Need help with PWM and LED control

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cg8798

Joined Oct 18, 2007
26
Still, getting into the NUD4001's, resistors, 4 pins?, I don't know. It seems like a lot of extra work for these. I'll do a quick MS paint and show you the setup. I'll post it in a minute.
 

Thread Starter

cg8798

Joined Oct 18, 2007
26

there ya go. Quick picture of the design. The way the are designed series of 4 will not work on the inside tails. Each set (24, 30) is a seperate housing so only the outter tails can use 4 (ovbisouly 6 sets of 4) The inner tails will have two leds left over if using a series of 4 leds. That why I went with three. It works out perfectly for all the tails.
 

John Luciani

Joined Apr 3, 2007
477
John,
The OP posted a link to the site where he purchased the LED's on page 2:
http://www.componentsuperstore.com/...entifier=HPWTMD00F4000LUMILEDSLIGHTING6901870
P/N: HPWT-MD00-F4000
70mA @ 2.6V
I have similar LED's from the same mfg.; but mine are red-orange instead of red, and different lens. That's why I suggested he check to see what his Vdrop was @ 70mA; my actual reading is 2.66v not 2.6 as per the datasheet.
You need to be careful with the forward voltage specification. The datasheet has
a range of 2.19 to 3.03V. If you get a batch of parts that runs high the NUD4001
won't have enough headroom. If you get a batch of parts that runs low you will dissipate
too much power in the NUD4001.

Measuring the LEDs is a good idea.

The idea I have is to supply each string of four LED's with a separate NUD4001 IC; that would put each of them operating at < 15% of their maximum rating; quite a safety factor. Although a single NUD4001 could power multiple strings, if an LED failed in either shorted, or even worse, open mode, it would result in the remaining LED's operating outside of their specs.
Having a separate NUD4001 for each group of four is very good idea. You do not need
to worry about how the current divides. With 2.6V LEDs and a voltage of 12V you
are at an efficiency of around 85% (the efficiency drops to around 70% at
14V.

An advantage of a PWM current source (versus the NUD4001 which is like a
current sink) is that you would get 80-90% efficiency over the 12-14V range.

(* jcl *)
 

John Luciani

Joined Apr 3, 2007
477
But for power dissipation, it would be best to use a printed circuit board.

They're really not hard to make - do you have a laser printer?
Especially if your PCB software is scriptable ;-)

This is a perfect application for my PCB replication script. I did an LED layout using gEDA/PCB and a script. With my script you layout a single circuit which in this
case would be an NUD4001, four LEDs + misc. You run the script and it replicates
the circuitry multiple times.

Checkout the documentation for the sch-matrix and pcb-matrix scripts at

http://www.luciani.org/geda/util/util-index.html



(* jcl *)
 

Thread Starter

cg8798

Joined Oct 18, 2007
26
so what the heck am I supposed to do? This is all just confusing me now. I figured it what be a matter of wiring the leds together, to a pwm, and with designated resistors. I never expected that I'd have this much to think through. It's just confusing me and frustrating me now. I need a clear way to do this. I know you guys are giving me the best ideas to making this work but I'm just not smart enough with this stuff to follow. I don't want to make you guys spell out everything but I need a clear cut idea. NUD4001 for every string? Do I need a pwm along with the NUD4001's? Resistors needed in between serires? What's going on?
 

John Luciani

Joined Apr 3, 2007
477
so what the heck am I supposed to do? This is all just confusing me now. I figured it what be a matter of wiring the leds together, to a pwm, and with designated resistors. I never expected that I'd have this much to think through. It's just confusing me and frustrating me now. I need a clear way to do this. I know you guys are giving me the best ideas to making this work but I'm just not smart enough with this stuff to follow. I don't want to make you guys spell out everything but I need a clear cut idea. NUD4001 for every string? Do I need a pwm along with the NUD4001's? Resistors needed in between serires? What's going on?
Do you have PWM current source that you were thinking of using?

