L.E.D.`s - series or parallel?

[#1]

If this were me, I'd put strips of LED next to each other to give maximum coverage and maximum LEDs. I'd then get a circuit that could control the brightness of these so it it is too bright and can be turned down or turned up for use with thicker paper. Just saying.

[#2]

Just a reminder that these LED strips need a PWM controller source to work, you just can't apply a voltage directly to them. So this would mean using the likes of an Arduino or Pi to control them. But having said that you could create yourself a very elegant solution if you do decide to use them.

PWM = Pulse Width Modulation

[#3]

@Mook  thanks for that very valuable information....I have just sent you a more detailed PM.

cheers

[#4]

@Mook 

I think that you are getting confused. Certainly the LED tape that I have used is a simple arrangement of parallel LEDs each fitted with their own individual series resistor.

I have just tried a length on an analogue variable dc power supply. They start to come on at around 7V and progessively increase in brightnes until I get to the rated 12V. There is no need for a PWM power supply, in fact a single series resistor of sufficient power rating would do the job fine.

[#5]

Fair enough @Baldrick1 maybe you could let @shutter know the part number and where you got them from please.

[#6]

@Mook 

Will do with pleasure. I'm wondering whether you are confused by companies selling PWM dimmers?

If so they use PWM rather than a series resistor because they will work with any length of tape up to the maximum rated current of the dimmer. If you use a series resistor it has to be 'tuned' to the length of tape. Analogue dimmers are more expensive to make these days and get hot.

It's not that you can't run the tape at a lower voltage.

[#7]

@shutter  @Mook 

If you've followed the last few posts then this is the stuff that I used but I have little doubt that they all work the same way.

https://www.ultraleds.co.uk/search/searchresults/?search_query=30+x+5630+Led+40+Lumen+per+Led+1200+L...

[#8]

My assumption @Baldrick1 was based on my own experience of these as I have them fitted  in my house (Philips Hue) and these require a PWM controller and the majority of others I've came across also require PWM and can be addressed and operated using I2C. Hence my reference to Arduino and Pi.

[#9]

@Mook 

That's the posh end of the market. As he keeps reminding us @shutter comes from Yorkshure!

[#10]

@Baldrick1  Thanks for the linky... looks as though most of that stuff on there is "high wattage" output.... so probably too (light) powerful for what I am looking for..

still considering using 5mm internal diameter plastic tubing in strips across the screen area... with the led(s) poked in the end... will be easy to fix in position and add further strips if the spread is not sufficient.

but... all is on hold for a while, as I have some other more important stuff going on now.

thanks to all for the input..

cheers

P.S.. talking of posh.... it "Yokshur"   .not Yorkshure !

[#11]

@Baldrick1  @Mook  and anyone else interested.

OK>.. so an update on (very little) progress..... I got some clear tubing, and tried the "tube light" situation... did not work... all right for an inch or two... but too much bother to cut up strips and insert an led into them all...

Next I bought a pair of these

https://www.ebay.co.uk/itm/Car-Day-Time-LED-Running-Lamp-DRL-COB-Strip-Light-12V-Waterproof-Drive-La...

WOW !  they are bright.... and would probably do the job, however... when I linked them up to a 12v 1amp switched PSU and plugged it in to the wall socket... just messing around to try various positions, and various light dimming surfaces.. after about 10 mins of use, the one i was using ( only needed one! )  got quite warm... not HOT.. but quite warm.. which is of concern, as I would want to use it for at least an hour at a time.. 

so... maybe I need some circuitry between the output of th12v 1a supply and the LED strip.? ? ? ?

They are supposed to be for "daylight" running lights ( sidelights to us old fogies)  on modern cars.. so should not run warm to hot, methinks.........

[#12]

In order for them to be 'day light' running lights they need to be so bright that they can be seen through the normal day light and in bright sunlight that's a lot of light so I'm not surprised they get warm.

[#13]

@shutter 

If you look at the spec then it gives the dissipattion as 6/7.5W so they are going to get hot. It also gives the voltage range as 10 to 16V. I don't know why there is a minimum voltage spec but suspect that what these are is a load of COB LEDs mounted serially edge to edge with no limiting series resistor. If this is the case then your options for dimming them is somewhat limited as once you get below the LED 'knee' voltage, presumably around 10V, they will probably just not light. If you want to try a series resistor to drop the voltage to 10V then you will need only a few ohms (4 max)  but watch the power rating of the resistor (1W minimum) as this will get hot.

They're cheap enough and are very unlikely to fail in a way that burns your house down so why not just go with them?

Another thought, to test whether they will work on lower voltage you could try connecting them in series to see what happens. Just remember to get the polarity correct.

[#14]

@Baldrick1  Yeah... I saw that wattage figure... but did not realise that LED`s would get "warm".. and was a bit worried about the location ( underneath an unventilated cardboard box, sitting on a plastic tablecloth !)  

I did try different battery voltages... and it would not light up or glow at 9 volts.. couldn`t quite fix up anything between 9 and 12 to see if it was a little dimmer...but.. as it is, running at 12v  with a 2nd diffuser paper between it and the screen, it works just fine for brightness.

If there is no "electronics" required, other than an "on/off" switch (or pull the power plug) .. I wonder how long it could run and how  hot it would get, and if that would present any fire risk.

[#15]

@shutter 

Have you considered making the back of your box out of an aluminium panel and fixing your leds to this to act as a heat sink? If you then put a spacer under this to lift the bottom off the table then I would not anticipate any thermal problems, especially if you put some ventilation holes in the aluminium and the box to get some air circulation.