Model Railway Forum

Hello

I have some 12v lights for the layout that are quite bright when connected directly to the 12v feed.

I have some resistors that I soldered but the lights are now incredibly dim!

I know resistors are colour coordinated and can post a pic of what I have if required but can someone advise me further please on types and the effect of various resistors.

Thank you

The brightness of your lights is going to be very subjective but the basic principal with resistive output devices like bulbs is halve the voltage will halve the output. This can be calculated using Ohm’s Law but other information such as the wattage or current draw of the lights is required.

However, there is a much easier way to get the right resistor. First you will require a multimeter, a cheap one linked below will suffice this should be a standard tool for railway modelling very useful for all sorts of fault finding:

https://www.amazon.co.uk/Multimeter-Bat ... 33&sr=8-40

Obtain something like a 1K potentiometer (1000 Ohms) you will probably get away with a lesser value. Connect the potentiometer in series with one of your lights and the power supply connecting the centre terminal and one of the outer terminals. With the power supply on adjust the potentiometer until you achieve the brightness you like, then use the multimeter to measure the resistance between the two terminals you connected it that will give you your required resistor. You will not be able to get an exact value to match but the nearest value above or below the reading on the multimeter will give as near as possible the same result.

Richard

Using the same principles but taking a slightly different approach - I supply each group of lights from a DC/Dc buck converter fed from a 12v DC bus. In simple terms these units (eBay or Amazon for about £5) take in 12v dc and include a potentiometer to adjust the output. Some have a digital display to show the output value. Switch them on, twiddle the screw adjuster and the output is fixed once it looks right.

I connected my string of lights to an old analogue controller speed knob output, twiddle that until I got the correct dimness and measured the resistance value - with it switched off at the wall. Next nearest resistor value was used.

Just as an aside, I do not know the value of your current resistors but if you use two and twist their wires together so that they are parallel that will in effect halve the resistance thus making the lights brighter. It’s worth a try you might get them to light at a suitable brightness.

Richard

dtb wrote: ↑Tue Oct 24, 2023 12:24 am Hello

I have some 12v lights for the layout that are quite bright when connected directly to the 12v feed.

I have some resistors that I soldered but the lights are now incredibly dim!

I know resistors are colour coordinated and can post a pic of what I have if required but can someone advise me further please on types and the effect of various resistors.

Thank you

Where 12 volts filament lamps are used on a 12 volt supply, the simplest method of reducing their brightness and increasing their longevity is to wire two lamps together in Series. Each lamp then receiving approx. 6 volts and will be reduced in brightness and reduced heat too.
Wire positive of 12v supply to one wire of first lamp then the other wire of that lamp connects to the next lamps wire and the second wire on that lamp goes to supply negative. With filament lamps it doesn't matter which wire of the two connects to supply or the other lamp. Down side is eventually when one lamp fails both will go out.

A 1K (1000 Ohm) resistor is often used in filament lamps on 12volts. Assuming a four band coloured resistor, then it is Brown -Black-Red the fourth colour denotes its % tolerance to the stated ohmage and is often Silver (10%) or Gold (5%). This calculator site is helpful Link to calculator Using a higher Ohm resistor will reduce brightness. Similarly a lower value will increase brightness.

Warning. Depending on the current flowing in circuit using a resistor will mean the resistor will get hot. Using a high wattage resistor will reduce this - 1Watt for example. But whatever resistor is used keep them away from anything flammable or that can be damaged. Cardboard of buildings etc or plastic which can melt!

One of these connected to a 12v DC input supply and feeding all of the lamps is perhaps a good idea - No heat, adjustable output voltage and a small display shows the volts out and its adjusted with the aid of a small flat blade screwdriver used on the blue trim pot. Link to voltage regulator from China. Or if preferred a similar item from a UK supplier, and it is just £1.22 more! UK seller voltage regulator link

dtb wrote: ↑Tue Oct 24, 2023 12:24 am Hello

I have some 12v lights for the layout that are quite bright when connected directly to the 12v feed.

I have some resistors that I soldered but the lights are now incredibly dim!

I know resistors are colour coordinated and can post a pic of what I have if required but can someone advise me further please on types and the effect of various resistors.

Thank you

You don't say whether the lights are filament or LED, this info would help in offering advice. If they are filament fitting a lower voltage bulb reduces the light and resistors for LED.

Mike

Hi all

Thanks for the input from everyone.
I had found some lighting I had purchased a while back, they are street / yard lights, some single and some double and some with resistors fitted whereas others not. These are some lights I have found in a box that I purchased a while ago.

All my layout lighting is working whether resistor fitted or not from one 12v supply. I have 3 engine sheds with int and ext lighting, water towers the same, street lighting, platform lighting for 2 stations, signal box int lights and more. This is where Brian's comments on lighting in series prompted me to create a second supply for the new lighting and to to look at the option to redistribute some of the other lighting.

I had a spare distribution board and found a voltage reg as well.

Cheers

What you really need to determine is whether the lamps are filament or LED?
Filament (Incandescent) can be obtained in various operating voltages and some may well be supplied as 12 volt or even 15 volt.
LEDs should always have a series resistor to limit current. Note there are some LEDs sold rated at 12 volts, but here they have a built-in resistor.

How do you tell the difference? Filament will always run hot to touch where LEDs remain virtually cool. Filament lamps work when connected to the supply either way around, LEDs only light one way (with the resistor fitted).

Advantages of Filament lamp There are none today really, other than they work either way around on the supply.
Disadvantages of filament lamps are - They run extremely hot and can damage plastic or burn card. They usually draw around 50 to 70 milliamps (0.05 to 0.07A) per lit lamp. Therefore a reasonable number of lit lamps can easily exceed 1.0Amp this then can cause some power supplies to trip out.

Advantages of LEDs They run virtually cool, they draw low current from the supply typically around 5 to 10 millamp (0.005 to 0.01Amp) per lit LED, so a large number can be feed from the same 12 volt DC power source as used for filament.
Disadvantages are they must have a series resistor and they only illuminate one way around on the DC supply.

that's great help thanks Brian.

See pics of the lights I refer to, tall single and double prefitted with resistors and short single and double that don't have resistors.

Looking at the lights am I correct that the tall ones appear to be LED and the short lights are filament?

As a side question, does anyone have any of the cream coloured Airfix lamp post bases they don't need as they help keeping the lamp upright due to only having a small circular base at the bottom.

Thanks again