainly not linear, so you
> need a graph. You could draw a graph for a resistor if you wanted like
> I said, but it would be a bit pointless because we know V=IR for a
> resistor.
Wait a sec - so you're saying there are devices which actually violate
Ohm's law?
>> OK... so... why not an open circuit then? That would have infinite
>> resistence?
>
> Then the input would be equally connected to 0V and V+ with near
> infinite resistances, that's called a "floating" input, because it can
> be persuaded very easily to either go to 0V or V+, depending on the
> exact details inside the IC (and maybe some state of the other inputs,
> who knows?!). Best to ensure you have at most around 100K of resistance
> to one of the rails to fix the input reliably.
Uh... so have it always connected to one rail, but through such a high
resistance that the rail can't affect it?
Well, you're the one with the engineering degree, but it *still* doesn't
make any sense to me...
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> Wait a sec - so you're saying there are devices which actually violate
> Ohm's law?
Yes, more exactly, there are devices which do not have constant
resistance/impedance.
> Uh... so have it always connected to one rail, but through such a high
> resistance that the rail can't affect it?
>
> Well, you're the one with the engineering degree, but it *still* doesn't
> make any sense to me...
Maybe it would help if you imagined that inside the IC there is a big
resistor (eg 1Mohm) connecting the input to one of the supply rails (but you
don't know which one). In order to ensure this resistor cannot affect the
input voltage, you need to make sure the input is always connected to the
voltage you want through a smaller resistor (electricity likes the lowest
resistance path!).
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scott wrote:
>> Wait a sec - so you're saying there are devices which actually violate
>> Ohm's law?
>
> Yes, more exactly, there are devices which do not have constant
> resistance/impedance.
Are you saying the resistence doesn't determine the current? Or are you
just saying that the current can cause the resistence to change?
>> Well, you're the one with the engineering degree, but it *still*
>> doesn't make any sense to me...
>
> Maybe it would help if you imagined that inside the IC there is a big
> resistor (eg 1Mohm) connecting the input to one of the supply rails (but
> you don't know which one). In order to ensure this resistor cannot
> affect the input voltage, you need to make sure the input is always
> connected to the voltage you want through a smaller resistor
Hmm, OK.
> (electricity likes the lowest resistance path!).
Now, see, I always thought electricity just takes *every* path, with the
current being determined by Ohm's law.
So, if you have a high-resistence path from A to B, and then you add a
second, lower-resistence path from A to B, does the current flowing
through the first path change? Or does it remain the same?
(Clearly the *potential* at A and B will be changed - there's more
current flowing now...)
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>> Yes, more exactly, there are devices which do not have constant
>> resistance/impedance.
>
> Are you saying the resistence doesn't determine the current? Or are you
> just saying that the current can cause the resistence to change?
If you want to look at it like that, yes, the resistance will depend on what
the applied voltage is, rather than always being constant. Nobody really
talks of "resistance" with such devices though, it's just accepted that V is
not linearly proportional to I, and that for any given V you can look up the
I from the chart.
> Now, see, I always thought electricity just takes *every* path, with the
> current being determined by Ohm's law.
It does, and if use Ohm's law to work out the voltage at a point that is
connected to 0V with a 1kOhm resistor and to 5V with a 1Mohm resistor, you
will see the voltage is essentially 0V. OK so it will actually be very
slightly above 0V, but that's why there is a tolerance range on the input of
such devices.
> So, if you have a high-resistence path from A to B, and then you add a
> second, lower-resistence path from A to B, does the current flowing
> through the first path change? Or does it remain the same?
It remains the same because neither the voltage across the high value
resistor, nor the resistance of the resistor itself have changed, and then
I=V/R. Of course you get some additional current flowing through the lower
resistance path now, but still the same amount through the high resistance.
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>> Are you saying the resistence doesn't determine the current? Or are
>> you just saying that the current can cause the resistence to change?
>
> If you want to look at it like that, yes, the resistance will depend on
> what the applied voltage is, rather than always being constant.
You you mean stuff like if you put power through a wire, it heats up,
and resistance varies by temperature?
> Nobody
> really talks of "resistance" with such devices though, it's just
> accepted that V is not linearly proportional to I, and that for any
> given V you can look up the I from the chart.
Hmm, how odd...
>> Now, see, I always thought electricity just takes *every* path, with
>> the current being determined by Ohm's law.
>
> It does, and if use Ohm's law to work out the voltage at a point that is
> connected to 0V with a 1kOhm resistor and to 5V with a 1Mohm resistor,
> you will see the voltage is essentially 0V. OK so it will actually be
> very slightly above 0V, but that's why there is a tolerance range on the
> input of such devices.
