LED Strip lights

[charliecoutas]

Does anyone have experience of LED strip lights? I don't mean the ones with the programmable chip, the ones I have are a single light colour and there are so many leds that you just see a continuous line of light. The strip is 8mm wide, sticky on the back.

The reason I am asking is that when I carefully apply a rising voltage (24V rating on the strip), nothing happens for quite a while then the brightness goes up quite suddenly. I can't see any resistors and the manufacturer says that 24V is OK, but no more. I have made a PWM dimmer and all seems good, but I am curious as to the finer details of their design.

Charlie

[RGV250]

Hi Charlie,
I have some I reclaimed from somewhere, never powered them up. I believe if you look they will be series'd in groups for the 24v. I will have a look later just to make sure I am not telling porkies.

Regards,
Bob

[david]

12V LED strips often use 3 LEDs in series and a resistor, repeating along the length.  If you can't find any resistors, then it may be intended for (voltage compliant) current drive.  Slowly increasing voltage won't do anything until you reach the combined forward voltage of the groups of LEDs (6 LEDs for 24V?) Depending on colour this may take 14V to 17V (for Blue and White) and then another couple of volts they are drawing well in excess of the typical 20mA rating per LED.
Brightness will be fairly linear with current but very abrupt when voltage driven.

Cheers,
David

[charliecoutas]

Bob, David

Yes, abrupt is the word. Having groups of leds in series makes sense, I cannot see any resistors. I am driving it with 24V PWM (18F26K22 and a power mosfet), although the 24V is only 20V at present.

Will experiment some more....

Charlie

[david]

Hmmmm.  You've still got no control over the current during the ON time of the pwm if there are no resistors in the strip.  You're just making shorter or longer periods of whatever current wants to flow.  You really need a switchmode current source which is like a boost converter, but it regulates output current rather than output voltage.

[Fanie]

Our experience with LED strips
Many buy them here for lighting due to the frequent power failures.
Is your use for decoration, or lighting ?

The led strips specified voltage greatly reduce life expectancy when exceeded.  If it is for 12V, and you have a charging circuit that will increase the voltage to say 14V, the led strip will not last very long, so do place a regulator on the strip supply.  Each LED group will have a resistor in series, but the value is low because the LED's themselves start up at around 10,5V on the 12V strips, like David said, so only 1,5V to drop with a resistor.

Even with the regulation, expect the LED strip to last only a few months.  I suspect this is due to overheating of the individual LED's, although it is not very detectable due to the strip construction itself.  You hardly get any specifications with these strips, and I suspect some may have a heat conducted sticky back you have to stick to an aluminum strip (or something else) for cooling which could increase it's expected life span.

You will also find the start LED's are brighter than the end LED's in the strip due to voltage drop over the length, very visible on the 5M and longer strips.  You have to apply the supply to the end also to achieve a more uniform brightness, and if solder pads are available you may want to connect some of them to the supply as well.

PWM works well, even a little dimming reduces the current (and heat generated) quite a lot without reducing the emitted light severely.
After a geyser, lighting is the second most power used in a household.

[charliecoutas]

Thanks Fanie, that all sounds good. It's for decorative lighting in the lounge.

I've done some fiddling: I cut a spare 4.5cm bit off the end of the 7 meter strip and put it on a variable power supply. It is a 24V strip, see photo (I carved some of the orange plastic covering the tiny leds to try to spot any other components).

Nothing much happens until 19V is reached, then it lights dimly and draws 30mA. As you increase the voltage the brightness increases all the way up to 24V. But the current always stays at 30mA. My psu was set for constant voltage. So there is a constant current device hidden away, in each "LED" I suspect.

Which means that my PWM method is OK, as you say Fanie. I know that running things like this at their "rated" voltage usually means a shorter life, that seems common these days. But tests show that a very nice, warm glow comes from around 20V so it should last. On the life-length - I bought a "Chinese Export (CE)" power supply from Amazon: 24V 5A Just under £10. We'll see how long that lasts.

There is no noticable difference in brightness at the far end of the 7 meter strip, it is only fed at one end.

The 7 meter strip is rated at 98 watts (at 24V). My offcut is 4.5cm long. So 4.5cm takes 30mA which is 0.6mA/mm  Therefore 7000mm will take 4.7A which sounds right.

What is a geyser?  In this country it's either a hot spring or some bloke in a pub.

Charlie

[Fanie]

What is a geyser?  In this country it's either a hot spring or some bloke in a pub

AI Overview
In England, a geyser, referring to a natural hot spring that intermittently spouts water and steam, is still called a geyser. However, the term is also used to refer to a domestic gas water heater in British English. This usage is less common now, as combi boilers have largely replaced them. Other terms used for a domestic water heater in England include water heater, hot water heater, hot water tank, or boiler.

