Battery Box / Power Supply FAQ
What power supply do I need for my scale model project?
The best power supply for your project depends mainly on a couple of factors and is generally a personal preference rather than anything set in stone.
1) Do you want to power it by a battery box or a mains socket?
2) What space do you have inside or around the model to house a power supply?
3) Will the lights be on for a few minutes to show off your model or for extended periods of time, say on display or in a museum etc.
Are 12 volt LEDs brighter than 3 volt LEDs?
I get asked this question a lot and the answer is no. The input voltage has no influence over the brightness of the LED. Without getting too technical the resistor (current limiting resistor) limits the current throuugh the LED, it's value calculated based on the colour of the LED and the input voltage. The higher the input voltage is the higher value the resistor needs to be but the current through the LED is always the same, therefore the brightness is always the same.
Okay, so why do 12 volt LEDs (mains supply) sometimes look brighter?
What can make a difference is using batteries versus a mains supply. Batteries will drop voltage over time as they get used up to the point where they will no longer have enough voltage to light up your LEDs. A 12v regulated mains supply always delivers exactly 12 volts and so the LEDs will never dim. Fresh batteries would resolve the problem.
How do I know what battery box to use?
In most cases, a 3v AAA battery box will be the best option. It is a good compromise between compact size and reasonable longevity of the batteries. In some cases, voltage may be pre-determined by other components in the model, say 6v in a remote comtrol car for example, or if you wanted to combine LEDs with standard 12v bulbs in a dolls house. If you have lots of space and prefer to use a different battery box then go with what works best for you and your model.
One misconception can be around 9v batteries. 9v PP3 batteries are not very high mAh compared to AAA's or AA's so will not last as long. They are generally used as back up batteries or low power applications. eg, a smoke alarm.
How long will my batteries last?
It is not an exact science and the battery chemisty, brand, type of battery all have an impact so it is impossible to give accurate durations, however, there are battery life calculators online and as a rough guide you total the mAh of the batteries and divide by the total current of the LEDs to get a rough approximation.
eg: 3v AAA batteries - (1200 x 2) / (20mA x number of LEDs) = number of hours
I found 6 LEDs lasted for 7 days left on 24/7 on a 3v AAA battery box but they will dim as the batteries run out.
The simple rule is the more LEDs you run off a battery box, the quicker the batteries will run out.
What's the difference between a 3v AAA battery box and a 3v CR2032 coin cell?
3v coin cells such as the CR2032 are low power batteries, usually used in car remotes and as back up batteries. They are not designed for realtively high power drain applications like powering LEDs becasue the mAh capacity of the battery is very small (approx 150-200mAh). However, scale models can be very small and sometimes that's the only size of battery that will fit, or even smaller ones such as the CR1220 12mm battery. What I suggest is limiting the drain on the battery to a maximum of 5 LEDs. My tests showed that although you can run more LEDs from them, the overall brightness of the LEDs was reduced and the battery only lasted a very short time. Coin cells will typically last 2-3 days if left on continuously.
In contrast, a 3v AAA battery box has 2 x AAA's which total 2400mAh so they can easily run 10 LEDs or more and if only on for short periods of time will last for months or even longer depending on usage.
When should I use a mains transformer?
Mains transformers are typically availabe in 5v and 12v options. They plug into a mains socket and regulate the output voltage to 5v or 12v. Most modern transformers are regulated but you msut ensure this is the case or the voltage can spike too high and blow your LEDs. They are ideal if you want to leave your lights on for extended periods of time, say for a display at an event or if you have them on every evening for several hours. In these cases batetries would run out too quickly or not be reliable enough for long term use.
Sometimes, mains supply is also just more convenient, if your model is next to a mains socket for example.
There are no hard and fast rules. Use what works best for you and your model.
What voltage of LEDs should I choose?
Always choose the same voltage of LEDs as the power supply. So if you are using a 3v battery box, choose 3v LEDs. Using a higher voltage power supply will fry your LEDs.
The only exception to this is if you want to dim your LEDs and might therefore choose a lower voltage for the LEDs than the power supply. This effectively means a lower current passes through the LED so it's dimmer. For example you could choose 12v LEDs and run them off a 9v battery. They will be half as bright. However, at some point the LEDs won't light up at all or will be extremely dim so I wouldn't suggest using a 12v LEDs with a 3v battery box.