LED question
Archiver
Posts: 46,084
Greetings,
I realize this question is not specifically related to a stamp, the circuit
will be run by a stamp if that counts. If anyone has another email list that
this would be better suited to, please let me know.
I want to power up an array of LED's that will be used as a lamp on an
automobile.
The LED's are 30ma at 2v.
using 10 LED's as an example, my current concept is to:
combine the LED's into 5 groups of 2 LED's each (in parallel)
combine the resulting 5 groups in series.
use a regulated 10v supply and a single current limiting resistor.
I have done this and I get a nice 30ma and 2v at each LED (or, real close).
Are there any major reasons I shouldn't do it this way?
Thanks,
Scott
I realize this question is not specifically related to a stamp, the circuit
will be run by a stamp if that counts. If anyone has another email list that
this would be better suited to, please let me know.
I want to power up an array of LED's that will be used as a lamp on an
automobile.
The LED's are 30ma at 2v.
using 10 LED's as an example, my current concept is to:
combine the LED's into 5 groups of 2 LED's each (in parallel)
combine the resulting 5 groups in series.
use a regulated 10v supply and a single current limiting resistor.
I have done this and I get a nice 30ma and 2v at each LED (or, real close).
Are there any major reasons I shouldn't do it this way?
Thanks,
Scott
Comments
I am pretty sure this exceeds the regulator limitations
now if you are going to provide buffering or use a pass transistor that is
a different matter. 60 ma per pin may even exceed the per pin limitation.
richard
Original Message
From: <scottsmith13@a...>
To: <basicstamps@yahoogroups.com>
Sent: Tuesday, March 27, 2001 12:26 PM
Subject: [noparse][[/noparse]basicstamps] LED question
> Greetings,
>
> I realize this question is not specifically related to a stamp, the
circuit
> will be run by a stamp if that counts. If anyone has another email list
that
> this would be better suited to, please let me know.
>
> I want to power up an array of LED's that will be used as a lamp on an
> automobile.
>
> The LED's are 30ma at 2v.
>
> using 10 LED's as an example, my current concept is to:
> combine the LED's into 5 groups of 2 LED's each (in parallel)
> combine the resulting 5 groups in series.
> use a regulated 10v supply and a single current limiting resistor.
>
> I have done this and I get a nice 30ma and 2v at each LED (or, real
close).
>
> Are there any major reasons I shouldn't do it this way?
>
> Thanks,
> Scott
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
could run it through a shift register or latch or something which should
have enough VA to run the led's.
On Tue, 27 Mar 2001, dakota wrote:
> 30 ma x 10 leds = 300 ma
> I am pretty sure this exceeds the regulator limitations
> now if you are going to provide buffering or use a pass transistor that is
> a different matter. 60 ma per pin may even exceed the per pin limitation.
>
> richard
>
>
Original Message
> From: <scottsmith13@a...>
> To: <basicstamps@yahoogroups.com>
> Sent: Tuesday, March 27, 2001 12:26 PM
> Subject: [noparse][[/noparse]basicstamps] LED question
>
>
> > Greetings,
> >
> > I realize this question is not specifically related to a stamp, the
> circuit
> > will be run by a stamp if that counts. If anyone has another email list
> that
> > this would be better suited to, please let me know.
> >
> > I want to power up an array of LED's that will be used as a lamp on an
> > automobile.
> >
> > The LED's are 30ma at 2v.
> >
> > using 10 LED's as an example, my current concept is to:
> > combine the LED's into 5 groups of 2 LED's each (in parallel)
> > combine the resulting 5 groups in series.
> > use a regulated 10v supply and a single current limiting resistor.
> >
> > I have done this and I get a nice 30ma and 2v at each LED (or, real
> close).
> >
> > Are there any major reasons I shouldn't do it this way?
> >
> > Thanks,
> > Scott
> >
> >
> >
> > Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
> >
> >
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
Sean T. Lamont, CTO / Chief NetNerd, Abstract Software, Inc. (ServNet)
Seattle - Bellingham - Vancouver - Portland - Everett - Tacoma - Bremerton
email: lamont@a... WWW: http://www.serv.net
"...There's no moral, it's just a lot of stuff that happens". - H. Simpson
---
Don Russell
---
Original Message
From: "Sean T. Lamont .lost." <lamont@a...>
To: <basicstamps@yahoogroups.com>
Sent: Tuesday, March 27, 2001 12:29 PM
Subject: Re: [noparse][[/noparse]basicstamps] LED question
>
> the 7805 is good to 1A, though the stamp can't output that much. You
> could run it through a shift register or latch or something which should
> have enough VA to run the led's.
>
> On Tue, 27 Mar 2001, dakota wrote:
>
> > 30 ma x 10 leds = 300 ma
> > I am pretty sure this exceeds the regulator limitations
> > now if you are going to provide buffering or use a pass transistor that
is
> > a different matter. 60 ma per pin may even exceed the per pin
limitation.
> >
> > richard
> >
> >
Original Message
> > From: <scottsmith13@a...>
> > To: <basicstamps@yahoogroups.com>
> > Sent: Tuesday, March 27, 2001 12:26 PM
> > Subject: [noparse][[/noparse]basicstamps] LED question
> >
> >
> > > Greetings,
> > >
> > > I realize this question is not specifically related to a stamp, the
> > circuit
> > > will be run by a stamp if that counts. If anyone has another email
list
> > that
> > > this would be better suited to, please let me know.
