In this post we are going to explore about power inverters that can power small AC appliances from a DC source which can be used for camping outdoors or during an emergency like long or short power outages.
In this post we will be exploring: what a power inverter is, types of power inverters, how a power inverter works and its circuit diagram.
What is a power inverter?
A power inverter is an electrical device which “inverts” a DC source (typically 6V, 12V, 24V or 48V battery) to a standard 230V AC at 50 Hz or 120V AC at 60 Hz or in other words a power inverter takes a DC input and outputs AC at a higher voltage than the input.
An inverter has a limited power output capacity and also has a limited operating time i.e. until the DC source (battery) runs out of power. We can power an inverter with not just batteries; we can also use solar panels, small DC hydro generators, windmills, even fuel cells, but typically most power inverters that we can find at homes and offices utilize deep discharge lead acid battery or lithium based battery.
What are the types of inverter?
A power inverter can be classified in the following ways:
1) Their output AC wave form.
2) The transformer technology.
Classification by AC output waveform:
A power inverter can be classified according to its output AC waveform. Power inverters output pure sine wave, square wave and modified sine wave. Below we have illustration and description:
Square wave:
The square wave type is the noisiest of the three but they are very easy to design, this was the first inverter type that achieved DC to AC inversion without any mechanical parts decades ago, before that we used DC motor and AC generators and they were called as electromechanical inverters.
Square wave inverter is suitable for resistive loads, loads that utilize SMPS for AC to DC conversion like chargers and small inductive that consumes power less than 100 watts like table fans, BLDC ceiling fans. Square wave inverter produces humming noise in inductive loads.
Modified sine wave:
The modified sine wave is sometimes also called modified square wave. The wave form has step shape to reduce the noise at the output, this is achieved by introducing pause before changing the polarity of AC; the step can be composite of multiple steps to reduce the noise further.
Modified sine wave is easier to design than pure sine wave type and cost effective too, this type of inverter can support most AC loads but on some inductive loads like motors, fans, we can hear (humming) buzzing noise from its coil winding and also heats a bit more than pure sine wave. All non-sine wave inverter available in the market are this type.
Sine wave:
Sine wave is the least noisy inverter when it comes to noise at its output. They are difficult to design and expensive when compare to other two types. Its output wave is similar or identical to AC mains and all types of AC loads are suitable with this type of inverter.
Classification by transformer technology:
Inverters can be classified based on the transformer type:
1)Iron core transformer.
2)Ferrite core transformer.
Iron core: As the name suggests, the inverter utilizes iron core transformer for stepping-up low voltage AC to high voltage AC and the conversion was done at 50 Hz or 60Hz. Iron core transformers are bulky and weighs more, this type of inverter is deployed where weight or mobility is not a problem like inverter installations at houses and offices buildings.
How an inverter works:
Any power inverter has the following essential stages:
1)DC source
2)Oscillator
3)Amplifier
4)Transformer (step-up)
5)Feedback system
DC source: It can be any DC source like battery / solar panel / windmill etc.
Oscillator: Oscillator is the stage responsible for converting DC into AC at low voltage level, at this stage the output frequency is determined at 50/60Hz.
Amplifier: The amplifier stage is responsible for strengthen the weak signal from the oscillator. The amplifier stage consists of power transistors or MOSFETs.
Step-up transformer: Step-up transformer is responsible for converting a low voltage AC from amplifier stage to high voltage (230V or 120V) AC.
Feedback: Feedback stage is responsible for taking samples of output voltage and feeding it to oscillator for automatic voltage correction.
Circuit diagarm:
Warning: The output is 230V AC, extreme care must be taken while prototyping and also while using it.
Circuit description:
The proposed circuit is simple consists of few active and passive components. The above inverter circuit has all the stages described in the previous section except the feedback stage and without one this inverter works with no issue unless you don’t overload the output, which can reduce the output voltage to a large extent, but this don’t pose any harm to the inverter.
The IC 4047 is configured as astable multivibrator which produces square wave at pin #10 and 11. The frequency of this stage is very important as it is going to determine the frequency of output AC. Use the provided 100K potentiometer to bring the output frequency to 50Hz / 60Hz depending on your country.
The IC 4047 has one drawback, when the battery voltage falls during normal operation of the inverter, the output frequency varies or in other words IC 4047’s output is voltage dependent. To overcome this issue we have used a fixed voltage regulator 7805 which provides a steady 5V for IC 4047.
The output of IC 4047 is fed to the amplification stage and this stage consists of BJTs and MOSFETs. BJT is responsible for converting 5V signal to 12V signal, in other words voltage amplification, since MOSFET is a voltage controlled device it needs sufficient voltage (above 10V) to operate efficiently.
The MOSFET is responsible for current amplification; now at the drain terminal of the MOSFETs we have sufficient voltage and current to drive the transformer.
The transformer converts the 12V oscillation to 230V AC output. At the output of transformer a MOV or metal oxide Varistor is connected, this is for arresting any high voltage spike that transformer could give out during start-up of the inverter and also while switching the loads ON and OFF. Please don’t ignore this component; it is for protection of the connected loads.
A 5A fuse is connected between transformer’s center tap and +12V supply, this will prevent short circuit if the oscillator stage fails to start.
The maximum power output of the inverter is 45 watts for 9V / 5A transformer and you can increase the power output by upgrading the transformer to 10A or 15A or more. Please increase the current rating of the fuse accordingly. Use at-least 7Ah battery or more.
What type of loads can I connect to this inverter?
You can connect any resistive loads, such as tungsten lamp, inductive loads like table fan under 100 watt (you may hear humming noise), loads that use switching power supplies like AC to DC adapters, chargers, LED lamps and LED batten etc.
NOTE: You should immediately disconnect any load if it misbehaves with this inverter’s output and NO medical or life saving device should be used with this inverter.
