Tony Davis Posted March 24, 2014 Share Posted March 24, 2014 After watching a youtube video by Everard Junction about fitting an LED to an Hornby Buffer stop I called into Maplins and bought these http://www.maplin.co.uk/p/3mm-red-led-10-piece-led-pack-n21gl and these http://www.maplin.co.uk/p/resistors-lucky-bag-n24bg. Mr Junction advises using a 10k ohm resistor with the LED's. I checked with an online Resistor calculator and couldn't find one in the bag (I found it really difficult to see the colours), so I used a multimeter to check each one and the closest I could find was 6.18 measured on the 20k setting. My question is, is that a suitable value for connection the LED's to a 12v DC supply? If not, could you, dear reader, suggest one that would be, please? I really want to connect 3 LED's, so do I need a resistor on each LED or could I just have one resistor in the circuit? Kind regards Tony Link to post Share on other sites More sharing options...
Suzie Posted March 24, 2014 Share Posted March 24, 2014 Try the 6K one first with the three in series and see how you get on. The value of the resistor just sets the brightness, you will be fine as long as it is more than 470R Link to post Share on other sites More sharing options...
Tony Davis Posted March 24, 2014 Author Share Posted March 24, 2014 Thanks Suzie, I'll give that a go Kind regards Tony Link to post Share on other sites More sharing options...
Crosland Posted March 25, 2014 Share Posted March 25, 2014 Three red LEDs in series with a single resistor will be fine on 12V. Start with 1.2K (approx 5mA LED current) and reduce until you get the brightmess you want. If connecting in parallel (e.g. you want to turn them on and off individually) then each LED should have it's own resistor. Start with 2k for a 12V supply (approx 5mA again) and adjust downwards for brightness. Andrew PS, I like Maplin's description of "assortment of 10 3mm red LEDs of randomly assorted values". Is this monetary value, or...? Link to post Share on other sites More sharing options...
Tony Davis Posted March 25, 2014 Author Share Posted March 25, 2014 Tried just the one LED with the 6k ohm and worked fine so will proceed connecting the other two now. Thanks again for the info. Whenever I see "random" selection on anything it reminds of going to Dreamland as a kid, I was very taken by robots of any sort at the time due to a childhood of Space Patrol, Fireball XL5, The Forbidden Planet etc. There was a booth there, a sign on it proclaiming something like "insert coin and receive a prize randomly selected by our wonderful machine". I pictured a complex machine inside, probably a cross between Robby the Robot and some Heath Robinson affair. I walked passed one day and the door to the booth was open, there was a chair in there, with a packet of fags and a stained tea cup and a newspaper, all recently abandoned by their owner. Not a robot I suspect. Shattered dreams etc.,,, But no, I don't think it's monetary value, more load of stuff we can't sell so lets bung it in these bags.. Link to post Share on other sites More sharing options...
Gm11 Posted April 2, 2014 Share Posted April 2, 2014 The maplin link said page not found, so I dont actualy know what Led your using, but the following is a rough guide. Depending of the colour of the LED it will have a Vf (forward voltage) value, the forward voltage of a LED is the amound of voltage that the LED will 'drop', so for example a standard red Led could be expected to have a Vf of around 2.2V, this means if you put 12V through the LED (forget resistors for a min) then at the other side of the LED you will measure 12-2.2V or 9.8V. Most small 3mm-5mm normal Leds have a max I (mA) of around 25 -30 mA, its best to aim for a max of around 20 mA this gives you a good margin for error. So we have 2.2V-12V or to put another way deduct the FV from your supply voltage, if you dont know the VF play safe and go for 2.2V. 2.2-12=9.8V to work out the resistor decide how bright you want the Led use 20 mA as a max for most Leds, so in this case you devide the 9.8V by the current in AMPS, this is important therefore 9.8/0.020 = 490 This is the resistance, so again in this case 490 Ohms you cant always get a resistor exactly to match the figure you get, dont worry just pick the next HIGHEST standard value. If you want a general rule of thumb, for 12V systems anywhere from 620 Ohm to 1.2K Ohms will be fine. One more point its best to use a resistor per Led Link to post Share on other sites More sharing options...
