Lamp with LEDs in parallel and very under-rated resistors?

[treez]

Hello,

I just purchased a “72 LED multi-purpose work light” from ASDA supermarket.
It says it was produced for “Status International (UK) Ltd” (£7)

This runs on four-in-series AA batteries (Alkaline batteries have on_load terminal voltage of 1.1 to 1.3V)

72 LED lamp schematic: (also below in PDF)
https://i41.tinypic.com/17zo6r.jpg



The LEDs are all basically in parallel as you can see. It’s three lots of 24 LEDs in parallel. These 24 paralleled LEDs have no added resistances to equalise the currents in them.
Does this mean that the LEDs must have resistance built in to their silicon?

I supplied one of these Work Lights with 4.4V, and found that the total current drawn was 447mA.
The voltages across each of the 10R resistors was 1.453V, 1.471V & 1.465V. As you can see, this means about 210mW being dissipated in each of these 0805 resistors. –That is above the absolute maximum allowed for an 0805 resistor. A 2010 resistor would have cost little more than 0805. Why did they not just use 2010 resistors instead of 0805s?
(remember that AA cells can sometimes be 1.3V,and that would have meant even more disspation in these 10R resistors….in fact , in a separate test, i supplied the product with 5V, and found the dissipation in each of these 10R’s was 412mW)

The LED forward voltage was 2.93V

With equal sharing, each LED will only have 6.2mA in it. Presumably these LEDs are 30mA rated LEDs (they look like it). So is this why they are OK with parallel operation?…i.e., because they are being operated at the lower end of the I/V curve where the dnamic resistance is much higher?

I note that regarding the three 10R resistors, one has the value text pointing up….but the other two 10R’s are “upside down”, and the value lettering cannot be seen. I suspect that this is a kind of “factory coding” to indicate that these units have been through production soak testing.?

I suspect that 0805 resistors have been used, because if any of the products have widely variant LED forward voltages, then there is more chance of the 0805 resistors burning out during soak test….and the staff can see that a resistor is burnt out , and simply bin the product, rather than ship it?

How are they getting away with so many LEDs in parallel and so badly under-rated resistors?

 

[tpetar]

I just look circuit what you link, I stopped reading text. Circuit is bad and wrong. That circuit is made by total amateur or kid.

Just look circuit use 0805 resistor. What is power of 0805 resistor ? :smile:

Paralleling led is another and next story...


Best regards,
Peter

 

[treez]

OK I see your point...but are we missing something here?....is their some hidden reason for it?.......I would have just put in a simple switch mode boost led driver and used 4 series leds in the output and driven 150mA through them...giving the same power with just 4 leds, instead of 72 leds.

or if 72 leds were wanted, i would have put three 24-in-series strings with a 510R resistor (1812 size) in series with each of them to equalise string currents....then driven that from the 4 cells with a switch mode, constant off time boost led driver.

but are they saving money by doing it like they are?...with no switchmode led driver?

maybe they just put the lamps on soak for 10 hours, then take a look at them to see if any leds are much brighter than others...and bin them if they have that problem..because it would mean poor current sharing.

so are we missing something?

 

[betwixt]

What you are missing is the current limiting...... meaning the battery can't provide enough current to overdrive the LEDs. It's the cheap and cheerful but horrendously unreliable way many Chinese products do it. The same is in 'keyfob' LEDs, they have an LED connected directly across a coin cell with no resistor at all. The cell simply can't produce enough current to do damage. If you run the Asda light for too long on a power supply something will cook pretty quickly.

Soak test?? If these things fit in the sales packaging they are deemed to be sellable! If one is returned as faulty they throw it away and give you another, it cost less than testing!

Brian.

 

[tpetar]

Typical power rating for 0805 is up to 0,125W.

If you say 4x1,3V (battery voltage at load) that is 5,2V.
5,2V / 10R = 0,52A (current limits) per string. 4x0,52A.... :-?
24 LEDs in parallel....

Circuit is bad and primitive compromising and endangering life of LEDs. Rather use dedicated IC for this job or adjusted circuit for LEDs.

This design you can only see in old bad chinese products from 90's.



Betwixt give good point and direction with battery capabilities. You cant expect from little batteries to produce high amount of current.


You can do additional measuring. Use ampermeter and measure current of one string, then try to measure current for first LED, then meter current of last diode, depending how they are linked on PCB.


