limiting current on LED''s (slightly off topic)
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Posts: 46,084
I've done it before. If you have the same batch of leds, it will
usually work. However, small differences in the led Voltage drop will
cause one to hog all the current and the others to be either dim or not
lit. So, if you are going to do this, you need to match the leds so
that all the ones on a single resistor will have the same Voltage drop
across it.
Or, you can just buy the leds with the limiting resistors in place,
installed inside the led housing. Digikey or Newark has them.
Original Message
From: smartdim@a... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=jCIVEalxTrLzHr7o9zBE74vKYv6Qo-ArT6OOwRkThjpp8Jp4ivVvARWEyWQwol3rQ-imHjfi]smartdim@a...[/url
Sent: Wednesday, May 14, 2003 12:53 AM
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] limiting current on LED's (slightly off
topic)
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.......
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usually work. However, small differences in the led Voltage drop will
cause one to hog all the current and the others to be either dim or not
lit. So, if you are going to do this, you need to match the leds so
that all the ones on a single resistor will have the same Voltage drop
across it.
Or, you can just buy the leds with the limiting resistors in place,
installed inside the led housing. Digikey or Newark has them.
Original Message
From: smartdim@a... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=jCIVEalxTrLzHr7o9zBE74vKYv6Qo-ArT6OOwRkThjpp8Jp4ivVvARWEyWQwol3rQ-imHjfi]smartdim@a...[/url
Sent: Wednesday, May 14, 2003 12:53 AM
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] limiting current on LED's (slightly off
topic)
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]
To UNSUBSCRIBE, just send mail to:
basicstamps-unsubscribe@yahoogroups.com
from the same email address that you subscribed. Text in the Subject
and Body of the message will be ignored.
Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/