wiring LED

[greatwestern]

Hi Making a gantry of 6 green and red LED's
+ - off each LED via a change over panel switch.
My question is can I wire up all the neg in line and fit one resistor which then goes to
a neg ring wire connected back to 3 to 12 volt varible transformer plug.
Each pos wire from each colour LED goes to panel switch ( on- off -on ) to change from green to red then
goes back to pos side of transformer plug.
I have seen that most advise that a resistor should be fitted to each pos side which would
mean a lot of resistors.
Does it make any diffrerance to which side of the LED the resistor is fitted.
The resistors are 3 volt and I am using a varible 3 to 12 volt plug.
What is the best voltage to set this at ?
Mike

[Tricky Dicky]

Starting with your next to last question, it does not matter which leg anode or cathode the current limiting resistor goes.

It seems you have a gantry with six two aspect signals and I take it that only six LEDs will be lit at any time be they red or green. Assuming you intend to run the LEDs @ 10mA that gives you a total current draw of 60mA at any one time so assuming you are using the 12V PSU then you need to drop approx. 10V across the single resistor power wise that works out at 600mW so a fairly hefty resistor will be required to dissipate the heat. You are far better off using a current limiting resistor for each LED with resistors available at less than a penny each it is not a major investment and really gets around a whole load of problems.

As for the power supply. LED voltage drops across them vary from just under 2V for red LEDs to just over 3V for Blue and white LEDs with the other colours somewhere in between, for most purposes 2V is a fair approximation in most circumstances. As to what voltage to use, anything above the working voltage of the LEDs will do. Do not be tempted to use say a 2V supply and run a LED without any series resistance. LEDs are not resistive and without a resistance in series can suffer what is called thermal runaway and you find they soon emit the “magic smoke” Using a power supply near to their working voltages means you do not have much scope resistance wise and need to be accurate with your calculations. 5V seems a good starting off point.

Since you have 3V or 12V, I would go for 12V and providing a resistor for each LED a 1000 Ohm or 1K resistor is a good starting point for each LED, if they are too bright you can simply increase resistance until you are happy.

Good Luck
Richard

[greatwestern]

Hi Richard,

Just to clarify I am using a varible plug in transformer which I can set from 3 to any voltage up to 12
As for LED's with other single lights of red and green only I would see about 20 being on at any one time.
Each independantly wiried.
Mike

[Tricky Dicky]

For safety sake I would still work on 12V, by ensuring you have adequate current limiting at 12V will also cover all other voltages. If you set your variable PSU at a lower voltage and calculate your current limiting resister at that voltage, there is always the potential to accidentally turn up the PSU to it’s maximum and it does happen and your layout could end up being a recreation of the blitz .

As to how many LEDs you can have depends on the current output of your PSU, this is normally a fixed value but with variables it could be represented by a range with the maximum current at the lowest voltage and lower current at the highest voltage. Different LEDs can run at various current draws, data sheets normally compromise on 20mA for test purposes but this can be quite bright and does not look right on a layout so most people settle on 10mA or even less especially with low current LEDs. Simply divide the PSU current rating by 10mA and that will give you the maximum number of LEDs. Therefore a PSU rated at 500mA (0.5A) would = 50 LEDs allow at least 10% overhead so as not to strain the PSU by running at maximum and that gives you 45 LEDs.

Richard

Edit: A useful electronics calculator can be found to download for FREE here;

https://www.electronics2000.co.uk/

It probably does more than what you may need but has a good LED resistor calculator included it is however a Windows app so needs to be run on a Windows enabled computer.

[Brian]

Richard has given you excellent advice.
If I read your first post correctly you have three signal heads each capable of showing a Red or Green aspect. Therefore you can connect each signal heads red and green LEDs Cathode leads together and take them to one series resistor, therefore three signal heads equals three resistors, as only one aspect per head can be illuminated at any one time, so the red and green LEDs per head can share a single resistor. On 12v DC a 1K (1000 Ohm) 1/4watt (0.25w) resistor will usually be fine.
Use a Regulated DC power supply ideally 12v DC. Such as are often sold under the heading of CCTV or LED power supplies. I would look for something offering 1.0Amp output which is capable of powering around 100 lit LEDs with 1K resistors. But as Richard has pointed out its always wise to allow some Headroom, so reduce the lit number to say 90 -92.

[teedoubleudee]

If I may add my two pennyworth, using a common resistor for two or more LEDs (where only ONE will be lit at a time) is fine except in my experience often one LED will shine a lot brighter than the other. I find that Green LEDs often shine a lot brighter than their Red counterparts. In this case a larger value R would be required in the Green circuit and each LED fed individually rather than connecting the cathodes together.

My advice would be to do a "bread board" test first with the chosen components to make sure you are happy with the light levels before commiting to adding them to the gantry. I know it sounds obvious but you should do this with the room lights dimmed for best results.

[Forfarian]

I built gantries for my layout during lockdown, I bought prewired LEDs with resistors from Bright Components and wired all the - together and all the other + wires back to my control panel. I used the Dapol kit gantries and extended them to suit the width of track.

[RAF96]

I made some station platform lamps each having a current limiting resistor.
These are powered by the speed knob of an old analogue controller so I can tune the brightness from off to bright to suit.
However to avoid blowing leds by accident in line resistors are still necessary.

[Brian]

If using a train set speed controller as the power source I would also recommend adding a single diode in one feed wire (1N4002 or 1N5402 etc) to prevent reverse polarity being applied to the LEDs and stressing them. Note also that many 'Train Set' controllers actually output far more than the nominal 12volts DC they are alleged to produce. It is not unusual to see 18v to 19 volts appear under low current loading. So it may be wise to increase the resistor value to a higher Ohm value - 2K2 may be needed?