Question about resistors and LED's

[SHMD]

The trouble with "online LED calculators" is that, people who do not know, think something magical or 'hard' is going on behind the scenes!

Nothing could be further from the truth.

 

This is what I do to get the LEDs limiting resistors value:-

 

 

 

1/ What is the voltage to be used to power the LED?

 

2/ Subtract 2 volts from this value. (3 if using a blue LED.)

 

3/ How bright do you want your LED?

 

1.0mA = Dim(ish).
2.5mA = Not Bright.
5.0mA = Bright.
10mA = Very Bright!
15mA = Too Bright!!

 

 

4/ Calulate the required resister thus:-

 

R = V/A,
R = (Vsupply - 2)/required brightness current.

Lets say 12V @ 2.5mA, Green LED

R = (12-2)/2.5mA
R = 10/0.0025
R = 4000
R = 4k Ohms

 

 

5/ Get the nearest commercially available resister. (In the %5 range)

Choose from the following:-

 

1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0,
3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1

3.9 is the nearest to 4.
The Multiply it by the required number of TENs
3.9 * 1000 = 3900

Therefore you need a 3k9 Ohm resistor.

 

 

6/ Identify the LED so that it is put in the circuit the right way round.

Make sure the Anode is more positive than the Cathode.

 

 

 

Steps 1 to 4 don't matter that much and +/-1 on any number here doesn't really matter.

Step 4 can have a tolerance of +/-%20 no problem!

Step 5 has to be right or you wont be able to buy the resistor or, worse, it will cost too much!!

Step 6 is VERY IMPORTANT!

 

 

LEDs come down to personal preference - especially on model railways.

Some like them dim (or even none existent!, some too bright.

 

Railways before the mid 1980s (in the UK) had dull hard to see lights. Soot and grime toned the lights down somewhat too.

Gas and paraffin lamps were even dimmer! Only at night, and especially in the rain, did these lights come to the fore.

Since then, high intensity lights have become common (compulsory) and are much more noticeable - even in bright sunshine.

 

 

Kev.

 

[Crosland]

To add to the last post, an easy way to decode a diode or LED symbol on a schematic:

 

--|<|---

 

Redraw it with a K for "Kathode"  --K|--- and see that the terminal on the left is the Cathode.

 

Andrew

[Ray H]

To add to the last post, an easy way to decode a diode or LED symbol on a schematic:

 

--|<|---

 

Redraw it with a K for "Kathode"  --K|--- and see that the terminal on the left is the Cathode.

 

Andrew

 

And that as they say is magic. I've been struggling to get to grips with general electronics for over 30 years and in all that time have never found a way to tell my anode from my cathode. Brilliant.

 

As for the suggested paper (or whatever) on basic electronics I'd more than welcome the suggested idea for it may finally allow me to get to grips in those (many) areas that I still struggle with because of all the talk of electrons and holes that confuse the h*ll out of me.

[DCB]

Maplins dont do anyone any favours! These Leds lead to confusion, a LED is NOT a voltage controlled device, as long as you meet the minimum required VF it couldnt care less how many Volts you shove through it. You can take a bog standard everyday Led and shove anywhere from 2.2V -400V through it without problems. They are a current controlled device, what a LED cares about is the amount of current it is fed, if you dont limit the current they will suck as much as they can untill they burn themselves out, put 2.5V through a standard Led with no resistor and it will burn out.

What they call 12V Leds are infact normal Leds with a inbuilt resistor as stated by someone else, in this case they are limiting the current to 8.5mA, to me thats about half brightness.

They are seriously a waste of cash, buy normal ones and add your own resistor. I am in the middle of writing up a full LED post, in it I will explain all about LEDS and how to controll them. I promise by the end of the article you will be able to master any LED easilly, I will also include some idea's of how to get the best from them and some circuits showing some methods of driving them. But PLEASE guy's dont spend your cash on these silly extra's, and no one should be paying that kind of money for a LED!!! Thats why we have China and EBAY

It will take me a day or so to write it all up, but by the end of it you will all be experts in LEDS

 

Please don't write a guide to LEDs Gm11 Not until you understand their behaviour in the 2 to 3.5 volt range.  Try connecting a standard LED 3 mm across a 2 AA battery holder with 2 X AA Nicads or NiMH batteries, wear welfding goggles and rubber boots and gloves if you must but the last 50 or more times I tried it a standard 3 mm  LED with 2 volts and no extra resistance it worked very happily and emits a pleasant glow ideal for lighting in a building,

I am afraid you are too used to using LEDs on 12 /16 or 240 volts to appreciate that LED performance varies greatly over a narrow voltage range between the voltage they begin to glow and the max voltage, 0.1 volt in the 2.2 volt range will double the current yet barely affect the brightness..

