(Another) AA/USB-powered LED lamp project by a novice

Thread Starter

Xenopus

Joined Apr 9, 2022
4
Hello,

I am an electronics novice seeking advice on my current project. I know my way around a wood, metal, or maker shop, and have done my own basic repairs on computers, video game consoles, and the like. But I'm not confident enough in my knowledge of electronics to be sure that my current project is going to be efficient and well-designed. I can make it work on my own, but I'd rather not make something that draws ridicule from experts.

I am trying to make a battery-powered lamp for a desk or bedside table. It will be a box-type lamp that throws light in all directions, bright enough to read by or for mood lighting, but nothing intense. I'd like to power it with rechargeable AA batteries because of their ubiquity, but have the option to use USB power as well. Here is what I have got so far, using some Aliexpress parts:

- A 5V, 5W warm white LED plate with 10 small SMD 5730 chips on it and some little resistors (see attachment)
- 4xAA battery pack with a female USB type A connector (currently using 4x1900mAh Eneloop AA batteries)
- A adjustable voltage regulator set to 4V, at which setting the LEDs draw a consistent 400mA. I currently have one with an illuminated LCD that shows voltage and current, with a pot for voltage adjustment only (picture attached). In the finished project, I would like to use perhaps an LM2596S step-down voltage regulator instead, and just set the voltage using the pot and a multimeter. (I don't know if an LM2596S is a well-known piece of electronics, so please let me know if more information on it is needed)
- The finished project would also have a simple power switch between the batteries and the voltage regulator. For now, the fancy voltage regulator I'm using has its own power button.

So, I've tested this configuration and it works well. The LED is 5V, but I am purposely limiting the voltage to 4V to keep the temperature low (the LED plate gets warm, but no more), to keep the current draw low (I am guessing that AA batteries prefer lower currents), and because the light produced is adequate at 4V. I also assume that this will extend the life of the LEDs (not that they're expensive, but I like the idea of long-life and durability as a design principle).

The lamp runs with an even, pleasing light for 3 hours and 40 minutes before the LED starts flickering and the voltage regulator gives up and goes dark. The starting voltage of each AA battery was about 1.38V, and at the time the lamp gives up, it was down to 1.12V per cell. And these are Eneloop NiMH batteries rated at 1900mAh, but they're many years old and well-used.

The lamp also works fine plugged into any powered USB type A jack on a computer or peripheral like my keyboard, drawing the same 400mA current at 4V.

So, my questions are:

- Is there anything in this configuration that is dumb, unsafe, unnecessary, or inelegant?
- Is my idea to ultimately use a cheap, accessible LM2596S step-down converter a good one? After all, if I want it powered with 4V only, then a 3xAA or 4xAA battery pack or a USB jack will all provide a bit more than 4V, so there's no point in using a regulator that can step up as well, right? Or are there better, more robust options for what I want?
- Am I being silly using a voltage regulator at all? I mean, could this LED unit, which has current-limiting resistors built-in, run fine with only a power switch and whatever voltage a 3xAA, 4xAA, or USB port power source feeds it (probably 5-8V range?), with no downsides except maybe a lower battery run-time than when limited to 4V?

And my most novice question, but not super-important for my project directly:

- If this is a 5W, 5V LED, then why does it not draw anywhere close to the amount of current at 5V (or 4V) that basic electricity forumulas tell me that a 5W LED should? Is 5W just the maximum amount of power it can take if you supply it with voltage somewhere over 5V and make sure it's cooled well?

Please, if there is any more information I can supply to make my questions clearer, let me know. Thank you in advance for any advice offered. It will be appreciated.
 

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LowQCab

Joined Nov 6, 2012
2,045
The first calculations that You need to make are how long do You expect this Light
to operate satisfactorily on the VOLUME of the Batteries that You might find acceptable.

Various Battery types have a generalized rating of watts per unit of volume.
The Battery manufacturers don't usually supply this number, and when they do it's usually exaggerated.

Then there is the efficiency of the LED-Current-Limiting method that You decide has
an efficiency-level that You find to be an acceptable trade-off.

