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One resistor - more than one LED - risk?

Posted: Thu Oct 22, 2020 5:20 pm
by bulleidboy
Having resolved the flashing light problem - for which many thanks to all. I thought it best to start a new thread.

I would like to put lighting into the buildings in the street surrounding the station. A 12v power supply (it has arrived) is about to replace the 9v supply currently in situ. The buildings have limited space, and for example the department store would need eight 3v LED's to illuminate all floors. It will be easy to run a cable from the bus up through the board and into the building - it would be relatively straightforward to put a resistor at the end of the cable on the ground floor and then, possible using copper tape, run the power to each floor and solder a LED as and where required. I know it is recommended that one LED/one resistor, but is there any way around this "norm". I'm sure the answer will be one LED = one resistor - but thought I'd ask. Does it matter how short the wires are on each end of the resistor? BB

Re: One resistor - more than one LED - risk?

Posted: Thu Oct 22, 2020 6:02 pm
by Brian
Hi Barry

As you say, its best to have one lit LED to one resistor. But you can connect LEDs together in series (Daisy chained) Cathode of LED 1 to Anode of LED 2 the Cathode of LED 2 to Anode of LED 3 and so on. Supply going to first and last LEDs the correct way around.

Eight LEDs would be too many for a single chain and one resistor.

If you use the LED resistor calculator linked below and state.... Source voltage as 12. Diode Forward Volts as 3. Diode Forward current as 10 (10milliamp) and the number in the chain as 8 (or whatever you're needing) then check the Wiring Diagram it will calculate the way to do it and give the resistor values needed too.

Wires on Resistors or LEDs can be as long or short as you wish.
Your Copper tape method would need to be feed with 12v positive and stop at LED 1 where that LEDs longer lead (Anode) is soldered to it. Then a second piece of tape not connected to the first has LED 1 Cathode (shorter) lead and it then goes to LED 2 Anode and stops. Then another piece of tape goes from LED 2 Cathode to LED 3 Anode and stops and then so on. The negative tape is on the last LED Cathode and runs non stop back to the supply wires negative feed via the one resistor.
Alternative is to run two tapes separated from each other and use either 12 volt rated LEDs or conventional LEDs which on the bench you have pre soldered a resistor to one lead of the LED and a fine flexible wire on the other LED lead. These then connect across the two strips the correct way around.

Note when soldering LEDs to a resistor don't allow the iron to remain for too long on the LEDs lead. Best is to pre tin one lead of the LED*** and one end wire of the resistor. Then trim their length to about 10mm each and place them overlapping by about 5-6mm (A 'Helping Hands' is great tool to aid this!) Pre wet with solder the irons tip with fresh solder and quickly place the iron tip on the joint. You should see the pre coated solder on both leads melt into one and make a solid joint. On the other LEDs lead do the same but instead of a resistor its a feed wire. On the other end of the resistor coat that lead with solder and trim to around 6 to 10mm long. Do the same to a wire stripped to around 3mm to 5mm of bare wire and once tinned solder to the resistor lead. After a few seconds and the solder has solidified you can insulate the whole lot with heat shrink tubing on the one lead with the resistor. Do the same on the other LEDs lead but direct connect it to a wire on that lead. It may not be necessary to insulate that lead if the other one is insulated. Ensure the heat shrink tubing insulation covers right up to the base of the LED and goes over the resistor and ends on the wires insulation before its shrunk in place. :D

*** Note I personally always fit resistors onto LEDs Cathode leads (short LED lead) but they can be fitted to the LEDs Anode lead. However, do try and maintain a common standard throughout.

Re: One resistor - more than one LED - risk?

Posted: Thu Oct 22, 2020 9:41 pm
by Tricky Dicky
Tacking resistors onto LED legs is in my opinion an unsophisticated method. Using Veroboard is a far better way a simple 3 X 3 hole piece will allow a LED, a resistor and the connecting leads. to be soldered on. Advantages of Veroboard are that the LED can be set right down on the board or made to stand off as required. Using a slightly larger Veroboard would allow you to include a screw hole.


Re: One resistor - more than one LED - risk?

Posted: Thu Oct 22, 2020 10:01 pm
by rick-h

I totally agree with Brian, however I have found from experience that it is better and easier (uses less space in the building) to put the resistors under the baseboard. What I do is put a LED(s) in each room / floor and connect all the cathode together and to a black wire, which then is connected under the baseboard to the negative. The LED anode are connected to red wires, run under the baseboard and each wire is then connected to its resistor, then connected to the positive. This way the lighting takes up less room and if you need to change the lighting level you can get at the resistors without having to destroy the building. One additional advantage is that you can individually switch on/off individual rooms/floors.

Hope this helps


Re: One resistor - more than one LED - risk?

Posted: Fri Oct 23, 2020 6:57 am
by RogerB
Great idea Rick. I mostly use the LED bulbs that are pre-wired and have a resistor already in place and heat shrunk in. The wires simply drop through the baseboard and attach to the the power supply via press fit connectors. My buildings are not fixed to the baseboard and [in most cases] the roofs lift off. Helpfully there are no high buildings so one LED per location is usually enough. R-

Re: One resistor - more than one LED - risk?

Posted: Sat Oct 24, 2020 12:28 pm
by bulleidboy
Many thanks for the help and suggestions. I am going to draw out a wiring diagram and see how I get on. I have moved the bookcase (thirteen years of Hornby Magazine :( ) from beneath the boards, so access is pretty good and I can sit comfortably and do any wiring. I also installed two "L" shaped copper rods, each arm(?) being about eighteen inches long, with the power connected at one end - this is mounted using chocblocs and with a couple of inches between each rod. All of the lighting/resistors has been soldered to the rods - each rod has suitably spaced red or black heat shrink, so I know which rod is +/-. So plenty of room for further connections. Barry

Re: One resistor - more than one LED - risk?

Posted: Sat Oct 24, 2020 1:57 pm
by Brian
Copper rods... Oh a proper Bus Bar ;) :D
Enjoy the lighting

Re: One resistor - more than one LED - risk?

Posted: Sat Oct 24, 2020 5:29 pm
by bulleidboy
I have just found several packets of 12v SMD LED's - each one is pre-wired, mounted on a paxolin plate with built-in resistor - this could make life a little easier. Steve M recommended these LED's some years ago, and they have just sat in a box all this time.

Re: One resistor - more than one LED - risk?

Posted: Sat Oct 24, 2020 6:29 pm
by Brian
They will be fine. Ensure you have them the correct way around on the supply. If you want them dimmer add a series resistor to one feed wire to the LED unit. Resistor value in Ohms will have to be determined on site.