Surface Mounted LEDs -How To...

OK, educate me on LEDs guys, especially the 0603; resistors, soldering tips, FAQs and answers.
In-wire-ing minds want to know.

Karin

I meant inqWIREing minds, duh!

For myself I am not interested in putting them into trains but am very much wanting to put them in buildings and scenery. So, John, a 800 Ohm resistor will take care of how many LEDs at once? What gauge of wire for the tiny stuff (0603's)? I don't like the shellac covered wire so need to know what size of vinyl covered wire to use? I think I can solder it okay but just need help for what connects to where.

- needed resistor = ((feeding voltage) - (voltage drop over led))/ (current wanted through led) The drop over the led depends on the type, but roughly 2V is a good guess unless some special types are used. The current can be anyting from 0.002A to 0.02A. The intensity will change a bit but less then you would think.

If you have enough voltage to start then you can put several leds in series and use only a single resistor. This only works well if all leds are of the same type. Replace in the formula above (voltage drop over led) with (n times voltage drop over a single led).

Take care of the power dissipated in the resistor: power = (current through les) x ((feeding voltage)-(voltage drop over led)) A 0603 resistor will take 0.1W, 0805 will take 0.125W, 1206 will take 0.25W. A traditional resistor with wires will handle 0.5W.

It doesn't matter where you put the resistor in the chain but it is easier for you if you always put it at the same place like at the positive supply side.

- soldering iron will need a fine point diameter 1mm is a good starting point. A 20-40W iron is fine for this. Clean it regulary. Set at 400°C, not lower or you will need to heat too long and that long roast is worse than a short high temp.

- get some fine solder wire 1mm max, 0.8mm is better. Smaller get fiddly.

- wire: you can use the enamel coated one, but then get the type with a coating that permits soldering. Sometimes know under the name of "roadrunner": www.rrunner.co.uk/wire/qse.htm and first tin the wire ends. Other thin wire that works fine is wire-wrap. Exsists with different type of insulation: teflon, kynar etc. If you use that, by a striping tool for that size and use it only for that.

Karin, my method is for dummys. yes, I use a 800 ohm resistor on the SMDs also. I try to use regular leds on building because its cheaper, but I guess you are trying it to make it less visible. The only thing I would avoid are those hair wires. They are very fragile. Bern used a very thin regular. Its similar to the ones on a decoder. Those work very well.

Different LED colors require different voltages. If you are using white LEDs, they need about 3.5 volts each. This limits how many can be placed in series using a single power supply and resistor. Calculate the resistor as the DC supply voltage minus sum of seriesed LED voltages divided by the desired current, usually between 0.005 and 0.010 amps (aka 5 to 10 milliamps). Then figure the resistor wattage rating by the resistor voltage squared divided by the resistance, OR the current squared times the resistance (same thing).

Example: 3 white LEDs in series need 10.5 volts total split up across the 3 LEDs at 3.5 volts each. The available power supply is 16 volts DC. The series resistor is required to drop the remaining 5.5 volts at the desired current in amps. To get a current of 0.01 amps at 5.5 volts across the resistor, ohms = 5.5V/0.01 amp = 550 ohms. Resistor wattage rating (aka smallest physical size needed to get rid of heat through air) = 5.5V x 5.5V / 550 ohms = 0.055 watts, 0603 resistor size is sufficient since most of the heat is developed in the 3 LEDs. OR 0.01 amp times 0.01 amp times 550 ohms is again 0.055 watts, same thing.

The 0603 size LEDs are a royal pain to work with. They can get lost under a fingernail, are very fragile, the polarity direction marking is invisible without a magnifier, and attempting to solder to them while holding them with tweezers often results in the plastic cover melting off under tweezer pressure. Unless you really need to fit into a tight space, better to use a larger LED with leads already attached.

That said, here is a way to solder to 0603 LEDs that will alleviate some of the aggravation. Use a mounting substrate for the LED such as a Z or N scale PCB tie (available from RLW etc for soldering rails when hand-laying track). Or for tight spaces use 1/32" or 1/64" thick pcb material and a cutting jig to cut it yourself into narrow strips, a laser might work for this. File a small groove across the copper to prevent shorting the LED connections together. Leave enough room on the end (about a millimeter) that a AWG 30 kynar-insulated wire can be soldered to the copper beside the LED.

Hold the tie in a clamp of some sort, alligator clip on one of those magnifying glass hobby stands, or even pliers with a rubber band wrapped around the handles. Pre-tin one of the copper pads with a small amount of solder. Then tweezer the LED onto the pad and very quickly melt the solder to hold one end of the LED onto the pad. With the LED thus partially secured, quickly apply a small amount of heat/solder to the other copper pad to complete the LED connection. Now you have a LED soldered both ends to the pcb, and if you held the LED in the tweezers by it's own base material, NOT the plastic, the LED should have survived.

Solder the AWG 30 Kynar wire to the exposed bit of copper pad at one end, then another wire to the other end. Then cut the pcb off with side cutters. The result is a LED and wire connections that is mechanically supported by the pcb tie material and will not break during handling.

Hi Glen, I could understand a LITTLE of what you said... I managed to solder some 0603's last summer but burned one up with too much power even with a resistor. Ask me anything about setting diamonds, but electronics will take me forever to read all I need to do it right.

If I have to I will just use one resistor for each LED and that way I don't have to worry about how many I can string together or not.

Just check the voltage drop of each led. If you connect them serially, just add the voltage drops together. This sum must be lower than the supply voltage. When you got enough leds together, you substract the sum from the supply voltage to get the voltage drop on the resistor. Then you can calculate the required resistor value, by dividing the voltage drop on the resistor with the required current. This supported current value is indicated on the datasheet of the led. Always use at least 20% less than the maximal rating, so you don't burn them up.

For soldering lights, I usually use uninsulated jumper wires. They are similar to the material paperclips are made of, just a bit thinner. They are very easy to solder to (since this is their original use) and strong enough to hold their own weight, so you can bend them to any shape you want. When the light circuits are ready, I glue them to their place and paint them with a plastic based paint, that insulates them. If you want to solder smd leds to this kind of wire, you can bend the ends 90 degrees to get solder pads. For bigger surfaces, you can use prototype circuit boards (the ones with copper strips and predrilled holes). You can glue the smd parts to the boards before soldering (only glue the plastic underside not the metal pads). Using small smd leds and black paper internal walls allows per room lighting, so when you turn on the light, only certain windows light up, and (if you use multiple circuits) you can literally switch rooms on and off.

ps: Our portable christmas tree with the train that I posted a few days ago had two chains of lights, driven from the accessory output. It's hard to see them on the photo because of the camera's flash. The chains are normal (cheap) leds with their wires bent and soldered together with a resistor on the end of each chain, then the metal parts were painted with green paint to hide them. You can also see the green feeder wires on the photo.