An elementary electronics question.

Fair enough - my initial diagram wasn't clear enough, my bad.

To draw a line under this (for me, at the moment) all replies have been useful, I can now see at least 4 (and counting) ways of doing what I wanted to do. That was never entirely the point though - I wanted to use the project to learn some basic electronics. I actually invested in a 3A 18V bench PSU which is one of the best toys ( per £ I've ever bought.

I've spent some time drawing out simple circuits with non-linear components and checking them in practice with the PSU and a multimeter. I now realise why Horowitz and Hill called electronics an art!

Robin.

For basic electronics and circuits, there are a series of little handbooks available by Forrest Mims III from the USA. Radio Shack stores in USA and Canada sold these books from the late 1960's to the early 1990's. If you google Mr Mims' name you can find many of the books as PDF downloads. Worth a read, and lots of very basic circuits are included for learning. Just food for thought.

I don't think the original schematic is representative. Fairy lights implies a number of LEDs. It seems unlikely that each LED is drawing 270mA. Conversely if each LED is taking 10mA or so, then each 2R7 series resistor will only drop a few tens of millivolts.

Presumably we are talking a consumer product here, so it seems unlikely the manufacturer would waste time and money adding a resistor to each LED when one will do.

Andrew

At least Robin's circuit didn't have all the LEDs upside down like mine did!

If it makes you feel better I've been there, done that, too.

Andrew

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… which is, incidentally, why driving the whole string, including its resistor, from an appropriate voltage source should be no riskier than using the designer’s Qty. 2 AA cells

MichaelG.

I think the "2R7" value might have been down to an inaccurate measurement - based on measuring the whole string? Seemed reasonable to me that there were resistor/led pairs along the string for current-balancing based on a variable battery voltage, and I'm pretty sure that that is what I saw on some led tapes I installed for workshop lighting - surface-mount devices embedded in the tape next to the leds themselves, although that might have been one resistor for 2-3 leds as it runs off 12V. Given that the Chinese produce the buck converter modules already discussed for less than the price (including postage) we would pay for the components, the incremental cost of adding SM resistors can't be that much, even at a very price-sensitive end of the market!

A resistor in series is a pretty crude current source but better than a constant voltage source in this case?

Looks like we need to see the string of lights that Robin is using

A picture is worth a thousand assumptions !

MichaelG.

LED tapes are a slightly different animal. They are often designed to slice off the length you want every 3 or so LEDs so the devices are bunched together with their own current limiting resistor and the supply running down the whole length with the clucters in parallel. Fixed strings are designed to be run as a unit so can have all sorts of internal arrangements dependent on what is convenient and or cheapest.

regrds Martin

I apologise for the inadequate diagram in my OP - what I represented as a single LED is in fact a string of 12 connected in parallel with a single limiting series resistor:

I can't see the colours on the resistor clearly enough to identify it from them, but measuring gives 3.1 ohms. Shorting the multimeter leads gives 0.3 ohms (1 sf) so I'm guessing the resistor is 2.7 ohms.

Anyhow, as I said earlier, from my POV my questions have been answered and my thanks again to contributors. With my new-found understanding I can now see better ways of creating the effect I want using a combination of 12V lighting and mechanical components and high intensity LEDs so I'll probably not use the fairy light string anyway. It's served its purpose as an example!

Jeff - thanks for the pointer to Forrest Mims, I'll have a look.

Robin.

LED tapes have got even more sophisticated than that. The ones I am playing with have 3 coloured LEDs, or sometimes three colours plus a white one, with a little chip for each group of 3 or 4 LED's. The chip controls the colour of its LEDs, and each chip has a data in and out connection, daisy chained along the whole tape, which can be many LEDs long. You squirt a long serial data steam in, the first chip reads and deletes the first instruction and then passes the rest on. The result is a strip of maybe 300 or so LEDs, each individually addressable for colour and brightness. This all happens at 100's of kHz, so you can get smooth animations of brightness and colour down the strip. All very sophisticated, and surprisingly cheap, although of course total overkill for the initial posters query.

John

Thanks, Robin

Exactly what I expected to see [but we do like ‘avoidance of doubt’]

MichaelG.

It doubles the time needed on the pick and place machine, every fraction of a cent counts.

Andrew

Robin,

This is a slight digression … but I think it’s in the spirit of this discussion

Have a look at this little board: **LINK**

https://www.adafruit.com/product/757

I think you will agree that it does a useful thing, within a tidy form-factor, at a reasonable price.

MichaelG.

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Now consider that I have just received ten of ostensibly the same board, shipped from China, for a total cost to me of £7.17

Edited By Michael Gilligan on 09/11/2022 12:18:58

I've not read the whole thread, so forgive me if I've repeated something someone else said. Looking at this, it's an ideal application for a "buck regulator". The problem you have is the relatively high power dissipation dropping from 12 to 2ish volts at 270mA.

That's always going to be 2.7 watts in any linear situation. Its not that you can't get a resistor or transistor to handle that, but it's wasteful - especially if you're using a 12V battery.

You can get a buck regulator ready made circuit from ebay, but you can make one too. Broadly it is an oscillator which switches a pass transistor through a diode and an inductor in a PWM fashion. You could use a 555 timer for the oscillator. There is a feedback loop and an error amplifier which are used to control the mark space ratio of the transistor drive. The error amplifier is just a simple op amp, and the transistor drive is a comparator.

If the voltage is too high then the transistor tends more off than on. If the voltage is too low then the transistor tends more on than off. The inductor smooths the output jiggles into a DC voltage.

By only allowing through the required charge, and switching very quickly, the resistive losses are eliminated. If you build the buck regulator from discrete components, it's quite a complicated task. If you buy a buck regulator chip you can just follow the instructions on the datasheet and it will work fine.

An example device is MC33063 which can be used in buck, boost and inverting configurations. It's a £0.40 8 pin part, easily available everywhere despite the chip shortage.

Edited By Andy Ash on 10/11/2022 01:45:52

That’s not a bad description of a buck regulator but for driving a string of LEDs if the feedback is taken from a current sensing resistor usually less than 1 ohm then the cct becomes a constant current generator and there is no requirement for any series resistor to the LEDs.

regards Martin

Further to yesterday’s digression … Here’s a short note about the usage of BSS138

**LINK** https://www.elprocus.com/bss138-mosfet/

… and I will also mention that the boards I purchased are now even cheaper mainly because they are listed with free postage.

I am now racking my three remaining neurones to think of uses for them.

MichaelG.

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[ digression offered in the sprit of Robin’s desire to learn ]

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Mine was the fifth post on the thread, Andy

With that said; your concise description should be very welcome.

MichaelG.

Thanks Andy. After mucking about with trial circuits and much excerise of my neurones (I have only two remaining by their reckoning, but they may be wrong) it eventually dawned on me that my original idea bought nothing, for exactly the reasons you have given. Before reading your reply I had actually ordered some Buck Converters which I hope will fit the bill. They should arrive tomorrow.

MichaelG - apologies if you were the first to suggest a buck converter and I didn't immediately take that on board - I'd never heard of the things and it went over my head.

Robin.

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No problem at all, Robin … it’s been interesting to follow the discussion,

and I’m sure you now have a much fuller understanding of why it’s probably the optimum solution.

MichaelG.