Hello all can some one give me simple instructions on how and where to solder resistors, that i have purchased for my Z scale layout, on to 6 volt lights that i also have for this layout. Do i simply solder each leg of the resistor to one end of each wire coming out of the light pole and then once they all have resistors then solder them on to two wires running from the power supply making sure that all resistors are in the same alignment?


Many thanks
MAd Peter from Adelaide

Edited by moderator 01 April 2016 23:18:25(UTC)  | Reason: Not specified

For led and resistors the standard practise is, as indicated on drawing, to solder 1 resistor onto the anode (+) of the LED (i.e. the longest pin). For normal light circuits it doesn't actually matter but in some circuits it would - hence general recommendation is to use the anodes.

If you want to wire LED in parallel each led should have is own resistor.

If you wire led in series, you would only put one resistor for the first led.

There is various online calculators who can help you with size of resistors. Remember a resistor size is not only how many ohms but also what watt it can cope with (heat build up).

Brgds Lasse

Ok, I got confused ... for soldering technique - there are youtube videos on 'how to solder'. For 'where to connect the resistors for lighting...

First, resistors have no polarity, so which way around you solder them on makes no difference.

If you have incandescent bulbs, those bulbs are rated for the voltage of the system typically - so on a 19v system, you probably are running 19v bulbs. The bulbs typically have their rated voltage stamped/printed on the metal screw in base in some form, or printed on the glass. This lamp acts as a resistor and gives out some of the energy as light and some as heat, like any bulb.

If you have LED's it is different, as LEDs are rated not for a voltage but are instead rated for a current in mili-amps typically. LEDs are not resitors, they will flow whatever current you give them, and that is why you need to always put a resistor inline with the LED's to limit the current that flows through them otherwise they burn out.

(+) --- R ----|>|------ (-)

An LED typically will drop about 1.7 to 2.0 volts in its conductive state, but as its resistance is very low, it will effectively convert to full current at that drop as if the resistance is 0, and flame out without a resistor.

An LED is typically rated at 20mA for example (typical for some small surface mount LEDs of today ). So you need to ensure that you never run more than 20mA through the LED. If applying 19v across the LED+Resistor combination, and say 1.7 of that is the drop across the LED, then 17.3v would be the drop across the resistor. So the smallest resistor to use then is R=V/I = 17.3V / 20mA = 865 ohm. Hence lots of folks just use a 1k (1000 ohm) resistor. Larger resistors result in a lower current, and less bright LED but are always safe.

The LED information sheet will tell you the maximum rated current, but using 20mA is a safe bet, so using a 1k ohm resistor or larger is a safe bet.



Now if you are connecting a bunch of LEDs in series or parallel, you can adjust the computation.

If you connect 5 in series, you only need 1 resistor for the 5:
(+) --- R --- |>|---|>|---|>|---|>|---|>|------ (-)

Here you have the same 1.7 v drop across each LED, so you get a 5*1.7v = 8.5v drop across the LEDs, so with a 19v supply you end up with 19-8.5 = 10.5v drop across the resistor. So to limit ot the same 20mA, you now have 10.5v/20mA = 525 ohm. If you use a 1k ohm resistor, the LEDs are likely to be about 1/2 as bright as the prior case, so choosing a 600ohm resistor is better to attain the same level of brightness.



If you connect 5 in parallel, you can also do with 1 resistor if you wire it up one way, or 5 resistors if you wire it up differently:

Parallel w/ one resistor: (sorry for the formatting but as I cannot just inject a <pre> </pre> html formatting command the spaces get crushed )

(+) --- R ---+--|>|---+
+--|>|---+
+--|>|---+------ (-)
+--|>|---+
+--|>|---+

Here all the LED's drop the 1.7v , so just as in the 1 LED case, the resistor has 17.3v across it, and you install a 1K ohm resistor, but the current is now split over the 5 LEDs somewhat evenly.. so the 17.3mA current gets split 5 ways, which is 8.66mA per LED.....so they will be much dimmer than running near 17mA.

