Re: A few novice questions

On May 2, 4:13 pm, Tomás Ó hÉilidhe <t...@xxxxxxxxxxx> wrote:
OK here's an example:

I have 26 LED's. Instead of them all being on at the same time, I'm
going to light each of them for 100 microseconds and then move onto
the next one (i.e. display multiplexing).

The LED's I'm using have a maximum current rating of 25 mA, but since
they'll only be on for one twenty-sixth of the time, I'm going to put
about half an ampere thru them. To achieve this, I'm not even going to
use a resistor, I'm just gonna go straight from Vcc to LED to ground.
(I've tested this in my project this year and it works perfectly).

Now my microcontroller pin won't be able to supply half an ampere, so
I'll use the microcontroller pin to turn on a transistor which will
allow the half ampere to flow from Vcc to the LED to ground.

So my question is, what's the best transistor to use? The supply
voltage is 5 V and it's powering an LED.

This is the second post in a week that wants to drive LEDs without a
current-limiting resistor. Is there something in the water that's
making people think like this? :-)

Regarding the transistor, just pick a cheap one that will take the
desired current and circuit voltage without stress - having a current
rating e.g. 50% or more higher than the current it will be switching,
same with the voltage rating. For example, the 2N2222 and plastic
versions are rated to 800mA.

Now, about those LEDs. Don't do it. In the first place, LEDs have a
maximum pulse rating as well as a continuous rating. For example, I
just looked one up that has a 30mA continuous rating, and a 160mA peak
pulse rating. That is to say, you can't just do the multiplexing
thing ad infinitum at the equivalent brightness of the continuous
rating. And while I've got your attention ;-) please, use resistors
to limit your LED current in a controlled way. In every circuit,
_something_ is limiting the current. Without resistors in your case
it can be the LED, the transistor, power supply voltage sag, or any
combination. You certainly won't have control over any of those
factors, and you are just as certainly stressing one or more of those
components, and they will start failing, probably right after your
devices are installed in Siberia or the Sahara. Add a resistor, let
Ohm's Law be your friend.

Mike
.

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