LED array

[John Fields]

How'd it change from 30 mA to .035A?

 

[David Wood]

That's a lot of LEDs. Whatcha makin?

Check out National Semiconductor's dedicated circuits for powering
white illumination LEDs. They're the best thing I've seen since I
started playing with LEDs. Commercial LED arrays frequently string a
series of up to six LEDs and then use as many of these series strings
in parallel as neccessary.That's a lot of LEDs. Whatcha makin?

 

[Watson A.Name \Watt Sun - the Dark Remover\]

You should measure the voltage across a set of the LEDs at 30mA. Average the
value, and call it Vf. Then your resistor value should be a standard value
close to

R = (12 - 4 * Vf)/0.030

Negative R means you won't get 30mA out of it.

Since you are running the LEDs at 30mA, I'm guessing they will get hot.
Since this is the case, and since Vf is dependent on temperature, you should
let them heat up before taking the measurements.

You can build a 30mA current source as follows:

VCC
+
+---+---+
| |
| .-.
| | |100
| | |
| '-'
| |
| |
.-. |<
1k | |<---| PNP
| | |\
'-' |
| |
| |
| LED
| |
| |
| |
+---+---+
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Hook the + lead of your multimeter to the collector of the PNP transistor,
and the - lead to ground, and put the multimeter into current measurement
mode. Adjust the pot to get 30mA. Then, hook up the LED between the
collector and ground, and measure the voltage at the collector (remember to
wait till it heats up). Thats Vf(30mA). It won't be exact, since the current
will vary depending somewhat depending on the voltage at the collector, but
it should be close enough for your measurement.

Regards,
Bob Monsen

I see no point in using the pot above. You know what current you want,
so just set the values for that current. Here is a circuit that will
limit the LED current to 30 mA, for wide variations in Vcc.

30mA current source:

VCC
+
+-------+---+
| |
| .-.
| | | 22
| | | or
| '-' two 47 in
Q1 >| | parallel
PNP |--[470]--+
/| |
| |
| |
| |
| |< Q2
+---------| PNP
| |\
| |
| |
.-. |
22k| | --- LED
| | V ->
'-' ---
| |
| |
+------+----+
|
===
GND

The [470] is optional, but it's a good idea to use it.
Transistors can be 2N4403s or BC327s.
Vcc should be less than 20 V so the power dissipated
doesn't exceed the transistor Q2's max rating of .6W.
You can also use NPNs by flipping the circuit upside down.



--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
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@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@
F
o
d
d
e
r

f
o
r

s
t
u
p
i
d

n
o
t

e
n
o
u
g
h

i
n
c
l
u
d
e
d

t
e
x
t

m
s
g

 

[Robert Monsen]

You should measure the voltage across a set of the LEDs at 30mA. Average the
value, and call it Vf. Then your resistor value should be a standard value
close to

R = (12 - 4 * Vf)/0.030

Negative R means you won't get 30mA out of it.

Since you are running the LEDs at 30mA, I'm guessing they will get hot.
Since this is the case, and since Vf is dependent on temperature, you should
let them heat up before taking the measurements.

You can build a 30mA current source as follows:

VCC
+
+---+---+
| |
| .-.
| | |100
| | |
| '-'
| |
| |
.-. |<
1k | |<---| PNP
| | |\
'-' |
| |
| |
| LED
| |
| |
| |
+---+---+
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Hook the + lead of your multimeter to the collector of the PNP transistor,
and the - lead to ground, and put the multimeter into current measurement
mode. Adjust the pot to get 30mA. Then, hook up the LED between the
collector and ground, and measure the voltage at the collector (remember to
wait till it heats up). Thats Vf(30mA). It won't be exact, since the current
will vary depending somewhat depending on the voltage at the collector, but
it should be close enough for your measurement.

Regards,
Bob Monsen

I see no point in using the pot above. You know what current you want,
so just set the values for that current. Here is a circuit that will
limit the LED current to 30 mA, for wide variations in Vcc.

30mA current source:

VCC
+
+-------+---+
| |
| .-.
| | | 22
| | | or
| '-' two 47 in
Q1 >| | parallel
PNP |--[470]--+
/| |
| |
| |
| |
| |< Q2
+---------| PNP
| |\
| |
| |
.-. |
22k| | --- LED
| | V ->
'-' ---
| |
| |
+------+----+
|
===
GND

The point was to be able to tune it up quickly to 30mA using a multimeter,
and then to use it to test a few LEDs. It was a part of a procedure for
helping him to choose a resistor value.

Regards,
Bob Monsen

 

[Watson A.Name - Watt Sun, Dark Remover]

Whoa! As if 144 resistors weren't enough!

Costas

Cost-wise, the 1/4W resistors are a hundred for a buck and a half at
Fry's or just about anywhere else. Same cost for the 1/2W. That's
not significant. Stuffing the board is a bit more hassle, but if the
holes are there and marked, it doesn't seem like a big deal.

I stuffed one board with 28 LEDs and resistors, but they got too hot
so I had to rearrange the LEDs and space them apart to get better
cooling. I think I'll get 60 on a board without too much overheating.
The PC board is Rat Shack 276-168 and is 2.8" by 3.7" (72 mm by 95
mm). Yeah, it's a bit tedious.

I think 144 LEDs is going to be one BIG heatmaker - over 4 amps! Real
bright, tho.

Yah!-Yah!-Yah!-Yah!-and 140 more!
Bright! That's what we want! Lotsa Bright!

I asked in another thread about where I could get an etched and
predrilled PC board for multiple LEDs and resistors. I didn't see any
replies, so I'm asking again. I did some searching and checked some
PCB places like FAR circuits, but I came up with nothing. Any ideas?
Thanks.


--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@

 

[Dr Engelbert Buxbaum]

Watson A.Name \Watt Sun - the Dark Remover\ wrote:

30mA current source:

VCC
+
+-------+---+
| |
| .-.
| | | 22
| | | or
| '-' two 47 in
Q1 >| | parallel
PNP |--[470]--+
/| |
| |
| |
| |
| |< Q2
+---------| PNP
| |\
| |
| |
.-. |
22k| | --- LED
| | V ->
'-' ---
| |
| |
+------+----+
|
===
GND

You can have that a lot easier, by using a FET (BF245 or similar). The
actual current depends on the amplification of the FET, A-types give
about 10 mA, C about 25:


o +5..30 V
|
|
G |____ S
----| BF245C
| |---- D
| |
| |
---------
|
|
____
\ /
\/ LED