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Old 20th Dec 2022, 1:12 am   #1
Jolly 7
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Default LED panel question

Can anybody please advise how to light up this 48 LED panel, which I have taken out of an old solar charged PIR light fixture. There was an 8.4V lithium battery inside it, but the battery was connected through the PIR unit and another board, which I think was for charging the battery. I fed it with 9V from my bench supply, with positive to V+ and negative to the ground but it did not light up. It doesn't light up with the negative supply connected to the middle pin either, as shown in the photos.The LEDs are fine, because they light up when both the PIR unit and battery charging board are connected.
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Old 20th Dec 2022, 5:54 am   #2
Terry_VK5TM
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Default Re: LED panel question

Try feeding + and - to the two outside pins as marked and short the FB pin alternately to the + and - pin.
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Old 20th Dec 2022, 10:44 am   #3
60 oldjohn
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Default Re: LED panel question

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Originally Posted by Jolly 7 View Post
and another board, which I think was for charging the battery.


I suspect this board may have contained some sort of Boost Converter to increase the voltage while reducing the current available.
Can you work out how the LEDs are connected?

John.
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Old 20th Dec 2022, 1:14 pm   #4
Jolly 7
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Default Re: LED panel question

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Quote:
Originally Posted by Jolly 7 View Post
and another board, which I think was for charging the battery.


I suspect this board may have contained some sort of Boost Converter to increase the voltage while reducing the current available.
Can you work out how the LEDs are connected?

John.
It seems that there are four rows of 12 series-connected LEDs in each row. The four rows are then connected in parallel and are fed power through a resistor from the ground side. I will look at the boost converter side of things later today. It might be operating on the 'joule thief' principle.

Last edited by Jolly 7; 20th Dec 2022 at 1:16 pm. Reason: Additional information
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Old 20th Dec 2022, 1:19 pm   #5
Jolly 7
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Default Re: LED panel question

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Originally Posted by Terry_VK5TM View Post
Try feeding + and - to the two outside pins as marked and short the FB pin alternately to the + and - pin.
Thanks Terry. It didn't seem to work that way, but I will try other ways.
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Old 20th Dec 2022, 2:16 pm   #6
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Default Re: LED panel question

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and another board, which I think was for charging the battery.




John.
It seems that there are four rows of 12 series-connected LEDs in each row. The four rows are then connected in parallel and are fed power through a resistor from the ground side. I will look at the boost converter side of things later today. It might be operating on the 'joule thief' principle.
I nearly suggested this may be the layout of the LEDs. Not sure about the joule thief, possibly Jacobs ladder. What ever circuit is used I would expect it to be contained in a chip.

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Old 20th Dec 2022, 5:40 pm   #7
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Default Re: LED panel question

Hi leds forward voltage of around 1.8 to 3.3 volts so with 12 in series =21.6 to 39.6 volts with a resistor in series to limit current , what voltage was the battery > Power this from a variable dc supply noting + - polarity monitor current of 10-15 MA ish x4 just noticed 8.4v batt so has a joule thief circuit to boost the b+ Mick
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Old 20th Dec 2022, 6:40 pm   #8
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Default Re: LED panel question

The ones I came across as emergency-lights, "Sustained Luminaires" in H&S-speak, always used series-parallel networks of LEDs and a boost-converter; they generally had 'series resistors' only for sensing the current through the LEDs in order to monitor and regulate the power from the converter.
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Old 20th Dec 2022, 7:45 pm   #9
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Default Re: LED panel question

Have you measured the voltage into the LED's with the PIR & Charging board connected in order to duplicate from you alternative PSU ?

Rog
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Old 20th Dec 2022, 7:54 pm   #10
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Default Re: LED panel question

You can achieve the illusion of continuous illumination using LED’s with astonishingly low on times.

You could easily knock up a generator of square pulses of the right amplitude and power for the panel to be illuminated, with a handful of components.

There ain’t no Joules to be thieved anywhere, but yer pulses will do the magic trick for you.

But not using DC from the battery you describe and bypassing the original circuit. It was there for a reason
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Old 20th Dec 2022, 8:01 pm   #11
Roger Ramjet
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Default Re: LED panel question

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Originally Posted by Al (astral highway) View Post
You can achieve the illusion of continuous illumination using LED’s with astonishingly low on times.

You could easily knock up a generator of square pulses of the right amplitude and power for the panel to be illuminated, with a handful of components.

There ain’t no Joules to be thieved anywhere, but yer pulses will do the magic trick for you.

But not using DC from the battery you describe and bypassing the original circuit. It was there for a reason
Hi Al,

Looks like I am on a learning curve here ..... do I take it that these type of fittings require a square wave supply to work instead of a DC one ?