[SHMD]

Folks - please ignore the above post.

 

This is NOT recommended and is definitely NOT good practice.

[Baby Deltic]

Hmmm I am not getting into that one, except I did say the Led would drop 2.2V and as stated above I deviate depending on who is reading. Having taught post graduate University students for a number of years, you reach a point where you realise to always be technicaly correct dosnt help. As this isnt an electronics forum I try to imagine how best to get the point over. In this case the point being you have to take the FV from the supply before the calculation. I also said a Led was not voltage controlled but current controlled, so while I did indeed say volts pass through the correct information was all there.

Yes I know its complicated but often its way better to fit the information to the person rather than try and fi the person to the information. The other point is this guide willo be for beginner's, professsionals will not need any information it contain's, therefore I see no point in trying to 'teach' electronics, I see more to be gained by giving a guide that the average modler can understand and use to there advantage. If anyone wants advanced information and guides includeing microcontroller circuits and programs let me know. I came here because I needed some information on models but decided to stay a while and help if I can.

Dont misunderstand me I dont mind being corrected as it often gives me the opportunity to set the record straight . For now I will probally stick with the format outlined, this one will be dealing with single Leds and maybe dual and tri colour. Later I will do a more advanced one dealing with all the nonsense on series and parralell Leds.

 

Any way Thank you for giving me the chance to clarify why I used the terms I did

Regards

GM

Go for it on microcontrollers. I know a lot of people go for Arduino's now, but straight forward PIC or AVR assembler techniques would be interesting. There is a 'computer control' group setup in the 'special interests' section.

[Crosland]

Please don't write a guide to LEDs Gm11 Not until you understand their behaviour in the 2 to 3.5 volt range.  Try connecting a standard LED 3 mm across a 2 AA battery holder with 2 X AA Nicads or NiMH batteries, wear welfding goggles and rubber boots and gloves if you must but the last 50 or more times I tried it a standard 3 mm  LED with 2 volts and no extra resistance it worked very happily and emits a pleasant glow ideal for lighting in a building,

 

The point you are willfully failing to grasp, despite being told more than once, is that you are relying on the internal resistance of the batteries to limit the current in this situation.

 

Andrew

[raymw]

http://data.energizer.com/PDFs/BatteryIR.pdf

http://www.learningaboutelectronics.com/Articles/Battery-internal-resistance

http://donklipstein.com/ledd.html

http://pdf1.alldatasheet.com/datasheet-pdf/view/418357/OPTOSUPPLY/OSWWY25111E.html

 

I would guess that David is using white LEDs, and the output voltage from the nicads is less than the LED forward voltage. It works for him, and I expect it would work for you too. Simple enough to try. It will not work for red LEDs. There are plenty of 'key ring' type LED torches, with no dropping resistors - they rely on the internal resistance of the button cells, for example http://allelcdn.upshotcommerce.com//mas_assets/theme/allelectronics/spec/GL-6.pdf

for the earlier red keyring lights, it was one cell.

 

 

There is theory, and then there is what works in practice.

 

Best wishes,

 

Ray

[Crosland]

I would guess that David is using white LEDs, and the output voltage from the nicads is less than the LED forward voltage.

 

Read a few of his previous posts and you will understand.

 

Andrew

[DCB]

I don't see what there is to disagree with, Without sufficient volage across the LED it cannot draw excess current. Everything I have ever seen suggests subtracting around 2 volts from the supply voltage to calculate the resistor required. Most people never think of operating LEDs from low voltage power supplies so the idea that "You need resistors," becomes almost a religious creed.  I found this published  diagram which approximates to a 3mm Red LED from Maplin I bought maybe ten years ago. It is a bit crude and the line actually corves continually rather than forms a straight line but the relevant parts are as near as damnit No current flow at 1.5 volts and 45ma at 2.3 volts, I measured 43.5 at 2.35 volts from two freshly charged though elderly NiMH AA size Nicads. through the 3mm Red LED.  These happily deliver 1 Amp plus when shorted so internal resistance is not restricting current flow.  A 3mm Yellow LED from the same era drew 2.2ma or 1/20th the current.  I use the 3mm Red LEDs as tail lights fed by pairs of button cells.  A 5mm Green LED I had to hand drew 28.3ma.  There is a lot of info about LEDs out there, Much suggests a supply voltage of 4 or 5 volts and resistors in the 100 ohm range, only railway modellers seem hung up on 16 volt supplies which they call 12 Volt.

