Scavenged Parts.... But how to wire LEDs?

[SeanStevens]

I've inherited tons of used white LEDs. They come in threes on a tiny PCB with 330 resistors between them. A large string of these was wired to the mains.

Most are broken (water ingress) but I've got a useable amount to make my projects.

The power supply will be a tiny rechargeable battery. These are 3.7v and range between 1.3 and 2 Wh per cell.

Question: What amount and arrangement of LEDs should I use for these tiny lamps? I intend using them within bottles.

Thanks in advance.

Sean

[Terry_VK5TM]

If the resistor is an smd type and the numbers on it are 330, then that is a 33 ohm resistor - 3 white Led's and a 33 ohm resistor commonly run off 12V.

A 3.7V battery will not be sufficient to run white Led's (a white Led's operating voltage is typically 3 V).

[paulsherwin]

Are you planning to remove the LEDs from the PCBs?

As always with unknown LEDs, the best approach is to experiment. Just wire one across the battery in series with a 5k pot, and adjust for the correct brightness. Measure the pot resistance and substitute a fixed resistor.

[SeanStevens]

Thanks all. Can I not use several LEDs at once and one small resistor?

I do t understand why 3.7 v won't be sufficient. I'm going to remove the LEDs from the pcb.

Sean

[paulsherwin]

You need a separate resistor for each LED. That's because they will all strike at slightly different voltages, so if you use a common resistor the first one to strike will carry all the current and go pop. However, if you have lots of them you could experiment and risk losing a few.

You say they are water damaged, but the actual LEDs themselves should be sealed.

[SeanStevens]

Here is the current pcb set up.

It might. Help determine the LED type

[paulsherwin]

That doesn't really help. You need to desolder a few and have a play around.

The resistors are 330 ohm. At a guess, it's expecting a 19V DC supply.

[cmjones01]

You'll generally get away with running a white LED from a 3.7V (I'm assume lithium here) battery. The forward voltage of reasonably modern white LEDs is about 3.3V, so that only leaves about 0.4V for the current limiting resistor to drop. If the LED should be run at 20mA, which is typical for a small LED, then the resistor should be (0.4/0.02) 20 ohms. I'd keep it a bit higher than that (say 33 ohms) to allow for some headroom when the battery's fully charged, and ensure the LED lasts a long time.

You'll want one resistor per LED. Trying to run multiple LEDs in parallel from the same resistor can be a bit unpredictable, especially if the LEDs are from mixed batches or the resistor is a low value. There's no guarantee that the current will be shared equally among the LEDs. Using one resistor per LED prevents this.

An alternative solution is to wire the LEDs in a long series string and power them from a little boost converter circuit. They're commonly wired this way for LCD backlights, with a total string voltage of about 30V (say 8-9 LEDs in series). I've used the Zetex ZXLD1937 (plus an inductor, current sense resistor and a couple of capacitors) to do the job in the past, but off-the-shelf modules may be available.

Chris

[SeanStevens]

Many thanks for all your suggestions.

Sean

[David G4EBT]

The last thing I'd want you to do is to discourage you from experimenting. There's much fun to be had from re-purposing items which might otherwise be discarded. That said, in case you don't already know, light strings of tiny ultrabright low-consumption LEDs are widely available quite cheaply. They're used extensively by woodturners (and other crafts), for example, to make Christmas baubles using glass globes illuminated with light strings, to which they attach wood-turned finials at the top and bottom to hang on Christmas trees.

A professional woodturner - Emma Cook - (known as 'The Tiny Turner' as she's just 4ft 10"), supplies a range of light strings quite cheaply. For example, a string of lights 2m long with 20 LEDs (colour of your choice), costs £2.50 including a switched battery box and two CR2032 batteries.

https://www.thetinyturner.co.uk/prod...f-fairy-lights

Or a quantity of three £6.50:

https://www.thetinyturner.co.uk/prod...f-fairy-lights

She also sells USB Sets of Lights with Remote for £7.50:

A string of 50 leds lights on 5m of wire (copper or silver) plugs straight into a USB socket either in the mains, on your computer or a powerbank. It comes with a remote control which allows you to turn the lights on and off, dim them, or choose from a variety of 8 different lighting modes: combination; wave; sequential; slow glow; chasing/flash; slow glow; twinkle/flash or steady on. They can also be plugged into a USB extender if you can't quite reach where you need to plug them in.

The listing includes:

o 1 x 5m 50 LED string of lights in your choice of colour
o 1 x remote (battery included)

https://www.thetinyturner.co.uk/prod...ts-with-remote

(I don't have any connection with Emma, other than having seen her demonstrate at the local woodturning club of which I'm a member).

Every good wish and success in your LED experimentations Sean.

[Oldcodger]

I may be off on a tangent, but I've found that for the needed voltage to light a LED , using a DVM on diode test will light LED and show the forward drop.Using a second meter in series to measure the current will let you see what brightness is with a set current
Then again ( granny and eggs comes to mind), the series resistor comes from
(V SUPPLY- V FD )/ CURRENT, with the current being decided by LED and battery capacity.

[paulsherwin]

There are lots of online calculators to calculate resistor values, e.g. https://ledcalculator.net/

[Graham G3ZVT]

Here's a vintage fuse box I made into an LED novelty item.

https://www.vintage-radio.net/forum/...d.php?t=162907

[stuarth]

I don’t really like running LEDs from a battery with just a resistor as current limiter.

For maximum efficiency, you need to minimise the voltage across the resistor, but then the LED current varies a lot depending on the charge state on the battery.

I bought some LED replacement 12V “bulbs” for cabin lights on a boat. The first ones I bought were very cheap and consisted of a small PCB with a bridge rectifier feeding several strings of 3 LEDs and a limiting resistor. All seemed well while moored up, but after an hour or so with the engine running, the lights began to flicker, and they were getting hot enough that one or two of the LEDs unsoldered themselves.

Testing them on a bench power supply, the current was zero until you got to a bit over 10V (3 LEDs plus the bridge) and increased dramatically between 12 and 14V (engine running or not).

The replacements I bought were rather more expensive, but had a basic “Joule thief” circuit and were rated for 12~24V operation (the cheap ones were rated as 12V only). The replacements started to light up at 9V, reached full brightness at 10.5V, and stayed at that brightness as the supply was increased to 24V, at which point the supply current had halved.

Joule thief circuits are very simple, there are lots on the web, and can be very small. I built one into the base of an MES lens-end bulb to convert a small torch using a single AA cell to LED. With a Joule thief, you should be able to run your strings of 3 LEDs in series from your Lithium cells.

Stuart

[kalee20]

Originally Posted by paulsherwin

You need a separate resistor for each LED. That's because they will all strike at slightly different voltages, so if you use a common resistor the first one to strike will carry all the current and go pop.

At low currents, I have successfully used white LED's directly in parallel! They were all from the same batch, but at 15mA between six, lighting a radio tuning scale, they work very well.

LED's don't 'strike' in the same way as a neon lamp does, where there is a running voltage that's less than a starting voltage, but there is a current-hogging mechanism where one LED can start to divert current from the others. It depends however on significant self-heating, however, so if the current is low enough, it doesn't happen.

[Graham G3ZVT]

Originally Posted by kalee20

At low currents, I have successfully used white LED's directly in parallel! They were all from the same batch, but at 15mA between six, lighting a radio tuning scale, they work very well.

Indeed. My novelty fuses I mentioned in Post 13 were cut from a Pound store string of 20+ parallel twisted copper(?) wire LEDs, fed from 2xAA cells and a single resistor.