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Old 20th Dec 2022, 8:28 pm   #12
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Default Re: LED panel question

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Looks like I am on a learning curve here ..... do I take it that these type of fittings require a square wave supply to work instead of a DC one ?
Hi Rog. What I'm rather saying is that Jolly asks how to illuminate the array from a <9V DC supply as he finds that won't work.

Others have already commented why.

I've suggested a remedy, which points back to the OP's insight about the role of the original circuitry.
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Old 20th Dec 2022, 9:27 pm   #13
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Default Re: LED panel question

Yes, I'd agree, it has to be worth checking the voltage the working circuit applies to the terminals when working off the original circuit. Especially since sometimes LEDs are actually a number of LEDs on one chip so the Vf actually has to be quite high on each module. I have one where I haven't mananged to get even one module to light even with a 20v Vf.

The previous comment about series and paralell combinations is for sure worth checking.

Regarding the feedback (FB) it could be a simple temperature monitor to ensure the array doesn't overheat. I noticed a single resistor mounted on the corner of the PCB, and the PCB is often used as a heatsink for the LEDs. Could it actually be a thermistor feeding back the temperature of the PCB?
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Old 20th Dec 2022, 11:10 pm   #14
Jolly 7
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Default Re: LED panel question

I have now successfully lit up my LED panel using a joule thief circuit I built some time ago for a different LED setup. The circuit I built is based on a BD948 transistor and a toroid core.
of unknown specifications. I wound the core with a bifilar winding using some enamelled copper wire I had lying around.

I had originally designed my circuit to operate from a 5V USB power source, but during my experiments today found that it works ok without any heating of the transistor or toroid at up to 9 volts. I was reluctant to go higher than that as I do not want to damage my LEDs and they are bright enough at 9 volts. I am not sure what is the actual voltage that's being output by my circuit, as my multimeter still measures 9V DC. However, this is not a problem as my LEDs are now working the way I wanted them to.
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Old 20th Dec 2022, 11:53 pm   #15
Jolly 7
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Default Re: LED panel question

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Have you measured the voltage into the LED's with the PIR & Charging board connected in order to duplicate from you alternative PSU ?

Rog
Yes, I tried to but could not get readings higher than 8.4V DC
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Old 21st Dec 2022, 8:58 am   #16
Roger Ramjet
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Default Re: LED panel question

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Quote:
Originally Posted by Roger Ramjet View Post
Have you measured the voltage into the LED's with the PIR & Charging board connected in order to duplicate from you alternative PSU ?

Rog
Yes, I tried to but could not get readings higher than 8.4V DC
Understood & it seems I am taking you down the wrong road on this suggestion. There are technical references in the above post that I don't really understand i.e. Joule Thief plus I assumed Jacob's Ladder related to a discharge up two vertical electrodes ? However it does seem to me the panel won't work from a DC supply. Any road, good luck with finding a solution.

Rog

Last edited by Roger Ramjet; 21st Dec 2022 at 8:59 am. Reason: typo
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Old 21st Dec 2022, 11:14 am   #17
60 oldjohn
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Default Re: LED panel question

Hi Rog, A Jacobs ladder has a few uses other than making a pretty lighting effect, the circuit is here, the circuit repeats itself hence, "Ladder"
https://www.elprocus.com/voltage-multipliers-working/

Joules Thief, here by big Clive. skip through video to get the idea. Please watch from 4.30 for 5 minutes to get the circuit explained.
https://www.youtube.com/watch?v=K53beWYdIpc
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Last edited by 60 oldjohn; 21st Dec 2022 at 11:19 am.
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Old 21st Dec 2022, 1:32 pm   #18
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Default Re: LED panel question

Instead of joule thieves, maybe they should be called "flyback ringers", a better description of how they work?
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Old 21st Dec 2022, 6:53 pm   #19
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Instead of joule thieves, maybe they should be called "flyback ringers"...
Yes, Andrew. I dislike the term 'Joule thief' as it suggests some kind of trickery that doesn't exist. I won't go into it as it's OT but the term has been widely appropriated in nonsense Free Energy circles.

The circuit takes minutes to build from point-to-point guides on the internet and has so few components that this simplicity feeds into this magical aura.

Ages ago, I was working through my understanding of oscillators, I built a blocking oscillator (another name for the same thing) using a valve design.

Others will know that this was established early last century and for example had a crucial role in Radar...

Here's one such design.


https://www.radarpages.co.uk/theory/...ec2ch9p150.htm

It would be a shame if the conclusion of this thread didn't properly illuminate (hah!) how the LED panel is being lit by high-frequency, high voltage pulses that are approximate square waves. ('High' being relative to the DC input voltage).
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