MarcD Posted March 7, 2021 Share Posted March 7, 2021 This might sound like a silly question. I'm just about make some small lights for my shunting layout to go in and on some of the buildings. The plan is to wire 3V LED's , in parallel, onto an old N gauge controller as a supply . Max voltage 15V at 300mA. There will be about 10 lights on the circuit but I have no idea what resistors I should be using to protect the LED's. I did watch a video on youTube which quoted 5,200ohm but when I used Ohms law my calcs came out at 40ohms, a bit of a difference. I haven't done any electronics in 20 years so my ohms law is a bit rusty can any one point me in the correct direction? Marc (A Frustrated Mechanical Engineer) Link to post Share on other sites More sharing options...
RMweb Gold ikcdab Posted March 7, 2021 RMweb Gold Share Posted March 7, 2021 I have found that LEDs are very tolerant! I use a 12v supply and 1000ohm resistor. But larger sizes are ok too. I would not go less than 1000ohm. I have found that you can't mix different colours of LED on one resistor. It's better to use one resistor per LED. They are very cheap and all I do is solder a resistor to the negative pin of the LED and put a bit of heat shrink over it. Then you are good to go! 1 Link to post Share on other sites More sharing options...
John ks Posted March 7, 2021 Share Posted March 7, 2021 1 hour ago, Furness Wagon said: when I used Ohms law my calcs came out at 40ohms Working backwards 9V drop across 40 ohms gives a current draw of 225mA If you have 10 LEDs in parallel fed by the 40 ohm resistor then each LED will draw 22.5mA 22.5mA would seem ok for Each LED Now a reasons why i think it is a bad idea to have all those LEDs in parallel If the LEDs are not perfectly matched the one or more will draw more than 22.5mA Assuming 22.5mA is the rating of the LEDs then the LEDs drawing more than 22.5mA may fail prematurely When one fails then the current to the remaining LEDs will increase causing another one to fail & you could get a cascading effect taking out all the remaining LEDs 1 hour ago, ikcdab said: I have found that LEDs are very tolerant! I use a 12v supply and 1000ohm resistor. But larger sizes are ok too. I would not go less than 1000ohm. I tend to agree with ikcdab & the following is meant to complement ikcdab's post The current rating of a LED is given to get the maximum brightness from that LED If you drive a LED at its maximum rating then you get the shortest lifespan Most times on a model railway you don't need the LED at maximum brightness You might be surprised how bright a LED is with 1K resistor If its too bright then larger value resistor can be used I have used 4K7 (or was it 47K,memory isn't the best ) resistors on a HST marker lights & they were bright enough With the price of resistors you can afford to have a selection on hand & use trial & error to get the resistor value that gives the best brightness for your needs. Start with 1K or 10K to see which is best for each application If you need bright LEDs then you will need to do some calculations to get the best value & I would tend to use one resistor per LED Hope this helps John 2 2 Link to post Share on other sites More sharing options...
cliff park Posted March 7, 2021 Share Posted March 7, 2021 By the way it matters not a jot which leg of the LED the resistor is wired in. It may be convenient if they are all wired the same when it comes to fitting them, eg negative feed always go to the resistor leg, but from an operational point of view it doesn't matter. 1 Link to post Share on other sites More sharing options...
RMweb Gold ikcdab Posted March 7, 2021 RMweb Gold Share Posted March 7, 2021 2 hours ago, John ks said: Working backwards 9V drop across 40 ohms gives a current draw of 225mA If you have 10 LEDs in parallel fed by the 40 ohm resistor then each LED will draw 22.5mA 22.5mA would seem ok for Each LED Now a reasons why i think it is a bad idea to have all those LEDs in parallel If the LEDs are not perfectly matched the one or more will draw more than 22.5mA Assuming 22.5mA is the rating of the LEDs then the LEDs drawing more than 22.5mA may fail prematurely When one fails then the current to the remaining LEDs will increase causing another one to fail & you could get a cascading effect taking out all the remaining LEDs I tend to agree with ikcdab & the following is meant to complement ikcdab's post The current rating of a LED is given to get the maximum brightness from that LED If you drive a LED at its maximum rating then you get the shortest lifespan Most times on a model railway you don't need the LED at maximum brightness You might be surprised how bright a LED is with 1K resistor If its too bright then larger value resistor can be used I have used 4K7 (or was it 47K,memory isn't the best ) resistors on a HST marker lights & they were bright enough With the price of resistors you can afford to have a selection on hand & use trial & error to get the resistor value that gives the best brightness for your needs. Start with 1K or 10K to see which is best for each application If you need bright LEDs then you will need to do some calculations to get the best value & I would tend to use one resistor per LED Hope this helps John If you go online to buy LEDs you will find that you are encouraged to buy ultra bright ones. That probaby isn't what you want. I look out the diffused ones which are much better. 1 Link to post Share on other sites More sharing options...
