I'm re-purposing an LED array from a motion light as a work bench lamp and I'm trying to figure out how to power it. The way it was wired originally was directly from the 6V battery pack in the base. I'd like to just power it off a 5V wall wart. How would I wire this properly? I have no idea what to use for a resistor value here.
Here is an album with pictures of the array, pcb, and battery pack.
Edit: I guess what I'm confused about is how to calculate the required resistor for a parallel array. If it were a single LED I'd go with 150ohm for a 10ma current (assuming 3.5V forward). I can't quite wrap my head around the math for for a parallel circuit. Would it just be 150/80?
You need to choose a resistor value such that the LEDs get the correct current going through the LEDs with the supply voltage you have to work with.
To figure that out, you'll need to figure out how that LED array is wired - I suspect there are additional components on the back of that LED array board (or on top which I can't see in the photo) in order to limit the current - otherwise, the LEDs would have already burned out the first time you turned it on
Grumpy_Mike:
The LEDs themselves should not be wired in parallel, each LED requires it's own resistor.
They almost certainly are in parallel.
I have seen countless commercial LED arrays (as in, lighting arrays like the one OP is talking about, where they're all turned on and off at once - when they're turned on and off individually, ofc, you need individual resistors), and they always wire the LEDs in parallel. In the distant past, you really did need a separate resistor for each LED, because of process variations between LEDs, but nowadays, process control in LED production is good enough that you can parallel LEDs without problems, provided they're all from the same lot (which they are in commercial products like this).
If the battery still works, I recommend you measure the current (and the voltage across the LEDs). If you don't have a multimeter, now's a good time to get one.
Is there a resistor now? Many cheap flashlights just rely on the battery's internal resistance, but that would be a bad thing to do with a power supply.
Would it just be 150/80?
If all of the LEDs are in parallel, you've guess the correct voltage, and you assume 10mA each, yes. 1.5V/(80 x 0.01) = 1.875 Ohms. You'd need about a 2 Watt resistor.
DrAzzy:
but nowadays, process control in LED production is good enough that you can parallel LEDs without problems, provided they're all from the same lot (which they are in commercial products like this).
That is a gross over simplification. The LED characteristics varies over the wafer let along over a whole batch. The trick in parallel lighting is to match the LED's characteristics even from within the same batch when making arrays. This video clip illustrates what happens when they don't get this right and the end up in the reject bit. Then someone fishes them out and sells them on eBay.
They're actually 40x2 I guess. The circuit is 40 pairs in parallel for what it's worth. The whole thing is pretty mickey mouse to be honest. The charging circuit is what looks to be a 2.5v zener and a solar panel. I've left it charging for most of the day. In decent sun I figured it should take about 8.5 hours to fully charge.
DVDdoug:
If the battery still works, I recommend you measure the current (and the voltage across the LEDs). If you don't have a multimeter, now's a good time to get one.
Is there a resistor now? Many cheap flashlights just rely on the battery's internal resistance, but that would be a bad thing to do with a power supply.
If all of the LEDs are in parallel, you've guess the correct voltage, and you assume 10mA each, yes. 1.5V/(80 x 0.01) = 1.875 Ohms. You'd need about a 2 Watt resistor.
Definitely don't have anything bigger than 1/4 watt resistors laying around. I'll see what I have on donor boards.
Running off the mostly charged battery and the solar panel it drew around 600ma @~6.8V and is far brighter than what I need to see what I'm soldering. Might be okay for brain surgery or something though.
What are my options here? I could run it off pwm with a bjt and a 555 or something. Even at 60% it'd probably be plenty bright. Probably doesn't change the fact that I'd need a hefty resistor though does it?
LED dies can be wired in parallel when they are all taken from the same wafer and on the same heatsink,
which is what the manufacturers of LED arrays do - all the devices on a given wafer have had identical
processing so the manufacturing spread is very small (semiconductor foundries have extremely tight
tolerances on process conditions). I've a number of 100 LED arrays and when you bring the voltage
up slowly they all start glowing within 0.05V of each other (at about 2V, full power at 3.2V).
A bunch of random LEDs you buy won't be from the same wafer (or even the same batch or manufacturer!)
so separate resistors (or current limiting) are needed.