 

My original posts were about running LEDs from batteries or low voltage power packs in the 2.9 volt range, these are essentially for building lights and apart from trimming resistors to get a red/green signal light in the same circuit to emit similar amounts of red and green light I do not use resistors in clusters of 15 or 20 LEDs wired in parallel intended to represent 1950s 60s yellow tinged lighting, rather than blueish neon lighting. .

[SHMD]

David, I am not sure that you advocating resistor-less LED circuits fits well here bearing in mind the Original Posts question.
(Yes I know you are only responding to posts #32 to #34.)

 

Most people on RMweb who request help on LEDs, as related to their layout, and will NEED a series limiting resister. They are not familiar, competent nor have time and parts for experimentation.
Your “solution” will require some experimenting IN EVERY CASE to ensure that it works, is reliable, or is even just SAFE!

 

Following the six easy steps, in post #26, will answer most modellers needs in %99 of all cases.
And for the last %1 of cases you can always ask here.
And yes, there are exceptions and useful more complicated ways of utilising different power supplies.

 

Also, I hope you are not overlooking the fact that LED life is directly related to current and temperature - and that you are “not controlling” the current at all!

 

Anybody fancy replacing 15 to 20 LEDs when they all fail at once!
(Opening up all those carefully constructed buildings shouldn't take too long!)

 

 

Kev.

 

[Crosland]

 

I don't see what there is to disagree with, Without sufficient volage across the LED it cannot draw excess current. Everything I have ever seen suggests subtracting around 2 volts from the supply voltage to calculate the resistor required. Most people never think of operating LEDs from low voltage power supplies so the idea that "You need resistors," becomes almost a religious creed.  I found this published  diagram which approximates to a 3mm Red LED from Maplin I bought maybe ten years ago. It is a bit crude and the line actually corves continually rather than forms a straight line but the relevant parts are as near as damnit No current flow at 1.5 volts and 45ma at 2.3 volts, I measured 43.5 at 2.35 volts from two freshly charged though elderly NiMH AA size Nicads. through the 3mm Red LED.

 

Instead of being so selective, go back to the same data sheet where you took the graph from and tell us the absolute maximum rating for the forward current.

 

A typical 3mm red LED data sheet can be found here http://www.rapidonline.com/pdf/55-0102.pdf

 

Looking at the second table "Electrical/Optical Characteristics" we se that the Vf is typically 2.25V, max 2.5V at a current If of 20mA. This variation is due to manufacturing variations. It is not saying that the maximum voltage that can safely be applied is 2.5V.

 

Looking at the next table "Absolute Maximum Ratings" we see that at 25 deg C the maximum allowabe If is just 25mA. If the LED is pulsed then this can be increased to the peak value of 130mA so long as the pulse is no more than 1/10000 of a second and repeated no faster than 1000 pulkses a second [footnote 1]. Note that no absolute maximum Vf is specified.

 

Now we look at the graph of Forward voltage against Forward current. LEDs, like other diodes, obey the "diode law" and will pass a current determined by the voltage applied, hence V being on the X-axis and I on the Y-axis. The current, however, is very sensitive to small changes in voltage, making voltage control of the current very difficult. This is the reason we talk of, and apply, LEDs as being current controlled. We can see 20mA equates to the typical Vf of 2.25V. The Vf only needs to increase by 0.1V for the typical LED to reach the absolute maximum If of 25mA If. The graph is only the starting point for a typical LED.

 

I leave it as excercise for the reader to figure out what happens with a power pack in the "2.9 volt range" with no resistors.

 

From this we can see that there is enough variability that anyone who says "it works for me without resistors" can safely be ignored.

 

Using a resistor is not a religious creed.

 

Andrew

[Gm11]

Please don't write a guide to LEDs Gm11 Not until you understand their behaviour in the 2 to 3.5 volt range.  Try connecting a standard LED 3 mm across a 2 AA battery holder with 2 X AA Nicads or NiMH batteries, wear welfding goggles and rubber boots and gloves if you must but the last 50 or more times I tried it a standard 3 mm  LED with 2 volts and no extra resistance it worked very happily and emits a pleasant glow ideal for lighting in a building,

I am afraid you are too used to using LEDs on 12 /16 or 240 volts to appreciate that LED performance varies greatly over a narrow voltage range between the voltage they begin to glow and the max voltage, 0.1 volt in the 2.2 volt range will double the current yet barely affect the brightness..