Best regards,
Peter

 

[treez]

the leds do look all the same brightness, and the voltage across the 10R resistors is given above.
I'd say they must have soak tested them ...just left them running for say three hours and then look at them to see if all leds look same brightness (all sharing current reasonably well).
In the Far East they unfortunately have 100's of staff all working for nothing (sadly), so they can do such extended soak testing.
I'd say I can make this a long life reliable product by simply changing the 0805 resistors for 500mW resistors..(or using two 1206 in parallel)..this wouldn't be that much cost...specially when you think the cost is £7, and three 500mw resistors are going to be a tiny fraction of that.

The 10R resistors don't provide GREAT current limiting though ......with 5V battery voltage, and 3V Vf, that gives 2V across the 0805 10R resistor....400mW in each 0805 res.

why have they used 0805?

it certainly isn't because the Chinese are stupid...the Chinese are better than the West now at Analog electronics and power supply kind of stuff.
The reason the Chinese sell this kind of stuff to us isn't because they choose to....its because Western Entrepreneurs who go to China insist that the Chinese do it like this.

 

[Audioguru]

Typical power rating for 0805 is up to 0,125W.

If you say 4x1,3V (battery voltage at load) that is 5,2V.
5,2V / 10R = 0,52A (current limits) per string. 4x0,52A.... :-?

No!
The 10 ohm resistor has 5.2V or more across it only when the LEDs are SHORTED. But the LEDs are not shorted, they have about 3V across them so the resistor has 5.2V - 3V= 2.2V across it for a current of 2.2V/10 ohms= 220mA. The resistor dissipates 220mA squared x 10 ohms= 0.48W. It might get hot enough to desolder itself.

I think cheap Chinese flashlights come with cheap carbon-zinc batteries that produce low current. Some of my cheap Chinese products came with "Super Heavy Duty" carbon-zinc batteries that produce almost no power.

Millions of cheap Chinese flashlights have many LEDs connected directly in parallel and they last for maybe a few minutes or more.
Of course many of them fail then are thrown away, not returned for a replacement or for your money back.

 

[tpetar]

No!
The 10 ohm resistor has 5.2V or more across it only when the LEDs are SHORTED. But the LEDs are not shorted, they have about 3V across them so the resistor has 5.2V - 3V= 2.2V across it for a current of 2.2V/10 ohms= 220mA. The resistor dissipates 220mA squared x 10 ohms= 0.48W. It might get hot enough to desolder itself.

I think cheap Chinese flashlights come with cheap carbon-zinc batteries that produce low current. Some of my cheap Chinese products came with "Super Heavy Duty" carbon-zinc batteries that produce almost no power.

Millions of cheap Chinese flashlights have many LEDs connected directly in parallel and they last for maybe a few minutes or more.
Of course many of them fail then are thrown away, not returned for a replacement or for your money back.

Its not my data Treez clearly give datas about voltage on load in his post #1, I suppose he do the measuring :

Hello,

I just purchased a “72 LED multi-purpose work light” from ASDA supermarket.
It says it was produced for “Status International (UK) Ltd” (£7)

This runs on four-in-series AA batteries (Alkaline batteries have on_load terminal voltage of 1.1 to 1.3V)

72 LED lamp schematic: (also below in PDF)
https://i41.tinypic.com/17zo6r.jpg



The LEDs are all basically in parallel as you can see. It’s three lots of 24 LEDs in parallel. These 24 paralleled LEDs have no added resistances to equalise the currents in them.
Does this mean that the LEDs must have resistance built in to their silicon?

I supplied one of these Work Lights with 4.4V, and found that the total current drawn was 447mA.
The voltages across each of the 10R resistors was 1.453V, 1.471V & 1.465V. As you can see, this means about 210mW being dissipated in each of these 0805 resistors. –That is above the absolute maximum allowed for an 0805 resistor. A 2010 resistor would have cost little more than 0805. Why did they not just use 2010 resistors instead of 0805s?
(remember that AA cells can sometimes be 1.3V,and that would have meant even more disspation in these 10R resistors….in fact , in a separate test, i supplied the product with 5V, and found the dissipation in each of these 10R’s was 412mW)

The LED forward voltage was 2.93V

With equal sharing, each LED will only have 6.2mA in it. Presumably these LEDs are 30mA rated LEDs (they look like it). So is this why they are OK with parallel operation?…i.e., because they are being operated at the lower end of the I/V curve where the dnamic resistance is much higher?

I note that regarding the three 10R resistors, one has the value text pointing up….but the other two 10R’s are “upside down”, and the value lettering cannot be seen. I suspect that this is a kind of “factory coding” to indicate that these units have been through production soak testing.?

