As some of you may already know I have recently started stocking led lighting modules - household stuff, brilliant product & ticks all the environmental friendly & energy saving boxes

However as with anything I sell I always want to fully understand the product & what makes it tick, hence I have been testing the modules with various different drivers, both constant current & constant voltage. With the constant voltage stuff things go pretty much as you'd expect in that the more of the 3 led modules you use the current goes up hence overall wattage goes up.

However when using the constant current drivers, for instance a constant current 1320mA, 22 - 38V driver if you drive 20 modules, they draw 1.32A @11.2V, which would imply a total draw of 14.8W (W = V x A) or 0.74W per module (the units have a nominal power of 1.5W each as given by the manufacturer). But if you drive 40 modules you get 1.32A @ 9.5V implying a total draw of 12.5W or just 0.31W per module - the light output of each unit remains very similar in both situations.

What am I doing wrong in terms of calculation? I am measuring voltage across all the units which are simply daisy chained in series; & current in line with all the units, how can 40 modules draw less wattage than 20 with the same power of light output??? It doesn't make sense! I simply want to be sure I am recommending people correctly in terms of which driver will be best for a particular application.

I have done similar tests with the lengths of flat led strip & similar results, more lights = lower overall power drawn - it appears to be contravening the laws of physics!

Please put me out of my misery & tell me what I'm doing wrong with the calculation or measurements!

Quantum physics is way out of my ever decreasing area of non expertise...

The more I think about things the more I realise the amount of stuff I know...

LED's light o/p is proportional to current and is usually set by the manufacturer of the LED. You can run them off a constant voltage but you need a resistor in series. The power supply however (to provide a given constant current) is at it's maximum internal dissipation when the voltage on the output is minimum (depends on the technology however). The voltage vs current of an individual LED chip is non linlear. It has a knee point (maximum efficient light output) about 1.2V so if you stick 10 of these in series you get approx. 11V. If the current source maximum voltage is 12V then that is the maximum number of LED's you can put in series.

To calculate dissipation for the load it is I x V. To calculate the load dissipation for the power supply is more complex it depends on the technology employed.

I hope this helps.

Id also comment that a constant voltage driver is not a bright idea for powerful LEDs. As they get hot they draw more current which makes them hotter.....

If you use a constant current driver you don't get the same temperature spiral....

Also worth noting that the human eye is terrible at discerning the difference between brightnesses of objects... so even if something is infact considerably brighter than the other...you might not actually be able to tell...

There's another consideration for LED's runnin of a rectified AC supply... they tend to flicker inline with the phase changes... a decent switched mode power supply should smooth that out somewhat

[Edited on 18/3/15 by tegwin]

I should have said these are manufactured using the 5630 chip, 3 x 0.5W (nominal) chips to each LED module.

theprisioner - you obviously know what you are talking about - is it likely that any 12v constant voltage led drivers would already have the resistor mentioned built in or that the led modules would have? As the ones I've been testing so far seem fine running anything from 1 module up to 22 for a 24W driver or up to 30 for the 36W driver - neither supplier of the driver or the led modules (different suppliers/manufacturers) has mentioned any requirement for a resistor. Also when driving the led's from a 12V battery there appears no limit to the no. you can daisy chain together (within reason!) without affecting light output - they simply draw more current - again, no resistors involved.

Are you saying that with the constant current drivers you can actually have the situation that I appear to be seeing that you can actually have less power drawn whilst getting the same light output from more chips as you approach this "sweet spot" - the knee point you mention - or am I misunderstanding?

And when you refer to 1.2V per chip as being around the ideal, then I assume that with a 3 x 5630 chip led module the ideal voltage would be around 3.6V, so, say, a 24 - 36V driver would drive 10 x modules each with 3 x led chips?

" To calculate the load dissipation for the power supply is more complex" - you're telling me!!!

Yes I do think you are suffering inconsistent measurements due to the knee point.

There a number of pitfalls using LED's in cars. The biggest problem is the vast majority of cheap automotive LED bulb replacement devices have built in resistors and are "designed" to work across 12V. My experience is this is not always a reliable way to design a product. The worst offender are the festoon type for internal lights. As you say they put several LED chips in series then connect whole lot in series with a resistor. The problem doing that is that the Alternator in a car varies from 11.5V to 14.5V this either under stimulates the LED producing a light that is not in the (designed) spectrum or it overheats the chip when the alternator puts out 14.5V. This produces a product that is less reliable than a filament bulb. I have experienced this, it is not just a theoretical possibility. The best way to run the festoon type LED replacement bulbs is to use a 12V - 12V regulator and put it in your lighting circuit for internal lighting.

I have had some LED type products that last extremely well especially the higher power LED ones in running lights etc. I assume the chip is either better cooled or they have a constant current chip built in.

Just to clarify this is household lighting running from rectified AC, everything I have works, I'm just having difficulty in getting my head round what I see!

As the manufacturer of the chips says they are 1.5W nominal I had expected that a 3W driver, say, would only drive a couple of modules, however in practice this defo isn't how it is working & in fact I have 9 modules in my understairs cloakroom being driven very satisfactorily with a 3W driver, it has been running without issues for 3 months now. I just wanted to be better able to have an understanding of what wattage driver should sensibly be used for what no. of modules/chips - it would appear trial & error is going to be the only accurate way - all very confusing.

I don't like selling a product I don't fully understand, I like to be able to give people sensible & accurate information & recommendations!

Any further info much welcomed!