 

 

 

Far from it, if you bothered to read what I have written, I am a Embeded design Engineer, the vast majority of what I do is in the 1.8V-5V realm! What you advocate is exactly why I wanted to write a guide, too many people think doing things this way is a good idea, it isnt.

But ok as you wish I will not bother to do a guide. Trusting the power source to limit the current for you is a classic sign of poor engineering pratice, I have no idea of your background but qualified in electronics I would think not. And yes the coin cell thing purely works because of the very high internal resistance. Your method is one of those classic arguments about smoking 'its cant be bad for you because my dad smoke 100 fags a day and lived to be 99', this kind of argument always neglects the fact had he not smoked, he may well have lived to be 130 .

I dont have time to argue silly points, I was trying to present correct information to beginners, this would not be required if people stopped posting advice about using Leds without current limit resistors! There is only one valid reason not to use a current limiting resistor, this is on certain micro controllers that you can controll the output current of the micro pin via software. For all other instances my advice to EVERY newcomer is use a resistor and do the job properly.

[Ian J.]

What I would like to be in any manual is how to deal with multiple LEDs and single/multiple resistors in a straightforward way.

[Tricky Dicky]

Gm11

 

Please publish your guide as I am sure the vast majority on here would appreciate it. I cannot believe this argument has rumbled on for so long over the use of a component that can be bought for a fraction of a penny and ensures correct LED operation.

 

Richard

[pauliebanger]

 

But ok as you wish I will not bother to do a guide.

 

Please don't give up on your suggestion.

 

From what you have said it promises to be a very useful resource, and I for one am looking forward to it.

 

I  find that explanations of the same knowledge given by different people in different ways helps me to gain a good grasp to the standard I require.

 

I agree with you that this should not be trying to 'teach electronics', and also with Andrew that for many, some 'Rules of Thumb' are perfectly adequate most of the time. I would like to see something in between, which will help me to understand the how and why.

 

Kind regards,

 

Paul

[Crosland]

What I would like to be in any manual is how to deal with multiple LEDs and single/multiple resistors in a straightforward way.

 

Series connected LEDs

This means conected like an elephant parade "nose to tail", anode to cathode in a long string. Add up the forward voltage (Vf) for all your LEDs, e.g. a red LED might be 2.2V at 20mA (blue and white have considerably higher Vf). If you have 5 of these then you get 11V. The power supply needs to be higher than this, so a 12V supply is appropriate in this case. Use ohms law (V = I * R or R = V/I) with the current (I = 20mA or 0.02A) and remaining voltage (V = 12 - 11 = 1V) and you need a resistor R = 1/.02 = 50 ohms. Choose the next highest "preferred value" - 56 Ohm.

 

With series connected LEDs you can drive multiple LEDs using a single resistor without using any extra power, at a given supply voltage.The power that would be wasted in a larger resistor with a single LED is used instead to light more LEDs. If one conection or component fails open circuit then all the LEDs will go out.

 

Parallel Connected LEDs

This means connected side-by-side.

Common anode means all of the anodes are connected together and thence to the positive supply. You light an individual LED by connecting its cathode to 0V through a suitable resistor. You need to "sink" current from the LED.

Common cathode means all of the cathodes are connected together and thence to 0V. You light an individual LED by connecting its anode to the positive supply through a suitable resistor. You need to "source" current into the LED.

In some applications the common anode or common cathode connections may be connected to the supply through further circuitry.

 

Each LED should have its own resistor. Call this a rule of thumb, or whatever you like, but please accept that this is the correct way of doing it. It's essential for maintaining constant brightness, if you are switching individual LEDs on and off.

 

Parallel LEDs can be driven from a lower voltage than the same number of series connected LEDs but the current required is multiplied by the number of LEDs. In the example of 5 red LEDs at 20mA each, we would need 100mA, but a 3V supply would suffice. Each LED would require R = (3-2.2)/0.2 = 40 ohms to run at 20mA with 3V. The next larger preferred value is 47 ohms. Each LED can be controlled individually.

 

LEDs on AC

To drive a LED directly from AC, derived from the mains via a transformer, you must use the peak voltage not the RMS voltage when calculating the resistor value. An AC power supply or transforner will be specified as RMS. Multiply this value by 1.4 to get the peak voltage.