I suspect that 0805 resistors have been used, because if any of the products have widely variant LED forward voltages, then there is more chance of the 0805 resistors burning out during soak test….and the staff can see that a resistor is burnt out , and simply bin the product, rather than ship it?

How are they getting away with so many LEDs in parallel and so badly under-rated resistors?

Best regards,
Peter

 

[kam1787]

1) Does this mean that the LEDs must have resistance built in to their silicon?

2) Why did they not just use 2010 resistors instead of 0805s?

3) is this why they are OK with parallel operation?…i.e., because they are being operated at the lower end of the I/V curve where the dYnamic resistance is much higher?

4) value lettering cannot be seen. I suspect that this is a kind of “factory coding”
to indicate that these units have been through production soak testing.?

5) burning out during soak test….and the staff can see that a resistor is burnt out , and simply bin the product, rather than ship it?

6) How are they getting away with so many LEDs in parallel and so badly under-rated resistors?

1) not at all, just plain LEDs

2) not familiar with the coding system you refer to but it would be cost or what was on hand or what was available, or to have brighter light

3) no, at same point they will likely start to burn out

4) no

5) never dump it, just patch it up and ship

I guess what you call a soak test, I call a burn-in. Unless it is in the buyers requirements it is not done - just a quick test to see that it works and then package and ship.

I doubt it would be worth trying to improve the circuit - replacing the resistors might prolong the life somewhat.

Perhaps the circuit was originally meant to be used with cheap recharge-able cells which have a lower cell voltage, and buyer asked for it to be used with batteries but at the lowest cost possible.

 

[betwixt]

To clarify point 2, the numbers are the width and length of the package so generally, bigger numbers mean higher power rating.

As I mentioned before and Kam1787 repeats, these things are made and shipped, they may not even test every individual one at all, sometimes they will only sample one in every hundred and even then only power it up for a few seconds to see if it works. I'm not critisizing China in the least, some manufacturers are very thorough and produce high quality goods but the ones that work to a price are generally less reputable. One customer of mine had a batch of Chinese made telephones returned to them with custmer complaints that they didn't ring, on inspection, none of them had the ringer loudspeaker fitted and in it's place was a 'tested' sticker!

Brian.

 

[Audioguru]

Hee hee, no ringer speaker.
Many "solar powered" calculators have a photo of a solar cell instead of having a real solar cell. Buyer beware.

 

[FvM]

How are they getting away with so many LEDs in parallel and so badly under-rated resistors?

What are worrying about? Do you want to copy the design?

As a user, you can be satisfied, if the device
- is not catching fire
- fulfills the lifetime expectations

Buying cheap chinese products is your decision. Personally, I banned chinese power supplies (except for controlled subcontractors of major manufacturers) after reviewing the design of a notebook supply that failed after a few weaks. Battery powered lamps aren't that critical safety-wise, I think. As you you don't know the specified resistor rating (some higher tolerance 0805 chips have 0.2 - 0.25 W), you can measure the resistor temperature to assess the design safety.

 

[treez]

Buying cheap chinese products is your decision

..sorry but it is not, people and companies are forced to buy Chinese if its cheaper otherwise they will not be able to compete with other people who are buying the cheap Chinese stuff....that's wy all shower manufacturers buy Chinese...because there competitors do, and if they don't, they cant compete.

 

[Audioguru]

American and European companies design a good product, find a Chinese manufacturer to make it then use Quality Control to make sure every item works properly.
Then the product is Name Brand and is reliable, unlike the very cheap junk that is designed over there.

 

[tpetar]

One thing is sure, in any case it will be light, the only question is light from LEDs or from fire. :smile:


Best regards,
Peter

 

[treez]

the very cheap junk that is designed over there

I don't think £7 is cheap for a 2W light...........(& £6.50 went to the uk entrepreneur who told the Chinese to make it to a tight budget)

 

[Audioguru]

I guess you are in the UK where EVERYTHING costs double or triple as everywhere else (to pay for your Queen?).
My son bought me a very bright LED light with adjustable focus for 4 bucks (2 pounds sterling) and it works very well from a single Ni-MH AA battery cell.

 

[treez]

yeah, I would have thought doing a 2W led lamp would have been better with 2 AA cells and a boost converter current regulator.......more efficient and cheaper in terms of less batteries.

with the existent 4 cell solution, with vin=4v4, theres 650mW lost in the 10R's, then the LEDs take 1.3W.