Please be gentle... I am a Mechanical Engineer and Electronics are not my forte by any means. I have dabbled in the past making a few simple power supplies but my experiences with LEDs are not the most reliable and I seem to end up letting out the magic smoke more than not. So I am seeking some advise before I blow these ones up!

I have been given an old LED light which was rechargable battery powered but it now pretty useless and I would like to re-purpose the LED banks to make them powered off the 6vDC mains adapter that came with the light.

I have dismantled it all and have the following kit which I need to wire up.
1 off 6vDC 0.5A power supply.
2 off PCB "strings" of CREE? LEDs wired in series as pairs of LED's in series with a "100" (ohm) surface mount resistor. There are 30 LED's on each PCB

What do I need to make in order to join the 2 PCB strings to the 6v Power supply and what do I need to do differently if I only want to connect one of the PCB to the power supply?

Thanks
Simon

 

Welcome to AAC!

What do I need to make in order to join the 2 PCB strings to the 6v Power supply and what do I need to do differently if I only want to connect one of the PCB to the power supply?

For 1 or 2 boards, connect the "2+" to the adapter positive and the "2-" to adapter negative.

SMD resistor marking "100" is 10 ohms. The adapter isn't going to be up to the task.

 

Agree. If the LEDs are only 20mA, you need 1.8A, and they likely take more current than that.

 

Oh ok, thanks... I assumed that if I wired them up directly to the power supply they would blow. Is that not the case?
Is the 10ohm resistor enough to protect them?

How can I work out what current the LED's will need and does the voltage matter? I have some other plug based power supplies.
Thanks
Simon

 

Did the original light have a current or power rating?

 

I assumed that if I wired them up directly to the power supply they would blow. Is that not the case?

As long as you use a 6V supply, they shouldn't blow up.

Is the 10ohm resistor enough to protect them?

Maybe. There are 2 LEDs in parallel sharing a current limiting resistor. They'll only share current evenly if their forward voltages are matched.

How can I work out what current the LED's will need and does the voltage matter?

I assumed a forward voltage of 3V and used Ohm's Law to calculate the current. I assumed that the current divided equally between the LEDs. That will only happen if the LEDs had their forward voltages matched. Otherwise, the LED with the lower forward voltage will hog current. If a single LED can't handle the full current, it may die. If that happens, the other will die because it will be taking the full current.

Voltage always matters. Even if you use a current source.

 

The pairs of LEDs are in parallel (NOT series). The voltage across a white LED is typicaly about 3.2 volts. This means that with a 6 volt supply you will have 2.8 volts across rghe 10 ohm resistors. So each pair of LEDs will be taking 280 mA.
As you say that there are 30 LEDs in total you have 15 pairs each taking 280 mA. 280 x 15 = 4200 mA which is 4,2 amps.
Check that the resistors are 10 ohms as there are two ways that the 100 maring could be interpreted.

Les.

 

The simple way, if there are two similar boards, will be to put the two boards in series and run them on 5 volts DC from a 5 volt charger, which many can deliver over an amp. The LEDs will light but probably not at the maximum possible brightness.

 

As white LEDs require about 3.2 volts putting them in series and using a 5 volt power supply would not work. It would work with a 12 volt power supply. Re reading post #1 the TS says that that they were originally powered by a rechargeable battery but he does not say what voltage the battery was. He just says he plans to use a 6 volt power source. If the original battery was not 6 volts then we need to re do any calculations.
Dnomis, can you give us details of the original battery ?

Les.

 

Many thanks to all for their help so far...
The original battery is a 3.7v rechargable...
I have attached pictures showing the LED panels., the battery and the original circuit board. The wall charger previously posted is the original from this light.
The original circuit no longer functions correctly. Originally the red charging LED only used to light when it was on charge and then go green when complete. Now it is on red all the time. The light was never usable when on charge. The button used to switch the light between off, panel lights or the single LED. I don't currently have a use for the single LED but if I could easily incorporate that then I am sure I could find a use for it.
The plan is to make a holder for the panels to fit either side of the gantry on my Ender3 3D printer.

 

This is the new calculation of current. 3.7 V - 3.2 V = 0.5 volts so the current trough eache 10 ohm resistor will be 0.5 volts / 10 ohms - 0.05 amps = 50 mA (So the current through each LED will be 25 mA) When the battery has just been charge it's voltage will be about 4.2 volts so at that point the current through each LED will be 50 mA
At the nominal battery voltage of 3.7 volts the total current for each panel of 30 LEDs will be 750 mA.
I would suggest connecting the two panels in series and using a 9 volt power supply and a series resistor of 1.8 ohms (9 volts - 7.4 volts = 1.6 volts. 1.6 volts / 0.75 A = 2.13 ohms) This valueof resistor will give an LED current of about 28 mA
(Total current 30 x 28 mA = 840 mA) The 1.8 ohm resistor will disipate about 1.3 watts so you would need a resistor with at least a 2 watt rating.