The "built-in" Resistors on the LED-Board that You have provided waste a lot of Battery just making heat.

A Switching-Current-Regulator is a far more efficient way to do it.
This means that You may want to go with individual High-Power-LEDs with no Resistors.
Individual LEDs will virtually always come with a Spec-Sheet which will help You immensely in
your calculations of required-Current and Light-Output.
A Switching-Current-Regulator is also an excellent way to provide Dimming with
little-to-no loss of efficiency.

Double-AA Batteries are not going to be adequate unless You parallel a big pile of them.
They will barely be able to supply ~5-Watts individually,
and will have a short life-span if pushed that hard.
There are certainly larger sizes available.
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Here's my version of the DIY solution ............
There are certainly other ways to do it, and this is not designed for absolute maximum efficiency.
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LED Lantern 3 FLAT  .png
 

bassbindevil

Joined Jan 23, 2014
422
A "5 volt" LED panel will usually run just fine from 4 volts. Less bright, but also less heat. Dimming can be done with an inexpensive PWM "motor speed control"/dimmer module. 4V (ish) can be obtained from a 1S lithium-ion pack of some sort, which are easily charged from 5V.
My approach to this sort of thing is to harvest COB or SMD LED panels from dollar store "switch lights" or "work lights". Three can be wired in series for 12V operation. While you can get 12V (ish) from a 3S lithium pack, charging is less convenient. Easiest is a 12.6 V "wall wart" that's specifically designed for lithium charging (available from ebay/etc or with some luck in a bin at a local thrift store.) Next easiest is a step-up charger that runs from USB-C (boards available...). And then there are step-down chargers based on chips like the SY6912 or MAX745; those require a 15V or so power source, but should also work directly from a solar panel.
 

Thread Starter

Xenopus

Joined Apr 9, 2022
4
Thank you for both of these replies.

So, AA batteries are not the best power source, then, despite their accessibility. And I think I will consider an individual, resistor-free LED as a light source.

LowQCab's DIY solution looks classy, but my, quite expensive. I think two 18650 batteries in parallel with a USB charging board looks attractive, though.

So, if I want to keep things rather simple and tidy, with an eye on efficiency, what about a configuration as follows: a USB charger board connected to a pair of 18650 batteries in parallel and then an appropriately-rated buck-boost voltage regulator set to ~4V, and this powers a single, resistorless 5V LED, with an on-off switch? Reading about buck-boost voltage regulators, I sense that they're relatively efficient. I get the same sense concerning switching current regulators, but I have less of a grasp on them.

A PWM dimmer would be nice, but was not something I considered originally. Would a dimmer need to be between the regulator and the LED to work properly? And would that not mean that I'd also need an on-off switch earlier in the circuit to cut power to the regulator when the lamp is off?

Also, charger boards for 18650 batteries look to be mostly designed to charge single cells. Would roping them into charging two in parallel just mean they do the job slowly, or badly?

Thank you once again for your expertise.
 

Alec_t

Joined Sep 17, 2013
12,808
Be careful when connecting 18650 cells in parallel. First make sure that they have the same (or very close) terminal voltage; otherwise there could be a large current flow between the two.
 

Thread Starter

Xenopus

Joined Apr 9, 2022
4
Be careful when connecting 18650 cells in parallel. First make sure that they have the same (or very close) terminal voltage; otherwise there could be a large current flow between the two.
So, if I wanted the power capacity of two cells, would going with two in series, and then dealing with accompanying higher voltage, be smarter? I guess one cell is okay, but, well... half the battery life, then.
 

LowQCab

Joined Nov 6, 2012
2,045
2 brand-new Cells from a reputable manufacturer,
(expensive, not from China), ( I recommend a company called "MaxAmps", made in America ),
will generally never have any issues when wired in parallel, and used only for 1 purpose,
not swapped-around into different devices etc..

If You want cheap, You will always end-up paying for it in the long run.

2 Batteries in parallel will have twice the Current-Capacity,
and therefore, will take twice as long to Charge with a limited Capacity Charger.
Virtually all Chargers will have built-in Current-Limiting,
if for no other reason than to protect the Electronics of the Charger.
The USB power-source will probably also limit the available Charging-Current.

