limiting current on LED''s (slightly off topic)
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Posts: 46,084
In a message dated 5/13/2003 8:53:49 PM Pacific Daylight Time,
jjghost@t... writes:
> This is something that I've been wondering about for a while now and
> was hoping one of you might know. Every circuit I see using LEDs
> uses a seperate current limiting resister for each. Yet, on the
> stuff I build, to cut down on parts I'm usually tempted to tie two
> or three LEDs to one resistor.
>
> Does anyone know of any drawback to doing this?
>
> Just curious, thanks!
>
Depending on how your circuit is wired, there is nothing wrong with many leds
sharing a resistor....
For example, If you have for example 2 leds and every time both will be on at
once, you connect the two leds in series and set the current through the Leds
with a resistor.
Each led will consume about 1.8 volts, therefore 2 will use up 3.6 volts. If
you are powering the leds from 5 volts and you want 15 mA divide 0.015 into
1.4 (5 - 3.6) and thats it!!
You can do this with any number of leds in series as long as the number of
leds leaves some voltage left over to use a current limiting resistor.
Another example....you have a fixed 10 volt supply to power the leds, you can
put 5 in series which will add up to about 9 volts. This leaves one volt to
set the current with an appropriate resistor.
Watch out though, the voltage used by the Led will vary with the about of
current. Typically, the more current you put through the led, the more
voltage it will use....this usually is not a problem. From my experience the
led voltage will be anywhere from 1.7 to about 2.2 volts for a simple run of
the mill led.
If you connect leds in parallel and use only one resistor, and you will not
have the same number of leds on every time, as the other "person" mentioned,
the brightness will vary depending on how much current you cause to pass
through the resistor...
Hope that now you are completely confused.......
[noparse][[/noparse]Non-text portions of this message have been removed]
jjghost@t... writes:
> This is something that I've been wondering about for a while now and
> was hoping one of you might know. Every circuit I see using LEDs
> uses a seperate current limiting resister for each. Yet, on the
> stuff I build, to cut down on parts I'm usually tempted to tie two
> or three LEDs to one resistor.
>
> Does anyone know of any drawback to doing this?
>
> Just curious, thanks!
>
Depending on how your circuit is wired, there is nothing wrong with many leds
sharing a resistor....
For example, If you have for example 2 leds and every time both will be on at
once, you connect the two leds in series and set the current through the Leds
with a resistor.
Each led will consume about 1.8 volts, therefore 2 will use up 3.6 volts. If
you are powering the leds from 5 volts and you want 15 mA divide 0.015 into
1.4 (5 - 3.6) and thats it!!
You can do this with any number of leds in series as long as the number of
leds leaves some voltage left over to use a current limiting resistor.
Another example....you have a fixed 10 volt supply to power the leds, you can
put 5 in series which will add up to about 9 volts. This leaves one volt to
set the current with an appropriate resistor.
Watch out though, the voltage used by the Led will vary with the about of
current. Typically, the more current you put through the led, the more
voltage it will use....this usually is not a problem. From my experience the
led voltage will be anywhere from 1.7 to about 2.2 volts for a simple run of
the mill led.
If you connect leds in parallel and use only one resistor, and you will not
have the same number of leds on every time, as the other "person" mentioned,
the brightness will vary depending on how much current you cause to pass
through the resistor...
Hope that now you are completely confused.......
[noparse][[/noparse]Non-text portions of this message have been removed]