 

You should connect another LED or diode in "reverse-parallel", i.e. anode to cathode and cathode to anode to protect the LED(s) from reverse voltage. They can both be connected through the same resistor. Each LED will be less bright than when powered from the same DC voltage so you may want to adjust the resistor value.

 

LEDs on DCC

A typical DCC loco decoder requires common anode conection as it has a comon positive connection (the blue wire). Each output, when on, connects the cathode to 0V via the resistor (that you supply) and a transistor (part of the decoder). The function outputs are DC

 

DCC is square wave AC, the peak and RMS voltages are the same, and there is no need for the 1.4 factor on the voltage if connecting an LED direct to the track.

 

A Note on Resistor Preferred Values

I am using the comonly available E12 series, see http://en.wikipedia.org/wiki/Preferred_number

 

A Note on LED Current

Modern LEDs are very efficient and you should probably start by assuming a 10mA, rather than 20mA current and work from there, gradually increasing or decreasing the resistor value until you are happy with the brightness for your application, staying within the LEDs maximum rating.

 

Andrew

[Gm11]

Series connected LEDs

This means conected like an elephant parade "nose to tail", anode to cathode in a long string. Add up the forward voltage (Vf) for all your LEDs, e.g. a red LED might be 2.2V at 20mA (blue and white have considerably higher Vf). If you have 5 of these then you get 11V. The power supply needs to be higher than this, so a 12V supply is appropriate in this case. Use ohms law (V = I * R or R = V/I) with the current (I = 20mA or 0.02A) and remaining voltage (V = 12 - 11 = 1V) and you need a resistor R = 1/.02 = 50 ohms. Choose the next highest "preferred value" - 56 Ohm.

 

With series connected LEDs you can drive multiple LEDs using a single resistor without using any extra power, at a given supply voltage.The power that would be wasted in a larger resistor with a single LED is used instead to light more LEDs. If one conection or component fails open circuit then all the LEDs will go out.

 

 

 

There is a slight Gotcha here, not Leds are born equal, in the datasheets the Vf given is a average Vf, for example a green 5mm Led may say in the DS 2.2V but in reality some will be 2.2,2.3,2.5 etc, There is often variation even in the same batch. Why does this matter? If your sensiable it dosnt, if you dont mix colours in a single string it dosnt. The issue you need to watch for in series driving Ledss is some will consume more current than others in the string, so as a basic rule of thumb stay well away from the max current, also allow some head room with the Voltage, if you try and drive 3 x 2.2Vf leds from 7V it may or may not work, in therory it should take 6.6V in reality it may be more or less. It isnt often a problem but be aware of it.

Also be aware that mixing different current Leds in a serise string is bad practice, They normaly have different Vf therefore will have diferent brightness levels or worse sstill one will burn out.

Often Electronic engineers will frown upon connecting in series, but in practice its ok as long as you leave a safety gap. The 20mA I mentioned at the start of my posting was a max guide, I agree for most of what you will use a Led for 10mA is plenty, you gain little extra light by adding another 10mA to it.

Most of these rules apply to normal Led types, High power Leds are a different beast altogether! And at the moment the bain of my life

[Crosland]

There is a slight Gotcha here, not Leds are born equal, in the datasheets the Vf given is a average Vf, for example a green 5mm Led may say in the DS 2.2V but in reality some will be 2.2,2.3,2.5 etc, There is often variation even in the same batch. Why does this matter? If your sensiable it dosnt, if you dont mix colours in a single string it dosnt. The issue you need to watch for in series driving Ledss is some will consume more current than others in the string,

 

For someone with your claimed experience you seem to be getting in a bit of a muddle :-)

 

Series connection:

Mixing colours is largely irrelevant so long as you choose LEDs that work over a broadly similar curren range, use the correct Vf for each LED in the string, calculate the total and use a power supply with a higher voltage than the calculated total Vf.

 

When connected in series some LEDs DO NOT consume more current than others. They are connected in series. EXACTLY the same current flows through all of the LEDs (and the resistor).

 

Also be aware that mixing different current Leds in a serise string is bad practice, They normaly have different Vf therefore will have diferent brightness levels or worse sstill one will burn out.

 

Series conection of LEDs is not "bad practice". It might not result in most efficient use of the LEDs and will as I said cause all the LEDs to go out if one fails open circuit. The chance of significant variation in colour or brightness in LEDs from the same batch is small.

 

So long as you set the current below the maximum value for any of the LEDs then none of them wil burn out.

