LED project

[dphens]

Hi!

I am new to electronics and could use some help determining what I need for my project. I apologize for the length of my very first post, but I want to provide plenty of detail.

My project is creating a number of acrylic panels that will be backlit with LED's. I expect to have at least 30 panels at this point. I have been experimenting with LED's for a couple months now and have reached a point where I need more power and assistance with this.

Up to now I've been using a 12v supply with 3.3v, 20ma white LED's and so I could have 3 in series without a resistor. What I need to now it to have more in series and then 30-35 parallel segments. My panels will vary in size, so some may only have 5 LED"s and other may have (need) 7. This tells me that I need a 24v supply, right?

I also realize I'll need different resistors for each segment depending on the number of LED's in that segment.

The other piece I want to add is being able to dim the LED's. I have a 12v PWM that I was using in my tests, so I assume I can do this with 24v, I just need a PWM rated for that.

Lastly, if possible I'd like to have the panels grouped but still running off one supply. In other words, 20 panels running through one dimmer and 10 running through another. Hope that makes sense.

Where I have problems is determining the other requirements I need to power my project. As I mentioned, I am new and just learning. I have some basics down, but I still struggle with the math portion of determining what I need in regards to the power supply. I'd be grateful is someone would be willing to help me out during this project while I learn more about this hobby.

Thanks!!!

 

[(*steve*)]

You MUST have a resistor in series with a LED driven from a voltage source.

It will make things far easier if you use the same number of resistors in each string. If you can manage to use a multiple of that number for each segment then you'll have less problems with differences in LED brightness (and less chance of error if there is only one value resistor!)

If you already have a 12V supply with PWM to vary brightness then I see no real reason to go to 24V. Just have more strings in parallel.

If your power supply is well regulated then 3 leds in series with a suitable resistor (100 ohms I calculate) times as many strings as you require.

Instead of 5 and 7 LEDs, can you use 6 and 9? they are divisible by three, so the strings of 3 fit well.

Designing a need to have prime numbers of LEDs in each group is probably the most perverse thing you could do

 

[dphens]

So you are saying that if I have a panel that needs 6 LED's I should use two strings of 3 LED's? That's interesting and I think I could go that route. I was going to mount the LED's directly to a PCB that I am making for each panel (using 5050 SMD LED's - which I've not tested yet and still need to do that). Using multiples of three would add some complexity to my PCB design, but if that would the best way, then I'll definitely try that.

Out town this weekend, so I won't be able to try this out, but will do so next week and come back if (when) I have more questions.

Thanks!!

 

[dphens]

Also, forgot to ask for confirmation on determining the amperage of the supply. Am I correct in thinking that for 180 LED's at 20ma, I'll need a supply with at least 4.5 amps? Also, it's my understanding that a higher amperage will not be a problem and the LED strings will only pull what they need. That correct?

Thanks again!

 

[barathbushan]

for led life longevity assume the forward voltage drop to be 3 volts and max current allowed to flow is 20ma [60 mw configuration]

now all you have to do is set the voltage across each led , and the led's draw their required current automatically , to vary their brightness just change the voltage across them .

1) for 5 led in series - use 5*3=15v supply .
2)for 7 led in series - use 7*3=20 v supply approx
3) for 35 parallel segments , the supply must have the required amperage , so get a
35*20ma=660 ma .[use 1A]

the above configuration is enough , so just get a 24v,1A power supply and start your work!!
you can use voltage regulators like lm7815 to get 15v off 24v supply

THUMB RULE - just make sure the given voltage is "ALL" dropped against all the led's connected in series , if there is excess voltage just drop it using a resistor , for example you need to drop 4v , and you know the current drawn by the led's is 20ma (approx) , then connect 4/20ma=200 ohms resistor in series with it

WARNING- if you overvolt your led's above 3.3v[white led] , then life decreases , anything above will surely kill it

 

[barathbushan]

180 led's in series or parallel ??

if in series more VOLTAGE and less CURRENT is required , if in parallel its the inverse

series or parallel depends on the source of led power

 

[dphens]

thank you! that is very helpful.

I thought I would have 5-7 LED's in series and then have 30 or so strings in parrallel. However, after Steve's reply, I am beginning to like the idea of having 3 LED's in series and just add multiples of 3 for each panel that needs more light.

I appreciate the helpful replies!

 

[(*steve*)]

Just to confirm that you calculate the total current required from your power supply as the current required for each string of LEDs multiplied by the number of strings.

So (repeating barathbushan) 180 LEDs in strings of 3 running at 20 mA will be 60 strings * 20mA = 1.2A

barathbushan speaks of "overvolting" LEDs. That is a concept that sounds good, but has very little practical meaning (and leads to some nasty design errors).

For LEDs you must consider their current, and take steps to limit (or regulate) that. In making calculations, you often (for low power LEDs) use the nominal voltage drop to calculate a suitable series resistor that will drop the excess voltage at a particular current.

Because the voltage drop across a LED is not linear with current, the voltage across the LED changes comparatively little with current.

For high power LEDs (say 1W and above) the power supply is typically a constant current source. This has the effect of reducing power consumption (and hence dissipation) as the LED heats up (which reduces Vf) and helps prevent thermal runaway.

(At a constant Vf, a LED will draw more current as the tunction temperature increases. This increases the power dissipated -- increasing the junction temperature further -- leading to a yet lower Vf, more current, more power, and an rapidly escalating temperature and current that will eventually damage or destroy the LED. I refer to this as a "Short but brilliant career". You do not want your LEDs to have a short but brilliant career!)

 

[dphens]

Thanks Steve! I think that makes sense to me, but just got back in town and need some rest so I'll need to come back and read it with a clear head tomorrow.

In the meantime, a thought occured to me about heat dissipation for the resistors. My plan is to mount the LED's and the resistors directly to a copper clad 1/32 board and that would be attached directly to the acrylic with pockets in the acrylic for the components. That assembly would then be mounted onto some aluminum sheet. What I don't know and am curious about is whether that will cause a problem with "trapped heat" from the resistors since there'll be no airflow over them in that design. If so, I assume I could put the resistors inline with the wiring going into each panel. ??

 

[(*steve*)]

The resistors will be dissipating very little power individually (20mA across a couple of volts?) and this should not be an issue unless there are very very many of them placed very very close together -- and even then, probably not.

Are you planning to use through hole or surface mount components? In either case, the printed circuit board will act as a heatsink, and the aluminium sheet behind that will also allow heat to escape.

Unless you're planning to use high power LEDs, or drop many volts across each resistor, you should not have a problem. Use 0.1W or larger rated resistors (assuming 20mA and 100 ohms as was calculated roughly earlier).

The actual dissipation is 0.04W and the overrating will allow them to operate at elevated temperatures (you could check the specs, but I'd imagine that you would be looking at 50C to 60C ambient, a figure I'd be surprised if you reached.

 

[dphens]

Thank you!

I plan on using surface mount components for this and will be going with your recommendation of using 3 LED's per string and just using more strings for larger panels.

Specs for the LED's I plan on using are here> 5050-PW6000.pdf

I still need to get the resistors I'll be using.

 

[dphens]

Would this resistor be suitable for my project?

CR1206-FX-1200ELF

 

[(*steve*)]

You might want to check out the voltage vs current graphs and the max current vs temperature graphs to make sure that everything will remain within the ratings of the components.

I don't think it will be an issue as long as the ambient temperature is 25C or lower (check the max temp for 20mA operation!)

edit: The resistor looks fine.