These Chinese fairy lights cost less than £5 somewhere on Amazon – you can get 3 for £8. In the ad there’s this lovely golden glow
but in practice the damn thing is dimmer than a Toc H lamp
These things are basically a throwie upscaled to a 50 LED string. Powered by two CR2032 lithium cells in series, the LEDs are in parallel, Current is limited by the internal resistance of the batteries. The whole thing is a disposable hazard to the environment, intended for a single use at someone’s wedding or party. It shouldn’t be allowed 😉
They make quite a nice distributed light in an outdoor shed, where I can fix the wire along the ceiling, just as well as the solid enamelled wire is going to break if moved too many times. I was surprised that you could put 50 LEDs in parallel. They are all fed from one end, and in the original configuration you couldn’t see any gradation along the string. However, putting 700mA through them generated a very welcome increase in light, and a slight gradation down the string, due to the voltage drop.I feared that would be bad for LED life, so I ran a third piece of enamelled wire through the string and fed one side of the LEDs from the far end and the other side from the near end, the drops along the string sort of cancel out. Using a light meter with the LED taped to it the original version as received gives a single LED output of 10EV, with 700mA it’s 15EV, a gratifying five stops more light – about 30 times more.
The 700mA is split among 50 LEDs, about 14mA per LED. I’ve never come across a LED (designed for illumination, rather than an indicator segment) that’s rated at < 20mA, so I figure I am OK. I was looking to upgrade this to 12V, powering off three 18650 LiIon batteries. The obvious solution was a Chinese LED current limiting switchmode supply, but the obvious solution comes with a nasty wrinkle for battery powering. Current rushes up as the voltage drops
a constant current driver is a very unkind battery load
Run off 12V it worked a treat. I used three LiIon cells scrounged from laptop packs and bits, and I found that this is a weapon of battery destruction – first I wrecked two cells out of three, then another two. Hmm. On the upside, at least I have now selected the strongest cells. On the downside, four LiIon 18650s have met their demise.
What’s up? The constant current LED supply is one of these
and I really should have been paying attention to that 5-35V spec, because as my Li-Ion’s fall from 11V down to 5V, it will say gimme, gimme, gimme more current, NOW.
And you don’t get to see that the batteries are running down from the LEDs dimming until it reaches less than 5V, because the driver is good for 5V. Oops. My bad. That’s why I am four 18650s down. Most things you run off batteries tend to draw less power as the voltage fades, but these suck the last dregs out of the battery in double-quick time, giving up just as it discharges the second weakest battery to below recovery.
I was imagining low-voltage disconnects and mucking around with P mosfets and PICs, then I spotted the PWM pin. You either leave this open, or ground the sucker to disable the output, the basic chip is the XL4001 from XLSemi. The EN wants to be < 0.8V to turn the thing off, and > 1.4V to turn it on. I had a vague recollection you could use a TL431 to get an active low power is high enough output, and Google delivered inspiration from ON1AAG on electro-tech online. TI also have a rather nice app note Using the TL431 for undervoltage and overvoltage protection which goes into some of the trials and tribulations of such misuse. One of which is quite a high Low condition voltage of about a volt or so – to wit
A lower bandgap reference voltage as seen in the TLV431, allows for a lower logic”low”output voltage without the need for external hardware.
Testing this with a LED showed it basically worked, but feeding the signal to the EN pin did ‘owt. As TI say, the resting Low voltage is over a volt. They’ve also got a Understanding Voltage References: Using a Shunt Reference as a Comparator blog series which points you at the TLV431 for this sort of thing. I needed to pad the output down with two diodes to get it just below 0.8V
There’s no hysteresis in this. I did first consider a 5.6V Zener instead of the two diodes, but that introduces a nasty pathology. The LVD turns the chip off at about 10.5V, but switches it back on again at about 5V, and the XL4001 goes way-hey, let’s suck the maximum current out of these dead and dying batteries. At least with the diodes it has to get down to < 3V and the XL4001 doesn’t draw half an amp like it does at 5V since 3V is out of its operating region which is 4.5V to 40V.
I’d be better off with the TLV431, but TL431 is what I have to hand. I’ll get some TLV431 next time I order some parts.
Recycling Neato 4/5AA batteries.
Looking for an alternative I hit on an old Neato XV robot hoover battery up for recycling. These get thrashed in this application, but the problem is there are two 7.2V battery packs with six 4/5AA size NiMH. To me these look pretty much like 18650 size. One of the cells has gone high resistance, but the remainder charge well on my MAHA battery charger/analyser
Although no chemistry appreciates full discharge, NiMh will tolerate the odd deep discharge. I’ve learned my lesson running constant current LED drivers off LiIon batteries, and while I have a LVD now I’m not taking the risk again.
I’ve also got a chance of trickle charging these via a solar panel. Battery University say you shouldn’t trickle charge NiMH at > .05C which is ~180mA or less. Mine is an old Maplin 1.5W @17.5V solar panel which would theoretically give 86mA in blazing summer sunshine. Which is not the time of year when you want lights in a shed, so I’m not going to be anywhere near endangering these batteries 😉 11 of them will give me a nominal pack voltage of 13.2V