Boiler is incorrect because the water in a geyser is not supposed to actually boil, although it probably can in which case you will get lots of steam spouting out the taps.  But yes, people use stupid terms sometimes, like here a traffic light is referred to as a robot.  You stop at a red robot, unless there is a flood then you stop at the row bot 

You do get current diodes which limit the current through them, very nice if your LED strip have them in, the supply voltage then is not critical.

[RGV250]

Hi Charlie,
From their web site.

Just note that this COB strip light is cuttable every 50mm, so round off your measurements every 50mm.

It looks there the are 6 LEDs in the lower part of your image so on that assumption they might do a series / parallel setup every 50mm?

Bob

[charliecoutas]

I didn't cut it on a 50mm boundary, it is therefore short. I think they only say that because there is nothing to solder to on the non-50mm cut end. I'm pretty sure that any shorter length would also work, the magic seems to be in the LEDs. Which supports Fanie's theory that the supply voltage is not critical, it and it alone determines the brightness.

Charlie

[david]

Judging by the 19V turn on voltage it's almost certainly using groups of 6 LEDs.  This could be 6 LEDs and a resistor or more likely 3 LEDs, a resistor, 3 more LEDs and another resistor. (effectively two 12V strings in series).  These are likely white LEDs which are blue LEDs with a yellow phosphor and hence the fairly high turn on voltage (~3.1V)
I still can't figure the constant 30mA.  Could they use a 2 transistor current sink?  LED brightness tracks current so it doesn't explain why the brightness varied while the current stayed constant.  Time to peel off a bit more of the plastic diffuser?

[Yves]

What about using a variable constant current circuit to power the LED. if you make a constant current circuit and adjust the current to  a pecify current you may avoid an over voltage,

Yves

[Fanie]

I still can't figure the constant 30mA.

https://www.mantech.co.za/Stock.aspx?Query=35M0380and

This one is a 20mA regulator but you can probably get them in other ratings as well.  When Manny (retired) was still running Mantech, he held a competition for the most uses of these diodes probably to promote the diode.  Won a multimeter if I remember right.

[charliecoutas]

You win the award David! A little stripping revealed the blue LEDs and, as you also predicted, a resistor! (I guess it's a resistor.) From the bottom I see six leds, a resistor, nine leds, a resistor, then seven leds which I guess would be nine if I hadn't cut the strip there. I'm struggling to see why the top seven are lit? This is at 19V



Yves, the PSU is a variable voltage unit. I think we have solved any problems, it's just a puzzle as to how these LED strips are actually arranged.

Charlie

[charliecoutas]

That might well explain it Fanie: it's not a resistor but a current regulator! That would also explain why different numbers of leds appear between each "regulator".

Charlie

[charliecoutas]

But hang on, if the current is constant, and the led volt-drop is fixed, how do you change the brightness? And my measurement of 30mA must be wrong. I'll have another look tomorrow.

Charlie

[david]

They look more like resistors to me but we would need a closer look.  0402 or perhaps 0603 - hard to get a scale.
I've attached some data for a typical small, "white" LED.  The forward voltage vs current curve is pretty steep, but it does have a slope so it has an intrinsic resistance even if small - about 25R.
The forward voltage between devices for a given fixed forward current shows quite a spread.  The risk of using a string of 6 LEDs (maybe 7) with 24V and a minimal series resistance is that every so often you may strike 6 LEDs at minimum forward drop and the current may then exceed the 30mA absolute maximum that applies to most of these small LEDs.  They will still work fine but may be hotter, a little brighter and have a shorter life span.  If it blows it will be like a missing tooth and the small section can be removed and another spliced in.

On the subject of LED ballast resistors - I have a small Aluminium pocket torch that runs on 2x AA cells and has a single modest power LED with no series resistance.  With new cells the end of the torch gets too hot to touch until the LED has knocked the tar out of the cells and the current drops to more modest values. Obviously working on just the LED slope and intrinsic resistance.
I also have a solar light consisting of a PV panel connected to a large LED array via a cable so both can be optimally placed.  The LED array is switched directly on to a single Li Ion cell (4.2V) so the LED with the lowest forward drop will be taking one for the team until it's voltage drop brings the others in to play.  Worse still - the PV panel is good for just over 6V at around 120mA and this is directly across the Li Ion cell.  (yikes!)
Just because we wouldn't design things this way doesn't mean that others can't sell them.

Cheers,
David

[charliecoutas]

Hmmm.  I just cut the rest off the extra length off and did some surgery on it. I went a bit too deep in a couple of places, and only those leds are now dark! See photo. 19.5V

I was also wrong about the constant current, it is only flat for a limited voltage range. So maybe those are resistors after all, they do look like it. But why are only the leds I damaged dark? They must be in a series/parallel config?

Charlie

[RGV250]

Hi,
Perhaps they are in the parallel part, I notice the ones next to them appear brighter?

Bob

[charliecoutas]

So they are. Fascinating. I will dig out one of the "resistors" and see what happens. Isn't science wonderful!

Charlie

In the hot South of England