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darbyvet

#1 Posted : 29 November 2020 02:44:51

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The next thing we need to consider is how are we going to power your electronics.
There are a few things we need to consider;

1. How much room do you have?
If you have a very small model you will either need a very small battery or an external power supply.This may be a battery stored in the base with wires feeding power into the model or a small batteey like a coin cell or watch battery.

2.What voltage do you need?
Once you have designed your circuit you should know what voltage you will need to power it.If you are just using a couple of LEDs you may be fine with 3v from 2 AAA batteries or a watch battery.If you have a lot of LEDs ,especially if you are using LEd strips you may need 12V. It would require a lot of batteries to get 12v.There are ways around thgat by using linear voltage regulators which we will discuss in another tutorial.A better solution would be a 12V DC power supply.

3.How much current do you need?
Again once your circuit is finished you should be able to calculate the current you will need.IF you are not sure you can measure the current draw using a multimeter.That was demonstrated in the "how to use a multimeter" tutorial.If you are using a bunch of LEDs you probably wont need a high current capacity pwoer source since LEds usually use about 20mA current.So if you had a circuit with 20 LEDs for you model you would need 400mA current.It is always a good idea to have a power supply with double the capacity of your circuit.Batteries and power supplies have a rated amp output, but it is maximum output and cant be constantly produced by the power supply or battery .

4.How long do you need the circuit to be on?
If you are planning to just light up the model occasionally it will not be a big drain on your power supply so you can use a battery with a lower power output capacity.IF you plan on leaving the model on for hours at a time you will probably need a DC power supply

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darbyvet

#2 Posted : 29 November 2020 02:53:09

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A little bit of electrical theory.I think it helps to get your head around power supplies.

I think of electricity like a water system in your house.The water is stored in a tank.The water flows through the pipes from the tank to the sink.

Voltage is like the water pressure in your system (size of the tank)

Current is the flow rate of the water through your pipes.

Resistance in the size of the pipes that the water flows through.

So controlling the power you need for you device is determined by the water pressure (voltage), flow rate (current) and resistance to flow (resistance).

The key is to match the power source that best fits the power requirement of your circuit.

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darbyvet

#3 Posted : 29 November 2020 03:24:34

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BATTERIES

Batteries are the simplest power sources.

The capacity of the battery tells you how much current it can deliver over a period of time before it is drained.It is measured in milliamp hours (mAh)

You can calculate how long a battery will last if you know the current draw of your circuit using the following equation;

Battery capacity/Current draw= Battery life

So if you have a battery rated at 800mAh and your circuit draws 400ma the battery life would be 800/400=2. So it would take 2 hours to drain the battery in theory.In practice it is less because once the battery starts to drain the voltage drops and the battery will decrease its current output.There are charts online which give you the exact amount of time a battery will last.

Button cell batteries provide 3v output and carying current from 25mA up to more than 1000 mA.Thye are perfect for very small models and circuits that dont draw a lot of current.

Regular batteries.
These are the classic AA, AAA,C,D etc... They come in varying voltages and current capacity.For a mid sized model thay can stored inside the model, or they can be stored in the base and power fed into the model through 2 wires. Here are the current capacities for the most popular batteries

9V 5 70mAh
AA (1.5V) 2870mAh
AAA (1.5V) 1150mAh
C (1.5V) 7800mAh
D (1.5V) 17,000mAh

You can combine batteries to change the voltage output and increase capacity.

Note that 9V batteries are pretty wimpy when it comes to capacity so although they are tempting to use because they are small if you have a lot of LEds the battery wont last very long.

Lipoly batteries
The newest kind of battery are the lipoly and Li-ion batteries.These are very popular in RC models because they have a very high capacity and can provide a lot of current.They can be quite dangerous though if not handled properly and can even set on fire if they are damaged.They also require special chargers and you need to monitor their volatge very carefully because if you let tghe volatge get too low you wont be able to recharge them.Unless you need a very high current discharge rate or you need a powerful battery in a small model I would not use a Lipo battery.

