Why don't these LED's work in parallel?

Question

noobie here as will be obvious in the next few sentences :)

I have this strip of LED's Amazon LEDs. I thought/hoped that since each led has its own resistor that this would mean i could cut off individual cells to rewire in parallel for a little project i was working on. So i cut one off the end, soldered some leads to it, hooked it up to the power supply that came with the strip (uses 4 AAA batteries) and it seemed to work!!! So i cut off several more, soldered leads to them also, and then wired them all together in parallel, but when i hooked them up to power, i got no lights and a faint electrical burning smell. When i plug the rest of the strip back into the power supply they initially come on, but quickly dim to almost nothing.

I'm reading 5v at the power supply and the LED cells are stamped as 3v. I have a basic understanding of circuits and electrical principals (I think :) ), but I don't have a great understanding of specifically what's going on here with respect to how the 5v at the power supply works with one or more of these 3v led's. I can't help but wonder if there was something in the circuitry of the power supply that regulated the voltage from 5 down to 3 and i somehow fried it and maybe that's what the faint smell of burnt electrical was?

I'm also not sure what i did wrong, whether it was a flaw in my plan? (ie. Wiring them in parallel like i did wasn't going to work in the first place for some reason?) I don't think i reversed the polarity on any of them... I'm not the best solderer so maybe I overheated them? I was hoping i could test them somehow, but i'm not sure how to go about it... I'm consistently reading 0.329k across the resistors of the cells i soldered, which is the same as for the cells i didn't solder, so that seems good. I tried taking two of the AAA batteries to make 3v and attaching the leads of my cells to the batteries, but no light there... Any advice, information, education or suggestions would be very much appreciated!

Thanks, Joe

Answer 1

I have this strip of LED's Amazon LEDs. I thought/hoped that since each led has its own resistor that this would mean i could cut off individual cells to rewire in parallel for a little project i was working on.

Absolutely correct, that's how those work. It looks like there's a cut every 25mm give or take.

The cut points are plainly marked, I know you don't see it, but you'll forehead slap when you do :) One thing I notice is that your loose individual piece doesn't appear to match up to a cut point. The cut point is exactly here. You should be lopping the solder pads in half. If you don't, you may hit circuit traces that matter.

I'm reading 5v at the power supply and the LED cells are stamped as 3v.

NEC 110.3(B) requires that you follow the labeling and instructions for equipment. They are plainly labeled 3V, so you seem to have violated NEC :)

what's going on here with respect to how the 5v at the power supply works with one or more of these 3v led's. I can't help but wonder if there was something in the circuitry of the power supply that regulated the voltage from 5 down to 3 and i somehow fried it and maybe that's what the faint smell of burnt electrical was?

Four AAA batteries are six volts unless they are NiCd/NiMH, in which case 5 volts.

I think that assessment is absolutely correct. The limiting factor is probably the PIR - they need 5-6V to make that work, so they are stepping down the LED power to 3V inside the power supply. The way to find out is hook it to the surviving LED string the way it came from the factory (the few LEDs that are missing are not an issue) and when it's running, stick a voltmeter on it and see.

FYI, 3V is two AA or AAA batteries.

I'm also not sure what i did wrong, whether it was a flaw in my plan? (ie. Wiring them in parallel like i did wasn't going to work in the first place for some reason?)

No, wiring them in parallel was correct. (assuming your understanding of "parallel" corresponds to the common one; but I would imagine so. If you hooked them in series by mistake they'd see 2.5 volts which would be non-destructive and light them dimly.)

I do that, by the way. I take 12V LED strips and connect them to 24V supplies by cutting the string in half and wiring them in series. I have a string that's burned 24x7 for 11 years and it's fine.

The strings have to be a dead-nuts match i.e. cut from the same string or ordered together with the same SKU. And exactly the same number of segments.