Basic wiring question

[Khaul]

I have started to work on getting all the platform, building and street lights to work with the same power source. The platform ones work at 12V and the rest at 3V. Today I got a solder less breadboard, a few resistors, wire for jumpers and terminal blocks to experiment. The initial test has been successful.

 

I have, however, a few questions. The photo below shows the 3V arrangement connected to one of the street lights. As you can see the light comes with a very thin and relatively short wire. I won't be able to run the wires for all the five street lights I am planning to the module to the same main board.

 

What is the standard practice? I imagine one possibility is to have a terminal block for each light and than run a longer cable to the main board. Does that terminal block need to be mounted in a little  board with it's own resistor there? Would it be better to somehow attach the block to the bottom of the module and have the resistors in the main block. Should I have one resistor for all the 3V system or perhaps one for each light?

 

Thanks in advance,

 

Pablo

 

[cteno4]

Pablo,

 

the best practice is a resistor for each led. If they are matched leds you can run three in serial with a resister and 12v, but if one led is a bit different on its current draw it can then overload and end up burning all three out. So safest is a resistor for each. I usually do what you suggest and have a local block that the led leads feed to and each circuit is fed off a buss of your input voltage. I just use the blank prototyping pc boards that you then just solder in jumper wires (extra resistor leads you cut off are great for this). You can get 2 pole terminal blocks for 5 cents or so in bulk if you don’t want to solder the led leads onto the boards

 

https://www.ebay.com/itm/Electronic-Ballast-Block-PCB-Connection-Terminal-Mount-Screw-Accessory-Tools/202382744410?hash=item2f1ef39b5a:g:SMIAAOSwg0hbWow2

 

https://www.ebay.com/itm/10PCS-5-cm-7cm-Prototyping-PCB-Printed-Circuit-Board-Prototype-Breadboard-BZ504/302691960571?hash=item4679d8c6fb:g:u1sAAOxyn~pRzalt

 

3v is a bit low for your power supply to white leds. They want usually 3v and change, but will usually run on 3v power supply. Just make sure as some have bottom limits that won’t start up at 3v, but most I’ve played with will. You will just need a very low ohm resistor. You can vary the resistor to make any dimmer if you want. I usually use 5v power supplies as then no issues with voltage and 5v power supplies are so prevelent these days.

 

i use little variable resistors to set each where I want it. I wire a fixed resistor to drop to 3v then the variable resistor innseries so with the variable resistor set at no resistance the led is going like 75% (I never want them full blast and they live longer lower and any small surge won’t fry things), the just fiddle the variable resistor where I want it. Much easier than changing a fixed resistor to change the light later. 1 or 2k variable pot usually give me the range I want.

 

https://www.ebay.com/itm/2K-OHM-Trimpot-Trimmer-Potentiometer-Pot-Variable-Resistor-RM065-202-x10/182762208248?hash=item2a8d79fbf8:g:ReEAAOSwkGVZuMxo

 

Quinn just posted an excellent example of this 

 

 

Cheers

 

Jeff

[Khaul]

Thanks Jeff. I did see the post by Quinn. Well, some work ahead...

[kvp]

3 hours ago, Khaul said:

Thanks Jeff. I did see the post by Quinn. Well, some work ahead...

I assume the lights doesn't have resistors built in. That would mean 3V is their forward (turn on) voltage and you have to go a bit above to have all on (thanks to manufacturing precision).

 

The classic way is to solder and shrink wrap the resistors to one led and a correctly polarized reverse polarity protection diode to the other wire. I would select a nice resistor for 12V that limits the current below absolue maximal value, then wire them in parallel and add a larger series resistor for dimming them further. You can leave out the protection diode if you never make mistakes and connect two wires in the wrong order. (i do this too many times)

 

What resistor value to use with 12V: ( http://led.linear1.org/1led.wiz )

- without a protection diode, the white LED forward voltage is around 3.3V, this gives 1000 ohm (1 kohm) 1/4 watt @ 10 mA

- with a protection diode made out of standard silicon, the white LED + diode forward voltage is around 3.9 to 4.0V, this gives 820 ohm 1/4 watt @ 10 mA

 

You could go higher than 10 mA, as the usual limit is around 20-25 mA, but that drastically shortens the lifespan of the LEDs and they will be very bright even at 10 mA. So once they are nice and lit up at 10 mA, you can add a variable (or preset) resistor series to the whole array, just add the wattage together (1/4 aka. 0.25 watt for each led, summed up will mean 20 leds will need a 5 watt common resistor). You can also use a 12V PWM dimmer circuit from ebay for very fine brightness control from zero to full.

[cteno4]

I always find the inline resistors a pain as they make feeding and dealing with leads a pain and also a pain to change out once an led is installed. Also I use 5v for lighting buss supply not 12v so I need to clip them anyways.

 

Hence im a big fan of the little circuit boards with a safety resistor (to make sure max ma is not exceeded with full voltage from your buss supply) and then the trimmer pot. Just takes care of so many issues at once and it’s like 10-20 cents per led circuit with a terminal block included. You can even minimize by dropping the terminal block and put inside the structure, or minaturize with smd trimmer pots.

 

as kvp mentions many times at even 10ma the leds are mini suns that can turn lighting into glowing.

 

As kvm says for situations like streetlights where you want all at the same setting then using something like a master, high watt resistor (do the addition and give extra and test its not getting too hot) or a 12v PWM dimmer circuit (and safety resistors for each led to 12v) and you can then have it all! The pwm are like $2 on ebay and usually have memory.

 

jeff