Thread Starter

rishsengupta

Joined Feb 24, 2021
2
Disclaimer: I'm not an engineer, just a DIY enthusiast.
So I have an cheap old plastic SMD LED desk lamp but its battery and circuit board are missing. I want to turn it into a USB powered lamp for my new apartment but I'm not sure of the best way to power it. It has 31 LEDs and the label on the bottom says "light source power: 2.5W" (written in Chinese which I translated using Google lens).
My goal is to power it with a usb cable either from a phone charger or a power bank. I'm not too keen on connecting the LEDs directly to a 5V USB power source because I feel it may draw too much current and ruin the LEDs. Then again I'm no engineer (I wouldn't be here otherwise) so correct me if I'm wrong.
Anyway, I was wondering if I should install some kind of protection circuitry between the LEDs and the USB power source, perhaps something that limits the current drawn to 500-600mA so it stays close to the aforementioned 2.5W?
I'd appreciate any and all advice I can get! Thanks a bunch in advance!IMG_20210224_123501.jpg
 

LesJones

Joined Jan 8, 2017
3,486
You will need to trace out the schematic of how the 31 LEDs are connected together. They may be connected in groups of a number of LEDs in series each with a current limiting resistor. As the total number is 31 not all the series groups can have the same number of LEDs in them so groups with different numbers in them will require different value current limiting resistors. Another possibility is that the missing circuit board contained a voltage step up converter that supplied a high enough voltage to allow all of the 31 LEDs to be connected in series. This would need to supply about 100 volts with some means of limiting the current through the LEDs.
I think it will be a better option to buy a new lamp. If you want to proceed with your idea then post the schematic when you have traced it out.

Les.
 

Thread Starter

rishsengupta

Joined Feb 24, 2021
2
You will need to trace out the schematic of how the 31 LEDs are connected together. They may be connected in groups of a number of LEDs in series each with a current limiting resistor. As the total number is 31 not all the series groups can have the same number of LEDs in them so groups with different numbers in them will require different value current limiting resistors. Another possibility is that the missing circuit board contained a voltage step up converter that supplied a high enough voltage to allow all of the 31 LEDs to be connected in series. This would need to supply about 100 volts with some means of limiting the current through the LEDs.
I think it will be a better option to buy a new lamp. If you want to proceed with your idea then post the schematic when you have traced it out.

Les.
Thanks for the advice Les. I want to continue with this project because I want to finally make use of some old parts.
As for the LEDs, they're all connected in parallel. I'm adding pictures of the front and back of the circuit board that they're soldered to. It's pretty clear that they're all in parallel and there are no other components on the board. IMG_20210224_164541.jpgIMG_20210224_164606.jpg
 

LesJones

Joined Jan 8, 2017
3,486
I had not considered that they may be connected in parallel. That makes it very simple as all it needs is a resistor of about 2.4 ohms in series with the supply. (This is assuming that they are white LEDs which will have a forward voltage around 3.2 volts.) The resistor will dissipate about 1.3 watts so I suggest using 4 x 10 ohm 1 watt resistors in parallel. (You cold use 0.5 watt resistors but they would run quite hot.) This is based on 2,5 watts to the LEDs but this gives a LED current of 24 mA. Most LEDs are rated at 20 mA maximum forward current. To limit the current to 20 mA would require a resistor of 2.9 ohms so using three 10 ohm resistor in parallel would be better to avoid exceeding the LED's ratings.

Les.
 

Walldredge

Joined Apr 2, 2021
5
I would hook it up to a variable DC power supply with current limiting (starting at 0 mA) and see where I like the light level while using a infrared camera (they're pretty ubiquitous now) to make sure I'm not burning anything up. I don't see any heat dissipation in the assembly, so it's possible the original board was a current limited switching power supply turning the LEDs on and off at a higher frequency than human sight in order to reduce heat build up. Using a constant DC power through resistors or a linear power supply has the potential to increase heat in the luminaire. Once you find the voltage and current you like, you can search for a cheap LED driver board online.
I really like that light. It would be super easy to solder in new LEDs to change the light output or color. Nice little project you got there.
 

Audioguru again

Joined Oct 21, 2019
3,509
Turning the LEDs on and off at a high frequency dims them (PWM) and reduces their average current. Then series resistors are not needed that get hot but the LEDs will still get hot.
 

MisterBill2

Joined Jan 23, 2018
9,243
With just a bit of cutting PC traces and adding a very few jumpers you can have two groups of 15 LEDs in parallel, and then put those two groups in series. That change will cut the power wasted in the series resistor in half.
With just a bit more cutting and jumpering you can have three parallel strings of ten LEDs, and put the 3 strings in series and not need a series resistor.. The one extra LED will need a series resistor about 270 ohms or so.
 

Audioguru again

Joined Oct 21, 2019
3,509
For a desk lamp then of course the LEDs are white that have a forward voltage of from 3V to 3.6V.
Two groups of LEDs in series need more voltage than 5V USB can provide. Three groups of LEDs in series need way more voltage than USB.
 

BobTPH

Joined Jun 5, 2013
3,655
I would assume a voltage of 3V because it will likely be higher than that.

That gives a resistor of 3.2 Ohms for 620 mA, which is likely the design current. Use a 3.3 Ohm 3 Watt resistor.

Bob
 

MisterBill2

Joined Jan 23, 2018
9,243
OK, I made an error assuming a lower forward voltage on the LEDs.
Clearly the very first step is to meaure the actual output voltage of that supply, as the claimed 5 volts may not be actually 5 volts.
 
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