How to make AC to DC Converter power supply?

Though we have seen few small circuits where we have connected more than one resistor, resistor with capacitor as well as cell or battery during learning these components, now we will build a basic power supply circuit to run 12V DC load like motor or lamp or any other circuit, and will also understand what each component contribute to get 12V DC from 220V AC supplied at our home.

We need below basic components:

• Center Tap Transformer 12V/500mA – 01no.
• Diode 1N4001 – 02nos.
• Capacitor 1000µf/25V – 01no.
• Resistor 67Ω - 01no. (optional)
• Zener 9.1V/0.5W – 01no. (optional)
• Some wires to power transformer and make connections.

Let us see how to make 12 volt DC power supply. We will make first DC power supply using center tap transformer, then will make same circuit using normal transformer. Caution: Handle 220VAC carefully as it is dangerous to life, don’t play with high voltage in absence of expert/guardian. A center tap transformer has three outputs in secondary side, one common (marked as 0v) and rest two marked as 12V. Since we are using a transformer to convert 220V to 12V, so we will use step down transformer i.e. primary winding will have huge number of turns in coil with thin wire and secondary winding will have small number of turns in coil with thick wire. Opposite to this step-up transformers are used in our home inverter where 12V DC source from battery is converted to 220V AC, this we will learn later.

So in the circuit we can see that the 220V AC supply is connected to primary of transformer, which is converted to 12V AC in its secondary side. So if you see the graphical representation, the waveform at primary side is sinusoidal wave with peak value of 220V and on the secondary side at point ‘B’ it is also sinusoidal wave with peak value 12V. The time period of these waves in primary and secondary side both are 20ms (Millisecond) as frequency of AC supply in India is 50Hz as we learnt in our previous posts.

During positive half of AC cycle, top diode ‘D1’ conducts as it gets positive supply at it anode (forward biased condition) hence it conduct and allows whole positive cycle to pass thru it to point ‘A’, and in the second half of cycle, lower diode ‘D2’ conducts hence passing the negative cycle to point ‘A’, hence we get the second negative cycle as positive cycle at point ‘A’ which we can see in the graphical representation, here each positive cycles are completed in 10ms hence the frequency is now 40Hz (doubled) as each cycle at point ‘A’ if we consider capacitor is not connected in circuit, otherwise point ‘A’ and ‘B’ are on same wire hence should have same waveform. Just for understanding function of each component in this circuit we are isolating capacitor. So, we can see from wave form in graph that we have converted the negative cycle of input supply into positive cycle. Now we have DC supply but very much fluctuating as it rises to 12V and falls down to 0V and cycle goes on, we need to smoothen this variation to get clean DC output.

Now we know that capacitor charges and discharges when required, hence now consider that the capacitor is connected, so when the voltage rises at point A to peak, the capacitor is fully charged and when the voltage starts falling after 5ms, the capacitor starts discharging thru any load connected to this power supply (currently not shown here, to be connected between point B and C). Even if the load is not connected, it slowly discharge but may take long time. As it discharges the next cycle comes up and again charges the capacitor to 12V and this way the cycle continues till 220V AC supply is provided to transformer input. So, now we can ignore the original highly fluctuating output at point ‘A’ which is shown as dotted lines in last graph which has very less fluctuation that what we saw at point ‘A’ the variation in voltage has been minimized by capacitor as it is working as a small rechargeable battery to feed power during falling part of cycle. Now we have DC supply between B and C which is near to 12V and with minimum fluctuation but still not very smooth DC supply.

Though this supply is enough to run small 12V motor or 12V lamp, but may show extra noise if we run some audio equipment with such supply, so let us see how we can regulate it further and smoothen the output graph. Note since operating voltage is 12V here hence we have selected capacitor have maximum voltage limit of 25V i.e. double the operating voltage. Since we have 12V, 16V, 25V capacitors available, hence we selected 25V.

In the second circuit with normal step down transformer, we do not have the center tap hence we need to use four diodes in the given fashion this is called bridge rectifier or bridge diode (diodes are often referred as rectifier). If we track properly during positive cycle    when top end of secondary coil is positive, diode D2 conducts as its anode is getting positive supply the current continues to flow the capacitor to charge it and then return to transformer thru diode D3 and the cycle completes. In the negative cycle, lower end of secondary coil is positive hence D4 conducts and charges capacitor and the current returns back to transformer thru D1 and the cycle completes. The capacitor works same as previous circuit.

Though not much used now a days, but for understanding purpose we will use

zener diode to regulate (smoothen) the DC output. Now a days lot of voltage regulator IC are available, which are cheap and very easy to use. Let us connect one resistor and a zener diode as shown in next circuit. This will be connected to 12V output i.e. point ‘A’ and ‘B’. Since we have 12V DC supply available with us, we will make 9.1V regulated DC supply. I have taken 9.1V zener as this is standard available in market near 9V. The resistance of 47Ω limits the current thru zener so that it operates in safe range. The 12V output is fed to zener diode thru this resistor hence the zener operates in breakdown region and hence will maintain 9.1V across it hence we get fix 9.1V DC out without fluctuation at point ‘E’ and ‘F’. Similarly if we need 12V regulated DC output, we need zener of 12V but then transformer should be of 16V and capacitor should have maximum voltage capacity of 32V or above.

So, here we made and understood the working of AC to DC converter to convert 220V AC supply to 12V DC and 9.1V regulated DC supply. 

Return to List of content