LED Light Panel DIY

Hi Guys,

thought id document my fiddles with the strange world of LED lights now!!
i have bought some strips of LEDs and put about 693 LEDs on one panel and 798 on another.
There is conflicting information out there on what current they pull so i have taken a Xbox psu and wired it for 12v and put an Ameter in series to see what current is being pulled. on the 693 leds im getting 2.31Amps which seems to work out as 0.03mA per led? i have read the white leds are ment to be 20mA each?
I'd definitely like to hear peoples thoughts on this please.

Also i'd like to be able to dim the lights if possible, could anyone suggest anything for this set please?
Im wanting to use for video so i dont want the lights to pulse.

regards

Spence.

If there are 3 LEDs in series, you get  231 parallel strings, and the current of each string (each LED also) is

2.31/231=10mA

LEDs have to be driven through a current source not a voltage source.

A LED will drop some volts of it's supply voltage, but if you don't limit the current it will only last for a few seconds before it will burn out.

In your case all the LED wired in series will drop your 12V supply voltage to such a low value that the current through every row of LEDs is limited by ohms law and / or what the power supply can deliver.

The brightness of a LED also depends on the current flowing through it. Less current - less brightness. When your LEDs are rated 20mA they will also light up with a much lower current but of course not as bright.

For status LEDs it is a common thing just to use a current limiting resistor together with the LED, but for lighting applications (or great amounts of single LEDs) you somehow have to switch to a regulated current source not to waste way to much power just in the resistors (besided other reasons like change in brightness with temperature etc....)

ok looks like i should be a little more specific i think just so i understand this fully:
both panels are using white leds.
on one panel  i have 38 rows of 7 groups of 3 leds (798 leds all together)
on the other panel i have 33 rows of 7 groups of 3 leds. (693 leds all together)
i am using an Xbox 12.2amp 12v power supply and getting 1.91amps on the 798 leds
and 2.31amps on the 693 leds!!!??
how is this possible that im getting less current on more leds?

also how could i implement a dimmer?

regards

Spence.

We don't know how the single LEDs are wired. Are three of them in series and than paralleled to groups of seven ?

As I said, you need a constant current source to drive that amount of LEDs correctly.

Dimming can be done by either adjusting the LED current or using PWM signal. Either can't be done  with your 12V power supply alone.

Do you have any information about how your LED strips are wired ? That would at least help a bit to figure out how the can be configured.

hi,

oh shit i didnt even think about that!!! yes the rows are parallel and the 7 groups of 3 leds so 21 leds in series!!!

so is it 20mA per 3 leds or 20mA per one white led?

if im getting 2.31Amps for 33 rows, 2.31 / 33 = 0.070 so its 70mA per row which would mean for each 3 leds it would be 10mA

does this sound right?

Spencerleehorton said:

so is it 20mA per 3 leds or 20mA per one white led?

if im getting 2.31Amps for 33 rows, 2.31 / 33 = 0.070 so its 70mA per row which would mean for each 3 leds it would be 10mA

does this sound right?

Click to expand...

If the LEDs are in series, the current is shared. So a 20 mA current source on top of three LEDs in series is sufficient, and of course the current through each LED will be 20 mA.

If the LEDs are in parallel, then each LED still have 20 mA If, but you need 60 mA total.

The difference is that the voltage at the top of the three LEDs in series needs to be higher than the forward voltages for each diode. So assume a 2.5 V Vf, you need 7.5 V at the top of the ladder.  For the three LEDs in parallel, you need only 2.5 V.

-a

sorry you lost me?

could you use my 33 rows of 21 leds as an example please?

Spencerleehorton said:

sorry you lost me?

could you use my 33 rows of 21 leds as an example please?

Click to expand...

OK, so 33 rows (parallel) of 21 LEDs in series. Assume diode specs as such: forward current of 20 mA and forward voltage of 2.5 V. You should definitely find out the real specs for the diodes, as some might have a Vf greater than 2.5 V.

You need to provide a voltage of 21 * 2.5 V = 52.5 V at the top of each column.

33 rows times 20 mA per row is 660 mA.

This is assuming you are turning all rows on at once.

ah ok, so why when i hit it with 12v and 12.2A psu and i put ameter in series does the ameter say its 2.31amps?

> 38 rows of 7 groups of 3 leds

What are "groups"?

I suggest these have no electrical significance. Just physical layout.

There are BAZILLIONs of "LED strips". Without specific product info, or more detailed measurements than you have done, calculations can only be wild guesses.

White LEDs ideally have a forward voltage near 3.5V and low internal resistance.

Three of these in series comes to 10.5V. If you connect to 12V, the excess 1.5V would cause "infinite" current flow and instant death.

