Valid point Phil, I had overlooked the fact that I would be operating from a battery source rather than a mains power source.
Jon,
1 can I assume that the 12v is the Battery? Yes
2 what is the TB, choc strip? Yes, choc block or any other type of terminal block,
3 I assume that the little handey things are the LED's? Yes - the base of the triangle is the anode, connected to positve, the long bar, on the battery, and the LED cross-bar is the cathode, connected to negative. An easy way to remember which pole is which on battery symbols is; the negative is a dash, the positive is the two dashes that make up a + the upright dash being laid horizontal next to the other one, thus - and --
4 as the resistors r2 and r3 are 'to be determined' how would I do that on the assumption that I may use such a setup for 15-20 LED's around buildings and lamps in my Garden Railway? I do not proffess to being able to do the calculations from memory, I use an on-line calculator and the link below will take you to one that also has a good tutorial on LEDs, there are others. My schematic was simply that, it wasn't a wiring diagram showing all the lamps or LED's and so no resistor values, as this will depend on how many LED are in use and in what configurations, again the tutorial part of the below link is very useful. You will need to know the characteristics of the LEDs that you are using, which is why on the schematic I had listed the forward and reverse voltages, the forward current and the power of that particullar LED, one which i generallay use for interior lighting and porch/canopy lights, other LED's will have different characteristics, the ones I use for signal spectacle lamps for instance, are different and will effect the value of resistor to be used. The other thing to remember is that each sub circuit is in parallel with the others, regardless of the configuration of the LEDs in the individual sub circuits.
www.electricaltechnology.org › Basic Electronics
How to Calculate the Value of Resistor for different types of LED’s Circuits Formula for finding the value of resistor(s) to connect LED’s in Series ...
The variable resistor set to give an output, in my case, of 3.5 volts means that in some cases I will not need a resistor. The variable resistor can be set to give an output voltage to suit your needs. 15 -20 LEDs if all wired in series would create a substantial voltage drop across them, and a 12v supply would probably not be enough, let alone needing a resistor. But if like me each building is to be treated seperately with LEDs in parallel, or series, then resistors will be needed. In practice when I come to do my lighting supply, I will probably calculate each individual circuit, insert the appropriate resistor, hook every thing up, turn the variable resistor to max, switch on and then turn the variable resistor back until all the lights come on or the resistor is at zero ohms with no or only some lights on. At which point I'll start reducing the value of the resistors in individual circuits until I get the brightness or effect that I want. Again without getting involved with ohms law the variable resistor will be whatever I have to hand, something like a 1 Kohm probably. Another thing I generally do with buildings is provide them with a plug and a matching socket on the 'foundation' making removal of the building easy; however, if like ours your buildings get put away for the winter you will need to devise a means of plugging or covering the socket.
Now if all this reads as if our railway is a profusion of lights after dark - it aint, not yet anyway. The buildings generally have the LEDs installed but are not yet powered. The only exeptions being a wayside station which, as an early experiment, uses the bits from 2 garden lights; the 2 solar panels wired in parallel and inset into the platform surface, supplying 2 AA cells in parallel, and controlled via the printed circuit board from one of the lamps, feeding an interior light and 2 under the canopy. (The batteries and pcb are mounted inside the building). Works reasonably well but as the panels are getting old and even though I occasionally polish them with car paint restorer, plus they tend to be in the shade for part of the day, which doesn't help, the batteries have to be replaced every couple of weeks, especially when the nights are longer. There's a signal box which at the moment has an interior light and just a 2 cell AAA battery box with a switch, and the 4 signals ditto.
Sorry, this has gone on a bit. but as I said, it is all just theory at the moment and the practical aspects have yet to be proven. When I get started on the implementation I'll try and keep people advised on the success or otherwise, or somebody else beats me to it!