RMweb Gold ikcdab Posted March 7, 2021 RMweb Gold Share Posted March 7, 2021 14 minutes ago, cliff park said: By the way it matters not a jot which leg of the LED the resistor is wired in. It may be convenient if they are all wired the same when it comes to fitting them, eg negative feed always go to the resistor leg, but from an operational point of view it doesn't matter. I agree. In their as-bought state, the positive leg is longer so you know. Once you start chopping them down you lose that. That's why I always fix the resistor to the negative and then sheath in black heat shrink so I know the polarity. 1 Link to post Share on other sites More sharing options...
MarcD Posted March 7, 2021 Author Share Posted March 7, 2021 The power supply has a pot omit so I can reduce the voltage and the brightest. I bought warm white whatever that looks like. I have ordered some 5.2k resistors I have some1k and 10k in my box of spares. Link to post Share on other sites More sharing options...
smokebox Posted March 7, 2021 Share Posted March 7, 2021 25 minutes ago, ikcdab said: I agree. In their as-bought state, the positive leg is longer so you know. Once you start chopping them down you lose that. That's why I always fix the resistor to the negative and then sheath in black heat shrink so I know the polarity. The negative side of the LED normally has a flat on it to aid with identification too. Link to post Share on other sites More sharing options...
Crosland Posted March 7, 2021 Share Posted March 7, 2021 With a 15V supply you could connect the LEDs in groups of 3 or 4 in series, use lower value resistors and use a lot less power. 1 1 Link to post Share on other sites More sharing options...
AndyID Posted March 8, 2021 Share Posted March 8, 2021 If you enjoy mucking about with electronics you could make an adjustable constant current controller. Probably "overkill" for your needs but it won't be very complicated. Link to post Share on other sites More sharing options...
d00m Posted March 8, 2021 Share Posted March 8, 2021 You might find it useful to type 'LED resistor calculator' in your search bar - it comes in handy when working out parallel or series resistors for led arrays. Link to post Share on other sites More sharing options...
Tricky Dicky Posted March 10, 2021 Share Posted March 10, 2021 On 08/03/2021 at 23:24, d00m said: You might find it useful to type 'LED resistor calculator' in your search bar - it comes in handy when working out parallel or series resistors for led arrays. A useful electronics calculator can be found FREE to download on the Electronics 2000 website, it is a Windows program so will only work on a Windows enabled machine. It has a very easy to use LED resistor calculator among other things. www.electronics2000.co.uk Richard Link to post Share on other sites More sharing options...
Philou Posted March 10, 2021 Share Posted March 10, 2021 (edited) I failed to use a resistor on a 3v supply and the magic smoke escaped. I learnt!! I use 1kΩ from now on and it's plenty bright enough even at 3v. Cheers, Philip Edited March 10, 2021 by Philou Link to post Share on other sites More sharing options...
MarcD Posted March 10, 2021 Author Share Posted March 10, 2021 Even as a mechanical engineer you know not to let the smoke out of the box. You learn that in beginners black masses and cross burning, sorry introduction to electrical/electronic engineering. Marc Link to post Share on other sites More sharing options...
Tricky Dicky Posted March 11, 2021 Share Posted March 11, 2021 15 hours ago, Furness Wagon said: Even as a mechanical engineer you know not to let the smoke out of the box. You learn that in beginners black masses and cross burning, sorry introduction to electrical/electronic engineering. Marc LOL Richard Link to post Share on other sites More sharing options...
RMweb Gold rodent279 Posted March 14, 2021 RMweb Gold Share Posted March 14, 2021 I would wire a resistor in series with each LED individually, which I know is more of a faff. You can assume around 10mA per LED, and a forward voltage drop across each of about 1.8V, so from a 15v supply you need a resistor of (15-1.8v)/10mA = 13.2/10 = 1.32kohm. If your LEDs are 3v, then you need 1.2kohm, but tbh it doesn't really matter, as long as the current is limited to a sensible value. 1 Link to post Share on other sites More sharing options...
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