Easy Mechatronics Based Projects: Power supply

Hey guys, here we are at the most basic subsystem of any system. The power source! In this post I will go through the procedure for building a simple power supply. When it comes to projects be it robotics or control systems you will need to provide a stable voltage to ensure that the system will operate without any issues. An unstable power supply can cause MCUs to reset unexpectedly. If you are working with audio amplifiers or any general purpose amplifiers a background noise will be generated if you do not filter the supply. In this post I will go through the procedure for building a simple power supply which should be satisfactory for most projects. So take a look

Before you go ahead there are a few basic terminologies which you must be familiar with

Voltage

Voltage is the potential difference between two points of a circuit like the + and - of a battery every component will have a maximum voltage beyond which it will fail to operate and may be permanently damaged. Cross check datasheet of the respective component.

Current

This is the flow of electric charge every device will have a current rating now unlike voltage current dose not enter the circuit just like that. Every device draws its rated current so an MCU that requires a 5V @ 30ma(milli amps = 1/1000A) can be connected to a 5v @ 1A supply. The voltage must be within the operating range. HOWEVER certain devices such as motors(not all), solenoids, high watt bulbs will require current limiters. this is because these devices will draw more current (more than what it can handle ) when started at full power. so a STARTER is required (initially) which limits the power going into the coils refer motor starter for more details. In robotics Brushless motors can draw huge amounts of power if directly started at full speed. However most speed controllers can monitor these conditions and provide a gradual acceleration so that the current flowing through the windings are under control. Another way to reduce the risk of burning the motors is to provide a smooth voltage control using PWM (pulse width modulation). so the motors can be started and stopped slowly(in milli seconds that is).

Polarity

This is denoted by a ‘ + ‘ and ‘ – ‘ sign. ‘ + ‘ is referred as the positive terminal and ‘ – ‘ is referred to as the negative terminal check the figures below

now moving on ->

Battery connections

Figure 1

Figure 2

The Figure 1 on the left has a single 9v power source when a multimeter is connected between the terminals ie ‘common to -’ and the other to ‘+’ u will get a reading of 9v. This is the simplest form of a battery arrangement. So 9v is great to operate LCD based projects (with a voltage regulator) But just not good enough for a heavy robot. Most relays and motors are designed to run from 12v and up (exception are there -> such as RC servo motors etc). In such a case we would go for a series connection (Figure 2) where one battery’s + connects to another battery’s ‘ - ‘ in this case the voltage would add up so if you connect two batteries of 9v @ 4.5A in series the available power would be 18V @ 4.5A notice only the voltage has doubled and not the current.

Now lets say you are operating a heavy load say an inverter or a UPS more than voltage you would need current in this case you would choose a parallel connection (Figure 3).

Figure 3

Here the ‘ + ‘ terminals of all the battires are connected together and the ‘ – ‘ terminals are connected together. So lets assume you have four 12V @ 7A battires (a typical UPS battery) connecting them in parallel would give you 12v @ 28Ah which mean your backup time would improve 4 times… however charging time will also increase by 4 times. Moreover this may over load the charging section of the UPS so you would need to provide a separate source for charging thereby reducing the load on the UPS . Anyway that was just an example back to our topic..

In case you need a dual polarity supply which means your power supply will have 3 output points ‘+, 0, -‘ You would need a circuit like Figure 4. Here two 9v battires are connected in series HOWEVER ground/ common/ 0v is not tapped from ‘ – ‘ but from the interconnection of both the battires. (Figure 4)

In this case

Figure 4

out1 = +9v

out2 = -9v

out3 = +18v

when you are working with these kind of power supplies (Figure 4&5) ensure that the common line is connected properly else it could result in component damage. These type of power supplies are used in amplifiers, computers (old ones) and so on. Most of the projects I post would use a single polarity supply

FIgure 5

A dual polarity voltage regulator based power supply can provide regulated voltages (+12 (not regulated), +5, -5, -12 (not regulated)) simply add more regulators to increase the voltage choices. (Figure 6)

Figure 6

Voltage regulators and filters

Now that you are familiar with the basic connections let’s move on to more fun stuff like using this power to our advantage. Now most MCUs will work on 5V, motors on 12V or 24V, LEDs on 3V and so on. So how would you get different voltages from the same supply? One way is to use resistors. For example to operate an LED from a 12v battery we would give a 1K 1/4W resistor in series to the LED. This is fine since it requires only a small amount of current. SO how on earth will you drive a servo motor which requires a power supply of 5V @ 2A? You would need a big rheostat (remember the ones in the physics lab?). Here we would use a voltage regulator. A voltage regulator basically regulates the voltage by internal circuity and ensures that only the prescribed voltage flows out. It is also more efficient than a rheostat since the rheostat dissipates energy as heat.

So how do you choose a voltage regulator? Simple! Look at the data sheet right?

Well first of you must ensure your requirements (like voltage, current, cycle)

Take a look at the classification below

Voltage regulators	Positive cycle	Negative cycle
Fixed	78xx series	LM 337
Variable	LM 317	79xx series

Note the ‘series’ check out the table below

Sl. No.	Voltage regulator	Output voltage
1	7805	5
2	7806	6
3	7809	9
4	7815	15
5	7818	18
6	7824	24

Sl. No.	Voltage regulator	Output voltage
1	7905	-5
2	7906	-6
3	7909	-9
4	7915	-15
5	7918	-18
6	7924	-24

Get the idea? The last two digits indicates the output voltage.

OK now lets move on to the final part of this post ill provide different diagrams for different types of supplies

simple bridge rectifier .... no regulator present the out put voltage is dependent on the transformer (good for a NON AUTOMATED lead acid battery charger)