Tony Davis Posted April 3, 2014 Author Share Posted April 3, 2014 Thanks GM - I found your post really helpful. I really need to read an electronics guide as I find this subject interesting. Kind regards Tony Link to post Share on other sites More sharing options...
Gm11 Posted April 3, 2014 Share Posted April 3, 2014 Be very careful what you read on the internet! there is alot of incorrect information especialy about Led's. When I get a chance I will do a Led guide, there are fair few tricks that make working with Leds easier! Link to post Share on other sites More sharing options...
Tony Davis Posted April 3, 2014 Author Share Posted April 3, 2014 I would appreciate that. Link to post Share on other sites More sharing options...
Pandora Posted April 3, 2014 Share Posted April 3, 2014 Maplin sell leds in a variety of colours and sizes designed to run directly from 12V. Here is an example from the online catalogue: http://www.maplin.co.uk/p/5mm-red-led-ck48c just think of them as a normal led with the correct value resistor built in. So no messing around with calculators and Ohms law They are also avaiable in 5V versions,againg in a variety of colours and sizes Link to post Share on other sites More sharing options...
Tricky Dicky Posted April 3, 2014 Share Posted April 3, 2014 The only problem with resistor installed LEDs is that generally they are designed to give quite bright emissions usually a lot brighter than required for railway modelling. I prefer to add my own resistors and have better control of the LED brightness. If you need an easy resistor calculator the link below is useful. http://www.electronics2000.co.uk Richard Link to post Share on other sites More sharing options...
Gm11 Posted April 3, 2014 Share Posted April 3, 2014 Maplin sell leds in a variety of colours and sizes designed to run directly from 12V. Here is an example from the online catalogue: http://www.maplin.co.uk/p/5mm-red-led-ck48c just think of them as a normal led with the correct value resistor built in. So no messing around with calculators and Ohms law They are also avaiable in 5V versions,againg in a variety of colours and sizes 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 Link to post Share on other sites More sharing options...
RMweb Gold SHMD Posted April 3, 2014 RMweb Gold Share Posted April 3, 2014 Hi GM11, I'm interested in your LED guide but not as a user. I am an Electrical/Electronic Engineer with 25yrs experience. I found that I agreed a lot with your thoughts especially when it comes to LED brightness. Most online LED guides/calculators do not give an indication of "resultant" brightness and assume an ambient lighting level of the average living room and "aim" their LED brightness for that scenario. Model Railways, as you have already said, do not need such levels of illumination and any such "guide" should give an indication of what to expect. I offer my services to proof read/appraise your guide - if you want that is.Please don't take offence as I have no idea of your abilities/experience is/are. I am just interested in helping fellow RMwebber. I won't take offence if you tell me to go an jump. After all, this is your show! Kev.(Weekends are busy times for me.) Link to post Share on other sites More sharing options...