Unless you purchase a PWM current source you have to build something to set
the current. The NUD4001 is simpler to build than the PWM current source but
you really should do a PCB layout. A resistor is simplest but the current changes
with the input voltage. If you have 108 LEDs you may want to do a PCB layout :D

If you decide on the NUD4001 you would use one NUD4001 for each string of
four LEDs. If your input voltage ranges from 12-14V and the LEDs are either on
or off you do not need a PWM current source since the NUD4001 regulates the
current.

Your choices ---

* Use resistors
low efficiency, unregulated current through the LEDs
* Use current-sink (NUD4001 type solution)
Same efficiency as resistors, regulated current through the LEDs, easily
modulated to change LED intensity, PCB is a good idea.
* Use PWM current source
Highest efficiency, regulated current through the LEDs, easily modulated
to change LED intensity, PCB is a good idea.

(* jcl *)
 

Thread Starter

cg8798

Joined Oct 18, 2007
26
I can't do PCB. One, I don't have the money to pay for the boards to be made. This is the PWM i was going to use. I've talked to the guy about it and he will sell them for $32 unmade with directions to make it or another $25 to make it before hand and test it. Plus he'll include the same leds I want to use. I was going to use that with resistors every three leds, as I explained in the beginning. So that was my idea but now I don't know if I should judging by what you guys told me.
 

John Luciani

Joined Apr 3, 2007
477
That circuit looks like it using a 555 timer to pulse width modulate a linear voltage regulator to control the LED intensity. Modulating individual current sources or sinks guarantees
even LED lighting but is more complicated to wire.

If you want to build a board similar to the one on ebay look for 555 PWM circuits
and a low dropout voltage regulator that has an enable pin. Something like the Micrel
MIC29151 or MIC29301 .

(* jcl *)
 

Thread Starter

cg8798

Joined Oct 18, 2007
26
No I don't want to build the PWM. I just want to buy the parts and wire it up. No extra wiring and creating. I barely have time to wire the tails (should take we at least two weeks to wire them all) with school and all. I'll only have time to do a little work each day so I'm looking for the easiest possible. Any chance you could diagram or something for how to wire a string with the necessary parts? 12V going into the PWM, then to the led string and with the resistors in between. I just need to visually see how to wire all that stuff, it's just easier for me to understand that way.
 

Thread Starter

cg8798

Joined Oct 18, 2007
26
I just wanted to post something else. A guy just posted on my other forum that he was using 1 27ohm resistor for every 5 leds. He said it dissipated about 115mW and he didn't feel any heat from the leds. Now does that math seem correct? If I were to do 5leds per string would that be better? There would be one string of 4 leds on the outer tails tho, since it only has 24 leds but the inner tails would be perfectly space with 6 sets of 5 leds. Thoughts? Math? Is that a better way to go to spread the voltage drop?
 

John Luciani

Joined Apr 3, 2007
477
I just wanted to post something else. A guy just posted on my other forum that he was using 1 27ohm resistor for every 5 leds. He said it dissipated about 115mW and he didn't feel any heat from the leds. Now does that math seem correct?
I am going to let you tell me ;)

Power is given by P = VI
The voltage across a resistance is given by V = IR

Plug in your values and calculate the power.

If I were to do 5leds per string would that be better? There would be one string of 4 leds on the outer tails tho, since it only has 24 leds but the inner tails would be perfectly space with 6 sets of 5 leds. Thoughts? Math? Is that a better way to go to spread the voltage drop?
The Vf of your LEDs is apx 2.6V. 5 * 26 = 13V. This is above the 12V of your
battery. You would need a boost converter for voltages greater than 11 - 11.5V.

(* jcl *)
 

John Luciani

Joined Apr 3, 2007
477
(should take we at least two weeks to wire them all)
If you did a PCB layout it would probably take you about 2-3 hours to assembly.
If you plan on doing other projects you may want to learn how to do PCB layout.

Since this project is a groups of leds and resistors and a connector it would be
an excellent board to start with. What is the physical size of each group of LEDs
and of the entire assembly?

(* jcl *)
 
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