BUT- real LEDs have some internal resistance and cheap LEDs have more. If we assume 25 Ohms in each LED's poor processing, 4V supplied across 3.5V+25r is 20mA. This is a reasonable current for the smaller LEDs. 12V is handier. 12V across 3.5V+25r series-three is still 20mA.

It is also possible, and not so uncommon, to use low-resistance LEDs with a resistor, user-supplied or built into a module. However I am seeing that high-R LEDs seem to be readily available as near-rejects, and are built-up into light strips at shocking low prices. As marginally acceptable parts, the actual performance will vary a LOT, depending on factory slop and module-parts buying specs. I would certainly expect +/-20% variation through a run of the same product, more if they are not the "same" modules.

What is your Real Problem? Is anything smoking? Is it bright enough?

Dimming raises more issues.

The most obvious and Dead-Easy answer is to switch-out 1/2 or 1/4 of the strings. Pick them staggered so you still have uniform light.

As we have 3.5V LEDs stacked to nominal 10.5V, anything below 10.6V will be "out" and anything over 12V-13V will probably pop the LEDs. You could contrive a linear voltage regulator over this range. Or a linear current regulator, assuming the many-many parallel paths share well. Using cave-man technique, start with 15VDC into a 3-pin regulator... oh, 2A is a lot for most TO220 regs. Pressing on: 15V-12V is 3V drop, 2A assumed current, 3V*2A is 6 Watts heat in the regulator. A heatsink but not a monster.

If you hadn't already built these panels, _I_ would put one dimmable household LED on a household lamp dimmer. See if it "flickers". I simply do not know if the LED ballast holds over an AC cycle. If so, 3 or 4 "60W" dimmable LED bulbs behind frosted plastic and a wall-dimmer would give large-area light source.

While I did not account for dimmability, when I costed-out lighting my garage with LED-strips or LED bulbs, it was not a real difference. The strips are very cheap in bulk but expect 12VDC power. Bulbs eat right off (my) 120V lines, so the wiring costs some more, but there's no power-pack. Yes, bulbs are fat and LED strips can make a thin panel, which may over-ride cost and performance.

right let me try and explain what ive got and where i want to get too.

the led strips are pure white 5050 and 3528 strips, i have cut them at where i would have 7 sgements. each segment has 3 leds and one resistor in it.
i have then got 33 rows on one panel ive made and 38 rows on another.
i have powered both panels with a 12v 12.2amp xbox power supply and i dont get any stobeing or flickering when videoing.
i have only left them on for about a minute or so and no smoke or leds going pop, so all good so far.
i havent yet measured the voltage but i have put a Ameter in series and it had read 2.31amps
i do have two of these i have bought:

http://www.ebay.co.uk/itm/171925218848?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

auto boost/buck converter, it claims up to 8A.

i could possibly use this unit to adjust the current right?

regards

Spence.

Also each resistor which is within the 3 led segment is 150ohms.
the adjustable trimmers on the buck booster are 200k for the voltage adjust and 200k for the current adjust.
if i had this after the xbox psu i could then put a pot and a limit resistor on one of these trimmers right?

Just tested the buck booster and have wired in a 25k pot with a 2.2k in series to set minimum voltage and can adjust voltage.
it needs 15v or more voltage source and i only have 12v so this isnt going to work for the moment.

I've just tested the Xbox psu and when connected to the light panel it is outputting 12.47v on the 693 led panel at 2.31amps.
i just need to figure out a way i can then voltage dim using this Xbox psu, anyone got any ideas?

hi,

i only get 5v max out of the buck booster with the Xbox 12v psu? 20k pot ad 6.8k resistor in series?
i think im going to try one of these

http://www.ebay.co.uk/itm/Wireless-12V-Remote-Dimmer-Switch-Controller-Mini-LED-Dimmer-Strip-Light-RF-HT-/401036985351?hash=item5d5faac407:g:bQUAAOSwyQtV0x0I

regards

Spence.

the next question that presents itself is can i run both the 693 led and the 798 led panels off the same psu?
in theory if they have both tested at 12v and both pulling about 2.5A with this 12.2A 12v psu that should be fine right?

Spencerleehorton said:

the next question that presents itself is can i run both the 693 led and the 798 led panels off the same psu?
in theory if they have both tested at 12v and both pulling about 2.5A with this 12.2A 12v psu that should be fine right?

Click to expand...

Yes, both in parallel, as long as the PSU can deliver the needed current at the rated voltage.

JS

Is there a way to diy my own dimmer?

Spencerleehorton said:

I still there a way to diy my own dimmer?

Click to expand...

PWM driving the control which turns on each column.