If you want each of the 5 LEDs to run full brightness, you need them all to run at their 20mA level, so you need to feed the parallel set (20mA*5=) 100mA of current. So you can treat that set of 5 parallel LEDs as one big LED that drops 1.7 V and can take 100mA of current. (rather than dropping 1.7v and taking 20mA current max).

So you can then use a 1.7V/100mA = 17ohm resistor, but thats the limit, you'd want to use a much larger resistor, say 100 ohm. AND you have to pay attention to the current handling capacity of the resistor here, something you could probably ignore before. If you run 100ma through a 17ohm resistor, that is P=I*I*R = .1 *.1 * 17 = 0.17W of power, so make sure the resistor is rated for 0.17 watts. For discreet resistors thats typically not a concern as they are rated for 0.25w or more.. but you need to start paying attention or risk burning out the resistor as well.

This is a possible usage if you are lighting a train platform, with 5 lamps. However, I'd recommend you still wire a resistor with each LED (example below) just because its more fault tollerant. If you ever mistakenly feed power directly to the LED in this case, you burn it out and then have to disassemble the light to replace the LED. In the below example, I'd mount the resistor very close to the LED, in the lap post, etc, so there is no chance of mistakenly connecting power directly to both sides of the LED.

Saving 4 resistors for this trickier solution when they are pennies in price isn't worth the potential consequences of mistakes in my book.



Parallel w/ 5 resistors:

(+) ---+---R---|>|---+
+---R---|>|---+
+---R---|>|---+------ (-)
+---R---|>|---+
+---R---|>|---+

Here, each individual resistor-diode pair is its own circuit from + to -, so they act as 5 independent 1 LED + 1 Resistor segments; use a 1k resistor.
If you build the resistor into the lap post, or into the locomotive, this is the likely arrangement - pair a resistor with each LED.

Generally when using incandescent bulbs you try and get bulbs that are rated for the voltage that you will be supplying to them. Since you are planning on using them for your Z scale layout I am going to make the assumption that you have a 10 volt supply and therefore, the bulbs have the wrong rating and would burn out very quickly if you connected them as is.

It is not usual to have to add resistors to incandescent bulbs, but is required for LEDs and that is why you have gotten so many responses regarding LED wiring and resistor sizing.

So if I understand correctly what you are trying to do, you are trying to solder in a series load that will drop 4 volts across it in order to have 6 volts across the light bulb. Now you are getting into OHM's LAW and you would have to know the characteristics of the light bulb as far as resistance is concerned, then calculate the proper resistor size, available current through the series circuit, etc. Is it really worth it?

My suggestion would be to buy 10 volt (or whatever output voltage supply you have) bulbs and not use the 6 volt ones.

My nickel's worth.

Cheers
Peter

Given we know they are lamps/bulbs:
1) Bulbs are rated for a voltage, running them at a higher voltage will burn them out quicker, often very fast
2) The resistance a bulb filament produces varies with the temperature of the filament. So when the bulb is off/just turned on its resistance is different than what it will be when its up to full heat/brightness. That makes sizing a resistor, if you need one to drop voltage, a bit trickier.

You definitely need the resistance when the bulb is on/hot/bright/operational, and that you cannot do with an ohm-meter. You need to measure the current running through it at full operation, then given its rated voltage (assuming that is what you are running it at its rated voltage), divide that voltage by the current to get a good value of its resistance... and from that you can compute the resistance you need for the voltage drop you are trying to achieve. Just remember to size the resistor for the power it will need to dissipate to get that voltage drop ( Power = Volts accross * amps through). Or better yet, if you need to step down the voltage, run a special lighting voltage line, and get a small step-down voltage transformer that gets you from your supply voltage down to your needed end voltage and feed the lamps with that.

Better still would be to use bulbs rated at your supply voltage, but you seem to be past that option.

Thank you HGK,
I have 60 lamps that i can use but hope not to have to use them all. Will all depend on what spacing I give them and most probably I will have them spaced and then located at alternate sides of the road way. If I get a chance I will add a photo and measurements of the layout.

Regards Peter