Les.

 

Thank you...
I have a selection of wall plug power supplies but none at 9v. I have some higher and some lower.
I also think I may only need 1 of the panels...
Can I use the supply below?

5v - 3.7v / 0.75 = 1.7ohm so also a 1.8ohm resistor? 1 watt?

or if I do need to use both panels, can I connect both panels in parallel as it says it can deliver 1.5A ?

 

The fact is that LEDs will illuminate at less than the rated voltage. Putting two in series across 5 volts may light them adequately, or maybe not. An older wall wart with a transformer rated at six volts could work quite well.

 

Using the two panels in parallel (Each with it's own 1.8 ohm resistor. ) from that 5 volt supply should work but I don't reccomend runing things a their maximum rating.

Les,

 

The lithium battery is 4.2V when fully charged and is 3.7V when at half a charge which is its selling and storage voltage.
The electronic circuit is its charger and disconnects the LEDs when the battery voltage drops below about 3V.

Near their power rating, the 60 LEDs will be very bright and very hot. I have 36 similar LEDs brightly lighting my computer room.

 

I'd think about cutting some traces and arranging it so there are 3 banks of LEDs in series, which should run nicely from 12V. 12V power bricks are very common, plus it's useful in vehicles with 12V electrical systems, or can be run from a 12V cordless tool battery. To really pimp it out, add a PWM dimmer module (aka DC motor speed control), which only costs a couple of bucks online.

 

You may have answered this already (I could have missed it) but is there a reason you don't want to use the existing PCB to drive the LEDS?

Also, the single LED might be nice on the toolhead facing down if you limit the brightness.

 

Thank to all for the suggestions.
The existing PCB didn't work to power the LED's it only charged the old battery. It would turn the light off while charging.
I like the idea of using the single LED as a toolhead light but left it out due to the added complexity! I am struggling to get my head around using the 1 or 2 panels which are the same let alone adding the complexity of an additional single LED!

The whole idea of this little project was to make something using as much of the stuff I have kicking around as possible. if I have to buy anything more than a couple of pounds worth then I might as well just buy a dimmable self adhesive LED strip from China for a few pounds.

 

Thank to all for the suggestions.
The existing PCB didn't work to power the LED's it only charged the old battery. It would turn the light off while charging.
I like the idea of using the single LED as a toolhead light but left it out due to the added complexity! I am struggling to get my head around using the 1 or 2 panels which are the same let alone adding the complexity of an additional single LED!

The whole idea of this little project was to make something using as much of the stuff I have kicking around as possible. if I have to buy anything more than a couple of pounds worth then I might as well just buy a dimmable self adhesive LED strip from China for a few pounds.

This really shouldn't be as complicated as it has become. It is almost certainly the case that the panels will light up reasonably using a direct connection to the 5V of a USB port. They will probably be overdriven, though, and be very bright and somewhat shorter-lived, but they would almost surely work.

As a matter of fact I cobbled together a similar salvaged light (though it was a linear strip) for an Ender-3 V2 Neo and just powered it directly from USB. I expect it will die sooner, but I don't really have much concern about that, it will probably last quite a while anyway.

If you have the CR Touch bed leveling, you can steal power from the LED on that board to make the toolhead light easier. A simple current limiting resistor to make it the preferred brightness would be easy. The downside is always on behavior. If you want to get fancy you can use an output from the mainboard and control it with g-code.

Anyway, step back for a second and do a little test. Hook the two panels up in series and try 5V from a USB source. If you cut off the end of a USB cable that doesn't plug into the computer, and strip it back, you will find four wires, including red and black which are positive and negative respectively. (Occasionally, I have seen the black as some other color the clue is the data is green and white, and the power lines are usually heavier gauge)

Hook up the panels and give it a go. You are very unlikely to do any harm to the LEDs (but hey, if you do, maybe that's an outside the box solution to the problem). If the brightness is acceptable, you are all set. If not, you'll have to do something but we can figure that out if it happens.

Also, applying 4V or so to the battery input would allow you to use the existing board and its switching though that may not be all that useful depending on the offered modes.

 

I hooked up the 2 panels in series to my PC via USB and they illuminated but only just, they were not bright at all.
Is this because the PC cannot deliver much current? Is the 10ohm resistor on the board enough to protect the LEDs if I connect it up to the 5v 1.5a supply or will the LED's just draw the current they need and no more?