The problem is what is the 3W referring to. Is it the total load or is it the maximum the driver is allowed to dissipate.

The only example I can find quickly

eBay item number:400614227266

It says 2 - 12V 0.3A 3W PSU

This means when the output voltage is 2V (1-2LED chips in series) output power = 0.6W
when the output voltage is 12V output power is 3.6W (10 LED chips in series).

In other words the power unit is delivering 1 - 3.6W if it is a switching technology and is in the order of 80% efficient then the consumed power will be 3.6/0.8 = 4.5W

Simples

Back to first post.

What exactky is this module.

Any electrical item is specified by its operating voltage and current it draws.

Find the current vs voltage graph via google for a led and y ou will see it being non linear. At low volt, hardly any curtent flows then it increase hugerly for smaller steps.

Constant current source does just that, and you would connect leds in series, no drop resistors are needed as there is a sense circuit to keep the current fixed.

It is not a good idea to connect doides in parallel as you cannot guarantee the share the current equally unless they are matched devices.

Constant voltage is for connecting leds in parallel. they need a drop resistor to limit the current.

Im guessing the modules you are selling are specified for a constant curruent, they wilk be specified for a minimum voltage so you know how many can be connected in series.

Or they are soecified for fixed operating voltage and current drawn, the latter allows you to work out hke many can be connrcted in parallel.

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Originally posted by 02GF74
Back to first post.

What exactky is this module.

Any electrical item is specified by its operating voltage and current it draws.

Find the current vs voltage graph via google for a led and y ou will see it being non linear. At low volt, hardly any curtent flows then it increase hugerly for smaller steps.

Constant current source does just that, and you would connect leds in series, no drop resistors are needed as there is a sense circuit to keep the current fixed.

It is not a good idea to connect doides in parallel as you cannot guarantee the share the current equally unless they are matched devices.

Constant voltage is for connecting leds in parallel. they need a drop resistor to limit the current.

Im guessing the modules you are selling are specified for a constant curruent, they wilk be specified for a minimum voltage so you know how many can be connected in series.

Or they are soecified for fixed operating voltage and current drawn, the latter allows you to work out hke many can be connrcted in parallel.

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although the modules appear to be in series when daisy changed, you are actually running them all in parallel, (+ & - ) both going to the next board.

the constant current limits the total current to 1.32A, shared between 20 modules, and the driver finds it can push that current at just 11.5v

if using 40 modules in parallel the driver can push that current at just 9.5v

if you were to connect 2 x20 modules in series
ie one string going to plus, the minus of that going to plus on the second string, and then back to the driver,
you should then find a current 1.32A, and a voltage of 23V

as others have said, brightness is difficult to judge, so you may need to get a light meter.

quote:

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Originally posted by gremlin1234
although the modules appear to be in series when daisy changed, you are actually running them all in parallel, (+ & - ) both going to the next board.

the constant current limits the total current to 1.32A, shared between 20 modules, and the driver finds it can push that current at just 11.5v

if using 40 modules in parallel the driver can push that current at just 9.5v

if you were to connect 2 x20 modules in series
ie one string going to plus, the minus of that going to plus on the second string, and then back to the driver,
you should then find a current 1.32A, and a voltage of 23V

as others have said, brightness is difficult to judge, so you may need to get a light meter.

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These LED substrates are configured for constant voltage, not the most efficient way of running multiple modules. You can see the series resistors in the images. The series linking of the design is just passing on the +12 to the next substrate.

As each module consumes 1.5W then 10 modules will consume 15W. Any variation will be due to the non linear nature of the diodes and the supply voltage.

Assuming 1.5W is correct and three chips. The current will 0.125A Total drop across resistor is 8.2V therefore there is 1W lost heat per substrate (10W for 10off). Hardly a green design.

If you were to remove the resistors you can run 10 modules off a power supply 1/3 the size.

Been doing some more testing with these today & also with the flat "ribbon" led strip, I've more testing to do & then results to compile, but they don't draw anything like 1.5W per module unless using them with a 12V battery rather than an LED driver, if used with constant current then there definitely appears to be a "sweet spot"where the power drawn drops significantly despite the fact that you are powering more modules. This was the main thing I was finding so confusing, but from the info you guys have supplied does in fact make more sense now. Many thanks for your input guys!

If this is an inefficient way of using LED's then I don't know what they are doing with LED bulbs in mass manufacture, I would say that comparing these with LED replacements that I've got for halogen bulbs then the actual lighting capability watt for watt is very similar to the mass produced bulbs

I've actually made a DIY LED "Bulb" which works brilliantly - I will post a youtube vid up at some point - should be of interest to Locost peeps!

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Originally posted by theprisioner
These LED substrates are configured for constant voltage, not the most efficient way of running multiple modules. You can see the series resistors in the images. The series linking of the design is just passing on the +12 to the next substrate.

As each module consumes 1.5W then 10 modules will consume 15W. Any variation will be due to the non linear nature of the diodes and the supply voltage.

Assuming 1.5W is correct and three chips. The current will 0.125A Total drop across resistor is 8.2V therefore there is 1W lost heat per substrate (10W for 10off). Hardly a green design.

If you were to remove the resistors you can run 10 modules off a power supply 1/3 the size.

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That is interesting, I am however retired now so I guess there have been some further developments in 10 years. Thanks

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Originally posted by gremlin1234

some modern LEDs run at about 3- 3.3V, and about 120mA
http://www.lumichip.com/Lumichip/Aurorics_files/LC_5630XXPN_G1X.pdf

they can also manage over 100lm/w

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