Hmm... my head hurts now! Heh.
>> So, if you have a high-resistence path from A to B, and then you add a
>> second, lower-resistence path from A to B, does the current flowing
>> through the first path change? Or does it remain the same?
>
> It remains the same.
Oh thank God for that! At least something about my understanding of
electricity is correct...
The whole "electricity chooses the easiest path" makes it sound like
when an easier path becomes available, all the current goes down that
and ignores the other paths! o_O
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> You you mean stuff like if you put power through a wire, it heats up, and
> resistance varies by temperature?
Yes pretty much, now imagine that in your design the resistance of the wire
changes enough to have a big impact on how your circuit works. If you
connect an LED to a voltage source and gradually turn up the voltage from 0
volts, you will see that almost no current flows, it's like nothing was
connected inside. But suddenly, you will get to around 2V or something the
current will shoot upwards, and from then on you only need make tiny
increments to the voltage and the current will keep getting larger and
larger very quickly until the LED blows haha
> Oh thank God for that! At least something about my understanding of
> electricity is correct...
Recipe for solving circuits with resistors:
1) Label each unknown voltage node as V1, V2, etc
2) Write out Ohm's law for each resistor in terms of V1, V2 etc
3) Solve the set of equations you wrote out in 2) for the unknown voltages
> The whole "electricity chooses the easiest path" makes it sound like when
> an easier path becomes available, all the current goes down that and
> ignores the other paths! o_O
If the "easiest" path is of *much* lower resistance than the others, then
yeh you can usually ignore the others.
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scott wrote:
>> You you mean stuff like if you put power through a wire, it heats up,
>> and resistance varies by temperature?
>
> Yes pretty much, now imagine that in your design the resistance of the
> wire changes enough to have a big impact on how your circuit works. If
> you connect an LED to a voltage source and gradually turn up the voltage
> from 0 volts, you will see that almost no current flows, it's like
> nothing was connected inside. But suddenly, you will get to around 2V
> or something the current will shoot upwards, and from then on you only
> need make tiny increments to the voltage and the current will keep
> getting larger and larger very quickly until the LED blows haha
Is that why LEDs display a non-linear brightness response too?
>> The whole "electricity chooses the easiest path" makes it sound like
>> when an easier path becomes available, all the current goes down that
>> and ignores the other paths! o_O
>
> If the "easiest" path is of *much* lower resistance than the others,
> then yeh you can usually ignore the others.
I presume you mean you can "ignore" them for the purposes of figuring
out the total current flowing through the system. Clearly the current
flowing through each path only depends on the resistence of that path
and the potential difference...
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> Is that why LEDs display a non-linear brightness response too?
Non-linear with respect to what? The brightness response is pretty linear
with respect to current, double the current you get double the brightness to
within a few %.
> I presume you mean you can "ignore" them for the purposes of figuring out
> the total current flowing through the system. Clearly the current flowing
> through each path only depends on the resistence of that path and the
> potential difference...
I meant if you have a point of unknown voltage connected to two (or more)
other known voltages via resistors. If one of the resistors is drastically
lower than all the others, then you can just assume the unknown point is
directly connected through the lowest resistance and treat all the other
resistors as essentially not connected.
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scott wrote:
>> Is that why LEDs display a non-linear brightness response too?
>
> Non-linear with respect to what? The brightness response is pretty
> linear with respect to current, double the current you get double the
> brightness to within a few %.
Well, I had a diagram for a circuit where you connect a lamp to the
battery through a linear potentiometer. The brightness of the lamp
varies roughly linearly. But the brightness of an LED... does not.
(I'm showing my age now! Can you imagine trying to buy a 3V lamp today??)
>> I presume you mean you can "ignore" them for the purposes of figuring
>> out the total current flowing through the system. Clearly the current
>> flowing through each path only depends on the resistence of that path
>> and the potential difference...
>
> I meant if you have a point of unknown voltage connected to two (or
> more) other known voltages via resistors. If one of the resistors is
> drastically lower than all the others, then you can just assume the
> unknown point is directly connected through the lowest resistance and
> treat all the other resistors as essentially not connected.
Right. Gotchya.
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"Invisible" <voi### [at] dev null> wrote in message
news:49196e5f$1@news.povray.org...
>
> Yeah - maybe I should get said to write open source software?
>
> Oh, wait...
You may not always get paid to write open source, but it's good practice at
writing real applications and working with (lots) of other people, and it
looks very good on a CV
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