LED Panel Circuit

[anearey]

Hey guys

I'm pretty new here so I'll try and be as descriptive as I can with my circuit here. I'm trying to build a circuit for an LED panel. Right now I've got a 12VDC AC power supply rated at 10A, which I want to use to power four separate banks of (180 LEDs wired in series), then wire the 4 banks to each other in parallel. (total of 720 LEDs) Thus supplying each bank with 3VDC (or so I think). LED ratings are Vf = 3-3.2V at 20mA.

I know I should have a resistor in the circuit, and I also want to throw a dimmer on there but I'm not sure what resistance ratings I should have.

I'd really appreciate your help!
Thanks

 

[anearey]

oops

Woops I meant 180 LEDs in parallel and the banks wired together in series.

 

[(*steve*)]

you're far better off wiring the diodes so that 3 are in series with an individual resistor, then repeat for as many strings as you need.

Wired this way, your 720 LEDs (240 strings of 3) will require 4.8A from your 12V power supply.

How well regulated is your power supply?

Dimming is another issue entirely.

If your power supply has an adjustable output voltage (and it a reasonably efficient switchmode regulator) I would tend to regulate the brightness by changing the voltage.

Otherwise you'll need a power mosfet capable of dissipating around 35W (so a large heatsink too) and a suitable control circuit to regulate either total current or voltage.

 

[anearey]

The power supply is basically just a printer PSU. So when you ask how 'regulated' it is, I have no idea. I would imagine not to well?

And if I had the 180 in parallel, would that not require 3.6A per bank since they are 20mA per LED? Or would each bank only need 20mA since they're only diodes? I apologize for my incompetence. The only reason I want to have them mostly in parallel is for the simplest PCB I can make.

Heres what i had in mind for mounting just the LEDs:

http://www.flickr.com/photos/40487333@N07/5013406812/lightbox/

 

[(*steve*)]

If you have 180 in parallel, bad things might happen.

Take my advice. Sets of three in series, each with their own resistor.

Even the Chinese make them this way. If there were a cheaper way, they'd do it.

Lots of little resistors also means the heat load is spread out rather than being in one spot. (which, incidentally, might be a cause for one of those "bad things" I mentioned above).

I have no idea how well regulated your power supply might be. Anywhere between very well and very poorly I expect. You'll need to supply more details. Is it a plugpack (or brick), or a unit taken from inside a printer? Does it have a heavy transformer or is it a switchmode power supply? What voltage do you measure when it has no load on it?

Each LED has 20mA going through it, likewise each string of LEDs. If you place them in parallel you sum the current, so three (individual LEDs or strings) in parallel would require 60mA

 

[anearey]

So 240 parallel strings of 3, each with a 150 ohm resistor?

 

[anearey]

Also, I dont have the PSU with me but if I recall correctly, the voltage output with no load was a little over 12V, like 12.43 or something like that. And its a power brick, its not actually from a printer, that was just a comparison. And it was actually 5A max draw, not 10. Heres the unit:

http://www.cctvhotdeals.com/power-s...c-power-adapter-for-surveillance-cameras.html

 

[(*steve*)]

Yeah, that will be well regulated.

4.8A is getting very close to 5A, so you'd want to make sure:

a) you get 20mA per string and not (say) 21 mA. Test one string of LEDs and chose a resistor that gives you just under 20mA (so maybe 180R rather than 150R.

b) ensure the power supply doesn't get hot (or too hot)

 

[anearey]

Does a 5A "max draw" mean that the unit will always be trying to push 5A or that is the maximum it will produce?

And are you insinuating that if 21mA per string rather than 20 would do damage to the LEDs or to the PSU?

Thanks for all your help, by the way

 

[(*steve*)]

21mA per string rather than 20mA per string would push your total current from 4.8A to 5.04A which would overload your power supply. The LEDs would probably be quite happy, but your power supply would not.

It will only supply the current required (up to the point that you overload it)

 

[anearey]

This makes everything make SO much more sense. Also, I do now like this idea of only three per string, which would make troubleshooting a LOT easier. Thanks for your expertise!

Now as far as a dimmer goes... couldn't I just use a linear pot coming from the power supply thats rated for say, 10A?

 

[Laplace]

Using a high-current rheostat as a dimmer is probably how Thomas Edison would have done it back in 1880. So you would be in good company.

 

[anearey]

hahaha im sensing some sarcasm in that.. is there a better, more stable way to go about doing this then?

 

[LTX71CM]

You would want to use PWM. What skill level do you have in regards to microcontrollers or 555 timers?

 

[anearey]

Im going to be using this LED panel for video lighting, and wouldn't a PWM circuit cause strobing that the camera would pick up? And I have essentially no background with microcontrollers.

 

[LTX71CM]

Here's an example 555 timer based PWM controller http://www.dprg.org/tutorials/2005-11a/index.html

Substitute your LEDs for the motor.

No the LEDs won't cause visual flicker if you choose a high enough frequency.

 

[(*steve*)]

You want to ensure that the frequency is at least several times 1/shutter speed.

If you're planning to use nothing faster than 1/1000 sec, then operate the PWM at around 5kHz and you should be fine.

Remember that a focal plane shutter will expose each part of the film/ccd for 1/1000 of a sec, the entire exposure may take between 1/60 and 1/200 of a sec.

As the slit moves across the film plane, some areas will see (say) 5 flashes, and others 6. That means you will have approx 1/3 stop variation in exposure, with maybe 5 to 15 bands across the image.

If this is noticeable, use a higher frequency or a lower shutter speed.

edit if it's a video camera then I doubt you'll notice anything since there is no shutter involved. Likewise if you use a cheapie digital camera with either no shutter, or a leaf shutter.

 

[anearey]

As far as layout and everything goes... does this diagram look like it would work?

 

[Leighcusack]

I would be increasing to main suppy rails to handle the power, or solder a length of heavy gauge wire down the length of them.

Aren't we talking total load of about 5 amps.

 

[anearey]

Yes, I was aware that that would most likely be an issue, however, I was working in a tight space. I'm now deciding to entirely revamp the project and now I should have room. You think 0.12" trace should be good? And what about the connections between the LEDs and resistors, wide enough there?

As far as the dimmer goes, its just going to be easier to buy a pre-assembled circuit for that, but thanks for the info