Gm11 Posted April 3, 2014 Share Posted April 3, 2014 Hi GM11, I'm interested in your LED guide but not as a user. I am an Electrical/Electronic Engineer with 25yrs experience. I found that I agreed a lot with your thoughts especially when it comes to LED brightness. Most online LED guides/calculators do not give an indication of "resultant" brightness and assume an ambient lighting level of the average living room and "aim" their LED brightness for that scenario. Model Railways, as you have already said, do not need such levels of illumination and any such "guide" should give an indication of what to expect. I offer my services to proof read/appraise your guide - if you want that is. Please don't take offence as I have no idea of your abilities/experience is/are. I am just interested in helping fellow RMwebber. I won't take offence if you tell me to go an jump. After all, this is your show! Kev. (Weekends are busy times for me.) Hi, you are more than welcome to add corrections etc, my background is as follows, very little knowledge of model railways, although I am building a custom controll system for a customer with a huge layout. My electronics background is 31 years in the job I hold the following qualifacation's CEng (Chartered electronics Engineer) PhD in embeded control systems. However untill I had 15 years experiance in the job I thought I knew most of it, then the next 15 years was spent realising I didnt! Feel free to chip in. I try and balance my information according to the audiance, so for example while discussing Leds I tolerate positive and negative instead of Anode and Cathode! unless it's of importance I would normaly discuss things in terms of conventional current direction. For non Engineers getting into the whole electron flow and conventional flow is pointless and leads to confusion , So be aware on ocasion I deviate from the engineering path in order to make things easier for the Audiance . I confess my pet hate is the current nonsense spouted on the internet regarding Leds, hence why I thought I would do a real guide rather than a theory type guide. I will sort it as a PDF and pass it over to you, take alook see if you think I have missed anything that may be of use to modlers. The one thing I want people to understand is that sight and Leds are logarithmic not linear, I might do a short video showing a Led and Oscilloscope using PWM, that way they can see that 50% duty cycle dosnt equate to 50% brightness! I also dont like seeing people being ripped off by buying 12V Leds from maplins and such! The main things I will include are the tips to know which is the Anode and Cathode both by Leg length and flat side on Led, also how to tell which is which when looking at a scematic. Also add in some easy ways to use Ohms law etc, and a couple of small tricks to adjust the lights in buildings etc to give a sense of distance. If that goes well I might add some simple circuits for detecting ambient light levels and adjusting the building light accordingly.I will stay away from micro's and try to keep it to cheap easy to get parts, Best Regards Gm Link to post Share on other sites More sharing options...
RMweb Gold SHMD Posted April 3, 2014 RMweb Gold Share Posted April 3, 2014 Yes yes, for this audience, conventional current only please! (Although there are a few members on here who could run rings...) Kev. Link to post Share on other sites More sharing options...
Roger.s Posted April 3, 2014 Share Posted April 3, 2014 Hi Gm , May I make the first correction or at least request that you stop trying to pass volts through a LED. One can apply a voltage across a LED and this will cause a CURRENT to pass through it. As you well know this be in Amperes or if we are lucky, or get our sums right, milliamps. Link to post Share on other sites More sharing options...
Gm11 Posted April 4, 2014 Share Posted April 4, 2014 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 Link to post Share on other sites More sharing options...
dajt Posted April 4, 2014 Share Posted April 4, 2014 I'm looking forward to the guide. Thanks, David Link to post Share on other sites More sharing options...
RMweb Gold SHMD Posted April 4, 2014 RMweb Gold Share Posted April 4, 2014 I think he is refering to post #12 where you wrote :- "...shove anywhere from 2.2V -400V through it without problems." Kev. Link to post Share on other sites More sharing options...
Gm11 Posted April 4, 2014 Share Posted April 4, 2014 I think he is refering to post #12 where you wrote :- "...shove anywhere from 2.2V -400V through it without problems." Kev. Yes indeed, The trouble you have with explaining electronics is to make sure your main AUDIANCE understands the concept. A beginer can measure the voltage before a Led (vf 2.2V) at say 12V, they can measure the voltage after the Led (9.8V) and they can measure the current through it (20mA). If you then say 'well actually its current not voltage' you open a can of worms and have to go deeper and deeper down the rabbit hole. My personal view is give people the information they need in order to get the job done. If this was an electronics site or if those with a deep interest in electronics wanted to know the theory etc then thats different, however my asssesment is most just want to know how to 'size' a resistor for a given Led and voltage. Again if I was teaching the subject then I would be getting into AC/DC methods of control and the use of capacitors when using Leds with Main's voltage. The point I was trying to make is Leds dont actualy have a voltage requirement above there VF, therefore no such thing as a 12V Led exists. For alot of beginners they will buy a '12V' Led from maplin etc and then not use it because they have 16V system's. So I was trying to point out that paying extra for a named Voltage Led is a waste of money. I once did a demonstration at a model railway group, where I used 400V (both AC and DC) to light a Led, I then used the same Led with a 'dead' battery and produced light. Again the point of the demonstration was simply that a Led once above it's Vf dosnt care how high the voltage is. This is the main reason beginners or modlers give up trying with electtronics, because Engineers have to point out the imperfection's in statements that are aimed at making things easier to understand. Lets get people using Leds and making up circuits, give them some confidence then perhaps we can start to polish the therory, but for now please lets not start addding confusion, hence why I also added I will use conventional current for now, Even though in reality electrons flow the other way, we use the term conventional to avoid confusing non engineers. Regards GM Link to post Share on other sites More sharing options...