 

Andrew

[Gm11]

Sorry but there is no confusion on my part, Try a simple experiment yourself. Get one each of the following Green,red,blue,white Leds. One by one connect to 12V with a 680 ohm resistor and measure the current drawn by each. You will notice that they will not all be equal, this is just a simple application of Ohms law. Should you wish to check my creditials pm me and I will provide the name and address of the institution I was professor of electrical engineering for and give you my chartered accreditation details and information on how to check it.

You may also wish to look up some of the 139 patents I hold in my name for various systems etc I have designed.

My mission is not o get into petty arguments however if you wish to question my authenticity or qualifacations then I view that more seriously, I am now semi retired and work solely as a consultant, however my reputation is of great importance to me.

Kind regards

G.M

[SHMD]

I think you two are arguing at slightly cross purposes.

[raymw]

Hi Gm11,

 

Please explain -

The issue you need to watch for in series driving Ledss is some will consume more current than others in the string,

 

Are you trying to say that some will not be as bright as others when connected in series, due to their differing current requirements for equal illumination? That I understand, however I always understood that for series connected components, the same current flows through each component, in which case the above quote is incorrect.

 

For what it's worth, I have no interest in ' Should you wish to check my creditials' whatever that may mean. I guess you can always tell a 'consultant', but you can't tell him much, so maybe there is little point in telling you to  read through your postings, before you hit the send button.

 

Best wishes,

 

Ray

[Ray H]

I hesitate to jump in, possibly with both of my big feet, but I do wonder if this apparent tennis match is getting anyone anywhere.

 

I profess immediately to having minimal understanding of electronics but even I can see that what is being said by each party could be viewed in many cases as either accurate or inaccurate, depending on the reader's interpretation of what has been written.

 

When I tell a computer, by means of a program, to do something specific in isolation I can be reasonably sure that it will do what it has been told for the computer does not have a mind of its own. However, if I tell someone to do something - like pick an apple off a tree, I know not whether they will pick the apple that I was thinking of off the tree that I was thinking of. The two people obviously put their own interpretation on what was said but may come up with different answers, neither of which is necessarily wrong - although I could have "accused" the other of picking either the wrong apple or going to the wrong tree.

 

Can I suggest that in the interest of what has the makings of a potentially useful thread (with possibly equally useful spin-offs) that those seeking to challenge what has already been posted pause briefly and consider whether what they think they have read is actually what is written and whether there can only be one interpretation on what is written?

 

For example, I see no contention in "a warning about possible different current consumption when connecting LEDs in series" and a statement that "the same current flows through each (series connected) component". Surely both are correct in the appropriate interpretation.

 

I bear nobody any malice and trust that what I have written will be viewed in the manner that it is intended.

 

Thank you.

[raymw]

Hi Ray

 

However, in things electrical and many other disciplines, there should be no room for different interpretations. (or at least they should be minimised).  In the case of LED's costing a few pence, it may be no big deal, but if the naïve extrapolate their results to higher voltages/currents, then more serious events may follow.

 

I am not surprised at the lack of understanding of very basic principles, and the sloppy way in which some folk seem to post on here. A newcomer may blindly follow what is said here, but they need to remember that free advice is what they are paying for, but that does not imply that advice they may pay for is  necessarily any better. 

 

wrt your apple analogy, I'd maybe ask which tree, which apple, but most likely it would be 'pick your own s*dd*ng apple, if you're being so fussy.'

 

Taking it a little further, assuming you drink coffee, for folk that know you, who've made coffee for you before, you could  just say 'could you make me a cup of coffee, please?'  but for the first few times you would have to specify the details wrt milk, sugar, sweetener, etc.

 

Giving instructions for an audience of unknown capabilities is not easy, but there is no need for them to be poorly written.

 

 

Best wishes,

 

Ray (not H)

[Crosland]

Sorry but there is no confusion on my part, Try a simple experiment yourself. Get one each of the following Green,red,blue,white Leds. One by one connect to 12V with a 680 ohm resistor and measure the current drawn by each. You will notice that they will not all be equal, this is just a simple application of Ohms law.

It's due to the different Vf of the different coloured LEDs, which is down to the diode law, not ohms law.

 

What is really at issue is your assertion that some LEDs in a series string (i.e. multiple LEDs connected in series, not connected one at a time) will take more current than others. They are connected in series. The same current passes through each LED.

 

I am always sceptical of anyone who insists on hiding behind anonymity in newsgroups, forums, etc.

 

Andrew