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darbyvet

#4 Posted : 29 November 2020 05:04:02

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DC POWER SUPPLIES

DC power supplies are a great way to power model lighting circuits. They come in various different voltages and amperages.Thye produce a very clean regulated DC output and never run out of power.They are more expensive than batteries and have to be outside the model due to their size.

All you need to do is pick the pwer supply that gives you the current and voltage you need. For a model with a few LEDs a 9V 1 AMP DC power supply is probably fine.If you are using a lot of LED and motors or microcontroller you may need a 5V 2A power supply.If you are using really high current requirement devies like stepper motors or neopixels you may even need a higher rated DC power supply like a 5A or 10A.

Polarity

One thing you need to be careful of with DC power supplies is polarity. An electrical circuit has a positive and negative side. You need to make sure the positive feed from the power supply is connected to the positive side of the circuit and the negative (ground) of the power supply is connected to the negative (ground) of the circuit.IF you get these connections wrong you may damage your circuit beyond repair.

ALWAYS CHECK THE POLARITY OF YOU DC POWER SUPPLY BEFORE CONNECTING TO YOUR CIRCUIT

So how do you figure out the polarity of a DC power supply?

There are 2 methods;

In USA you can look at the back of the power supply for some symbols to help.I am not sure if it is the same in UK, but if it isnt you can use the second method .

The back of the power supply will have the input voltage (100-240V) ;
Underneath that should be the OUTPUT voltage and max current (CD +12v 5A).The symbol for DC power is a long line with 3 shorter lines underneath.

The next symbol consists of 3 circles.There is a dot in the center circle and a "-" in one circle and a "+" in the other.This symbol tells you the polarity of the power supply.IF the plus is in the right circle the power supply is positive polarity.

When we say positive polarity what that means is the tip of the output jack is positive and the barrel of the output jack is negative.So when you solder the socket that the jack plugs into to your circuit you know that the center of the socket will get the positive feed and the outside ring on the socket will get the negative output.

In many cases you can buy a power supply that has the corresponding socket with the connectors labelled for you.

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darbyvet

#5 Posted : 29 November 2020 05:26:54

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The second way to check polarity is to use a multimeter.

This is a power supply I bought for a project.It came with a socket to plug the jack into.The socket has the 2 terminals marked as + and -. This is a center positive polarity DC power supply that provides 5v output with 10A current.

So in theory All I need to do is connect the socket to ther circuit with the positive of the socket going to the positive side of the circuit and the negative from the socket going to the negative or ground of the circuit.

Now this supply has a lot of current output.It would surely damage any circuit if it was not correctly wired.

So I tested the power supply with the multimeter.Here is the procedure step by step;

1.Switch the multimeter to V.

2.Connect the wire coming out of the positive terminal of the socket to the red probe of the multimeter.

3.Connect the wire coming out of the negative terminal of the socket to the black wire of the multimeter.

4.Plug in the power supply to the socket and then into the mains power.

5.The voltage on the multimeter reads 5V.

So this tells me the power supply is indeed center positive and the labelling on the socket is correct.

But what would happen if the power supply was mislabelled or the socket was the wrong type.

I connected the positive output from the socket to the negative (black probe) of the multimeter and I connected the negative output from the socket to the positive (red probe) of the multimeter.When I plugged in the power supply this time the multimeter it reads -5v.Them minus sign tells you that the polarity is reversed.

So once you have tested your power supply and socket you should know the polarity of the power supply and which terminal of the socket is the positive and which is the negative.

I always recommend testing the polarity of your dc power supply.They are not all the same in my experience and the last thing you want to do is destroy your circuit.

The next section will be discussing the different types of LEDs and how to connect them to make a circuit.That will be the last theoretical tutorial.Once we have discussed LEDS we will start making some models with lighting effects. We will start of with simple lighting effects that dont require circuit boards and then move on to more complex lighting systems that use circuit boards and electrical components and finally we will move on to using microcontrollers to create much more sophisticated electronic effects inclduing the use of sound effects and motors.

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