> > >
> > > I want to power up an array of LED's that will be used as a lamp on an
> > > automobile.
> > >
> > > The LED's are 30ma at 2v.
> > >
> > > using 10 LED's as an example, my current concept is to:
> > > combine the LED's into 5 groups of 2 LED's each (in parallel)
> > > combine the resulting 5 groups in series.
> > > use a regulated 10v supply and a single current limiting resistor.
> > >
> > > I have done this and I get a nice 30ma and 2v at each LED (or, real
> > close).
> > >
> > > Are there any major reasons I shouldn't do it this way?
> > >
> > > Thanks,
> > > Scott
> > >
> > >
> > >
> > > Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
> > >
> > >
> >
> >
> >
> >
> > Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
> >
> >
> >
>
> Sean T. Lamont, CTO / Chief NetNerd, Abstract Software, Inc. (ServNet)
> Seattle - Bellingham - Vancouver - Portland - Everett - Tacoma - Bremerton
> email: lamont@a... WWW: http://www.serv.net
> "...There's no moral, it's just a lot of stuff that happens". - H. Simpson
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
series, they will consume a total of 30mA, but have a voltage drop of 10V.
To get good regulation of the current through the LEDs, you will want to
have a large enough voltage left to pass through your resistor, so if you
are using a 10V supply, I would suggest only 4 LEDs in series.
That way, your resistor calculation would be:
Vsource - Vled
I =
R
or 30 mA = (10 - 8) / R, R = 67 ohms
The more LEDs you have in series, the more potential change in voltage
drop you will get due to (potential) lot to lot variation in the
manufacture of the LEDs or operating temperature. (I think that the
current through the LEDs will change with temperature, due to a change in
the voltage drop). So you want a large enough voltage left over after the
drop so that your variations will be within whatever your tolerances will
be. (If this is for brake lights, for example, I imagine there will be
strict tolerances on light output variability).
If you have LEDs wired in parallel, then you sum your current.
For the example you gave, you had 5 LEDs in series in parallel with
another 5 LEDs. This will consume 60 mA and drop 10V.
I think it is a good idea to "ladder" your LEDs as you planned, as this
gives you some protection against variations in individual LEDs.
I read some of this info on the Lumileds website a couple of months ago
(they make LED traffic lights among other things - some of their product
literature was interesting reading. www.lumileds.com).
The other thing you might want to think about is using premade cluster
LEDs - Digikey sells some, for example. Although each LED is not as
bright as the brightest individual LEDs on the market, they are
conveniently packaged. For example, they sell a red 9 LED cluster (3x3
square) that uses 30mA with a 5.4 volt drop (each LED is rated 2000mcd),
for $1.99.
Hope this helps.
Nick
On Tue, 27 Mar 2001, Sean T. Lamont .lost. wrote:
>
> the 7805 is good to 1A, though the stamp can't output that much. You
> could run it through a shift register or latch or something which should
> have enough VA to run the led's.
>
> On Tue, 27 Mar 2001, dakota wrote:
>
> > 30 ma x 10 leds = 300 ma
> > I am pretty sure this exceeds the regulator limitations
> > now if you are going to provide buffering or use a pass transistor that is
> > a different matter. 60 ma per pin may even exceed the per pin limitation.
> >
> > richard
> >
> >
Original Message
> > From: <scottsmith13@a...>
> > To: <basicstamps@yahoogroups.com>
> > Sent: Tuesday, March 27, 2001 12:26 PM
> > Subject: [noparse][[/noparse]basicstamps] LED question
> >
> >
> > > Greetings,
> > >
> > > I realize this question is not specifically related to a stamp, the
> > circuit
> > > will be run by a stamp if that counts. If anyone has another email list
> > that
> > > this would be better suited to, please let me know.
> > >
> > > I want to power up an array of LED's that will be used as a lamp on an
> > > automobile.
> > >
> > > The LED's are 30ma at 2v.
> > >
> > > using 10 LED's as an example, my current concept is to:
> > > combine the LED's into 5 groups of 2 LED's each (in parallel)
> > > combine the resulting 5 groups in series.
> > > use a regulated 10v supply and a single current limiting resistor.
> > >
> > > I have done this and I get a nice 30ma and 2v at each LED (or, real
> > close).
> > >
> > > Are there any major reasons I shouldn't do it this way?
> > >
> > > Thanks,
> > > Scott
> > >
> > >
> > >
> > > Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
> > >
> > >
> >
> >
> >
> >
> > Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
> >
> >
> >
>
> Sean T. Lamont, CTO / Chief NetNerd, Abstract Software, Inc. (ServNet)
> Seattle - Bellingham - Vancouver - Portland - Everett - Tacoma - Bremerton
> email: lamont@a... WWW: http://www.serv.net
> "...There's no moral, it's just a lot of stuff that happens". - H. Simpson
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
Nick had the idea of what I was looking for. The critical question was the
configuration of the LED's; the reasonableness of my plan to use a single
resistor instead of a bunch of them. I plan on using a MOSFET or other means
to actually provide the power, the stamp is just providing the control.
Thanks,
Scott.