While we're here, does anybody comprehend what VCC, VSS, VDD and related 
symbols mean?

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On Wed, 19 May 2010 16:16:28 +0200, Invisible <voi### [at] devnull> wrote:
> While we're here, does anybody comprehend what VCC, VSS, VDD and related  
> symbols mean?

http://en.wikipedia.org/wiki/IC_power_supply_pin



-- 
FE

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>> What you mean is, they draw *up to* 40 mA?
>
> No idea. The spec sheet just says "forward current".

It's probably under a section titled "typical operating conditions" or 
"absolute maximum ratings".  There should also be a graph showing a curve of 
current against voltage, from zero up to the absolute maximum current 
rating.  Usually with LEDs the light output is fairly linear with respect to 
current, but certainly not with voltage.

>> How much they actually draw depends on what value series resistor you put 
>> in there.  Using Ohm's law and the V-I curve of the LED from the 
>> datasheet you can work out exactly what resistor size you need for a 
>> given current.
>
> Heh. And here I was thinking it's just a case of "if the voltage is less 
> than X, you don't need a resistor"...

If you want to run the LED at 10 mA you look up on the V-I curve in the LED 
datasheet what forward voltage corresponds to 10 mA.  Say for example 2.8 V. 
If your logic output is 5 V (and the other side of the LED is connected to 
0V), then you want 2.8 V across the LED and hence 2.2 V across the series 
resistor.  So your resistor needs to drop 2.2 V with a current of 10 mA, 
that makes it a 220 Ohm resistor.

Connecting the LED directly with no series resistor to a voltage source is 
usually a bad idea (even if you think the voltage is correct), because even 
slight changes in the voltage cause large differences to the current the LED 
draws which might damage your circuit.

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> content of this datasheet means! :-D

I can send you a longer one if you want :-)

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scott wrote:

>> the content of this datasheet means! :-D
> 
> I can send you a longer one if you want :-)

Why thank you, you're too kind. :-}

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>> Heh. And here I was thinking it's just a case of "if the voltage is 
>> less than X, you don't need a resistor"...
> 
> If you want to run the LED at 10 mA you look up on the V-I curve in the 
> LED datasheet what forward voltage corresponds to 10 mA.  Say for 
> example 2.8 V. If your logic output is 5 V (and the other side of the 
> LED is connected to 0V), then you want 2.8 V across the LED and hence 
> 2.2 V across the series resistor.  So your resistor needs to drop 2.2 V 
> with a current of 10 mA, that makes it a 220 Ohm resistor.
> 
> Connecting the LED directly with no series resistor to a voltage source 
> is usually a bad idea (even if you think the voltage is correct), 
> because even slight changes in the voltage cause large differences to 
> the current the LED draws which might damage your circuit.

With my old electronics kit, if I'm remembering this rightly, you 
connect through a resistor if you're using the 3V power source, and skip 
the resistor if you're using the 1.5V source.

Oh, and *don't* connect the LED to the 9V source at all. I did that once 
by mistake... that particular LED doesn't do anything now.

Ah, life seemed so much simpler back then. :-(

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Fredrik Eriksson wrote:
> On Wed, 19 May 2010 16:16:28 +0200, Invisible <voi### [at] devnull> wrote:
>> While we're here, does anybody comprehend what VCC, VSS, VDD and 
>> related symbols mean?
> 
> http://en.wikipedia.org/wiki/IC_power_supply_pin

OK, thanks for that...

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> Oh, and *don't* connect the LED to the 9V source at all. I did that once 
> by mistake... that particular LED doesn't do anything now.
>
> Ah, life seemed so much simpler back then. :-(

Reminds me of an interesting story.  We got a complaint from a customer that 
about 25% LCDs they have from us have a dark edge to the picture, either on 
the left or the right side.  When we checked the faulty ones we found that 
always the left or right LED was not working in the backlight (I think there 
were 8 in total).  This puzzled us for a long time, why would it always be 
the far left or far right LED that stopped working, and why were they 
breaking in the first place?

In the end we figured out what happened.  Actually *every* LCD we shipped 
had one broken LED, it's just they never noticed it when it was one of the 
central LEDs.  And it turned out the equipment at our outgoing inspection 
test was set to the wrong voltage, so the tester was just blowing one LED on 
every backlight that went out :-)

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Am 19.05.2010 16:04, schrieb Invisible:

> Most of the books I've seen only cover the 7400, 7401, 7402, etc., and
> don't mention things like the 741G00. Could be useful to know. (Except
> that Maplin don't seem to sell this stuff... *sigh*)

AFAIK they usually come in SMD packages only - which in turn means 
they'll usually be sold in bulk. And anyway - why would you buy, say, 
one 74x1G00 NAND and one 74x1G32 OR gate when you can get a 74x00 4xNAND 
IC for the same (or even lower) price, and wire up three of the gates to 
simulate the OR gate? (The 74x265 is also a neat beast - with its two 
(N)AND gates and two inverters/drivers it's an ideal candidate to 
simulate quite a wide variety of 2-gate combos you can imagine).

I guess the single-gate ICs are mainly targeted at applications where 
space, routing, speed or EMI constraints are serious enough to forbid 
sharing the gates of a single multi-gate IC among different sections of 
the circuitry.

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Am 19.05.2010 15:52, schrieb Invisible:

> http://www.maplin.co.uk/Media/PDFs/N24AZ.pdf
>

> content of this datasheet means! :-D

Ah yes, the typical cheap alphanumeric LCD.

I guess most of the datasheet information says, "this thing does exactly 
the same as other manufacturers' devices of this type".

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