Crosland Posted April 4, 2014 Share Posted April 4, 2014 I once did a demonstration at a model railway group, where I used 400V (both AC and DC) to light a Led, I then used the same Led with a 'dead' battery and produced light. Again the point of the demonstration was simply that a Led once above it's Vf dosnt care how high the voltage is. LEDs care very much about the voltage across them. If you tried that trick with no current limit then you would very quickly damage the LED. The point is that the current limit resistor also drops the voltage (simple application of ohms law) so an LED, operated within it's normal range, will never have more than its rated Vf across it. Confusion arises because people think that the resistor is there primarily to drop the voltage (rather than limit the current) and therefore the remaining voltage is what is important to the LED. Andrew Link to post Share on other sites More sharing options...
Pandora Posted April 4, 2014 Share Posted April 4, 2014 From the above posts , we need an article to cover basic electronics such resistors, voltages , current and power in watts. Diodes, including Zener types. Ohms law, Ohms law applied to LEDS, ie calculating the value of the current limiting resistor. and finally Voltage regulators Link to post Share on other sites More sharing options...
meil Posted April 4, 2014 Share Posted April 4, 2014 After watching a youtube video by Everard Junction about fitting an LED to an Hornby Buffer stop I called into Maplins and bought these http://www.maplin.co.uk/p/3mm-red-led-10-piece-led-pack-n21gl and these http://www.maplin.co.uk/p/resistors-lucky-bag-n24bg. Mr Junction advises using a 10k ohm resistor with the LED's. I checked with an online Resistor calculator and couldn't find one in the bag (I found it really difficult to see the colours), so I used a multimeter to check each one and the closest I could find was 6.18 measured on the 20k setting. My question is, is that a suitable value for connection the LED's to a 12v DC supply? If not, could you, dear reader, suggest one that would be, please? I really want to connect 3 LED's, so do I need a resistor on each LED or could I just have one resistor in the circuit? Kind regards Tony Look at this site: http://led.linear1.org/led.wiz Link to post Share on other sites More sharing options...
Crosland Posted April 4, 2014 Share Posted April 4, 2014 Look at this site: http://led.linear1.org/led.wiz That site is not perfect. http://led.linear1.org/how-do-i-drive-leds-from-alternating-current/ takes no account of the reverse breakdown voltage of the LED (often as little as 5V) when driven by AC. If you want to drive an LED from AC then conect another LED, or a normal diode, in parallel but the opposite way around, sharing the same resistor. Andrew Link to post Share on other sites More sharing options...
Gm11 Posted April 4, 2014 Share Posted April 4, 2014 Yeah and this is where it all goes wrong. I totaly dissagree with the whole teach electronics thing. There are plenty of resources for those who wish to learn it, but for many they simply want to accomplish something simple like light something with a LED, That is what I will focus on. As for trying that trick without a resistor!!! I would appreciate some credit, I am not a beginner, as stated before I am a Chartered Engineer with a PhD in electronics, I have taught masters degree's in electronic engineering, I have 5 books published and 367 Engineering papers published, I have 30+ years experiance in the industry. I didnt come here to willy wagle or to argue silly points about Leds, I simply offered to do an article that the average modler could understand. If you think you can 'Teach' Leds in a simpler way that makes sense to mr average, then go ahead. Regards GM Link to post Share on other sites More sharing options...
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