With a "Switch-Mode" Current-Regulator You can achieve the highest efficiency.

If You use a "Switch-Mode" Current-Regulator adding-on a dimming function is not terribly difficult,
but since the "Color-Rendering" qualities, and a few other specifications,
will change when operating the LEDs at reduced Current,
it's a better plan to add a PWM Circuit if dimming is desired.
This will mean that the instantaneous Current flowing through the LEDs will not vary,
but the percentage of "On-time" and "Off-time" will create the illusion of less brilliance.
PWM-dimming will, of course, extend the Battery run-time,
almost in exact proportion to the amount of dimming used.
Example, 50% dimming = almost twice the Battery run-time between re-chargings.

Battery-Protection-Circuitry is ALWAYS required,
Lithium-based Batteries don't like it when they are discharged below ~2.8-Volts,
and must be disconnected from any loads when they get below the set cut-off-Voltage to avoid damage.
.
.
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Example of using a standard "Buck-Boost" Chip as a Current-Regulator ........
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5A 60V Current Source .png
 

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bassbindevil

Joined Jan 23, 2014
422
So, if I want to keep things rather simple and tidy, with an eye on efficiency, what about a configuration as follows: a USB charger board connected to a pair of 18650 batteries in parallel and then an appropriately-rated buck-boost voltage regulator set to ~4V, and this powers a single, resistorless 5V LED, with an on-off switch? Reading about buck-boost voltage regulators, I sense that they're relatively efficient. I get the same sense concerning switching current regulators, but I have less of a grasp on them.
The voltage needs of white LEDs are a good match to lithium-ion cells, so the voltage difference is small and there's really no need for a switching converter. Unless you need constant brightness and colour temperature for photography, a resistor has the advantage of gradually dimming as the battery runs down, and won't just suddenly go out and leave you in the dark. If the LED module already has a resistor sized for 5V, it'll probably be just fine at 4V.

A PWM dimmer would be nice, but was not something I considered originally. Would a dimmer need to be between the regulator and the LED to work properly? And would that not mean that I'd also need an on-off switch earlier in the circuit to cut power to the regulator when the lamp is off?
The PWM dimmers include an off switch in the pot (just like the volume control on most old portable radios). If you were using a switching regulator (boost or buck), yes, you should have a switch that turns it off completely. One thing to watch for: not all of them will work down to 3V; it might be necessary to bypass an on-board linear regulator or something.

Another option is a flashlight driver module. These have a tiny microcontroller that drives the LED through one or more constant-current drivers and can dim the LED by varying the duty cycle. While they're ordinarily used to drive a single high-power LED, there's no reason they can't drive a bunch of paralleled LEDs like a COB or SMD array. They need to be powered through a push switch which can be briefly interrupted to change modes.
One example is the NANJG105C; it comes with eight AMC7135 drivers for 2.8 amps max drive (I removed half of those since I figured 1.4 amps was plenty). The board has has star-shaped jumper points to select different operation modes (many of which are actually useful) (I picked the one that just gave me low, med, high), or the ATtiny microcontroller can be reprogrammed if you have the right hardware. There are cheaper more generic flashlight drivers, but many have stupid modes, no memory for last mode used, pwm frequencies that cause visible flicker, and cannot be reprogrammed.

Also, charger boards for 18650 batteries look to be mostly designed to charge single cells. Would roping them into charging two in parallel just mean they do the job slowly, or badly?
Charging more than one lithium cell in parallel just means that charging with a given current takes longer. If you use the popular TP4056 or TC4056 USB charging modules, note that most of them lack the input resistor that the data sheet recommends, so the chip can run unpleasantly hot. And some include protection chips for the lithium cell, so your load should be taken from the pads on the charging module, not directly from the cell(s).
 
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Thread Starter

Xenopus

Joined Apr 9, 2022
4
Thanks again for these replies, they are very helpful. I've ordered a bunch of parts to toy around with and test a few designs. When I receive them and get a chance to work with them, I'll report back with my progress.
 
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