In this post we are going to learn how to construct a simple transformer-less inverter circuit which can power loads up to 1000 watts.
We will see:
- What is Transformer-less inverter?
- Advantages of transformer-less inverter
- Stages of Proposed Transformer-less inverter
- Circuit Diagram
- Block Diagram
- How to Test and Operate this Circuit
- RMS calculation of this Inverter
What is Transformer-less Inverter?
As the name suggests, the inverter is not equipped with a standard step-up transformer like traditional inverter for converting low voltage AC to high voltage AC.
Transformer-less inverters utilizes high voltage DC as input which is converted to high voltage AC output by oscillators and H-bridge drivers, since there is no transformer great efficiency more than 97% is achievable, the only loss is from the MOSFETs of H-bridge. The high voltage DC source can be solar panels or battery.
As the transformer-less inverter cannot boost the voltage to power nominal 230V/120VAC appliances, DC current of 120V or 230V or more is applied at input.
Brief Insight on High Frequency Transformer Inverter:
There are products marketed as transformer-less inverter with a tiny transformer in it, which can step-up from 12VDC to 230VAC, these are called high frequency transformer inverter, but they are not truly transformer-less, yet the terms can used interchangeably, since the real life efficiency, weight and other advantages are similar.
You can build a High Frequency Transformer Inverter Circuit, click here
The high frequency transformer works like this: The low voltage DC is converted to high frequency AC and step-up by the tiny ferrite core transformer, ferrite core transformer can handle much higher power at better efficiency than bulky iron core low frequency transformer.
The high frequency AC is converted to high voltage DC around 320V and fed to high voltage H-bridge and switched at 50/60Hz in sine PWM form or SPWM form and you get 230VAC sine wave.
The high frequency transformer inverters are not bad either, they offer light weight and good efficiency above 90%.
Now you know the difference between a true transformer-less inverter and high frequency inverter which looks like transformer-less.
Advantages of Transformer-less inverter:
In this section we are going to understand why we need a transformer-less inverter, this can also be applied to high frequency transformer inverters.
Transformer-less inverter are relatively inexpensive due to absence of bulky iron core transformer which is the most expensive part of the inverter and now, there are no losses related to transformer so more efficiency.
Now the inverter gets lighter and makes it portable which makes less challenging to install.
Stages of Proposed Transformer-less Inverter Circuit:
The proposed inverter is very simple, it consists of stages:
- DC Power Source/Battery Bank
- Oscillator / Multivibrator
Full Circuit Diagram:
We have designed yet another best Transformerless inverter circuit which can output modified sine wave at 230VAC, click here for the circuit diagram.
Block Diagram of Transformer-less Inverter Circuit:
DC power source:
The power source / battery bank consists of (12V 7Ah) 19 batteries connected in series. A fresh fully charged lead acid battery reads 13V so 13 x 19 = 247 VDC output.
The combined 19 batteries gives total power of 12V x 7Ah x 19 = 1596 watt hour (Wh) of energy.
For those who home’s power supply is 120VAC, you can connect (12V 7Ah) 10 batteries in series.
Which gives 13V x 10 = 130VDC.
The combined 10 batteries give total power of 12V x 7Ah x 10 = 840 Watt hour (Wh).
The excess battery voltage will get drop due to the MOSFET and you will get nominal operating voltage of 230V and 120VAC.
When the battery reaches 11.1V per battery you can consider it as low battery condition.
The AC voltage output at low battery will be 11.1 x 19 = 210.9V or 11.1 x 10 = 111V
So from full battery to low battery condition the output varies approximately 36V for 230V system and 18V for 120V system. The connected electronic appliances will happily work in that voltage ranges.
You can also replace batteries with appropriate rated solar panels.
NOTE: There is a separate 12V battery for the oscillator.
The oscillator is the circuit which generates appropriate waveform for the inverter’s output. Here we are using a simple Astable multivibrator using NPN transistors. The oscillator is powered separately from a 12V battery.
The circuit is tuned to generate 50Hz square wave output but due to the tolerance of the resistor and capacitor network we will get close to 50Hz. For 60Hz you can replace R2 and R3 resistors with 25K ohm resistor.
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The frequency of this Astable multivibrator can be calculated by:
F = 1 / 1.38 x R x C
F is the Frequency in Hz
C is capacitance in farad
R is resistance in ohm
Output of this Astable multivibrator circuit:
The H-bridge is the one which converts the high voltage DC to high voltage AC current, the oscillator switches the H-bridge.
The H-bridge consists of four power MOSFETs, couple of IRF740 and couple of IXTP10P50P which are rated at 400V 10A and -500V -10A respectively, which is enough for this inverter.
Now let’s see how an H-bridge functions:
The H-bridge changes the polarity across the load which makes direct current to alternating current.
In the left image above, S1 and S3 are closed, now the power flows from S1 to load and through S3 to complete the circuit, note the polarity across the load.
Now look at the right image above, the S2 and S4 are closed. Now the power flows from S4 then to the load and completes the circuit through S2, now look at the polarity across the load which is reversed from previous cycle.
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This cycle continues and generates alternating current in square wave form.
At no instant S1 and S2 should be ON simultaneously, if such instants occur it will lead to short circuit, this is same with S3 and S4.
The switches are replaced with MOSFETs as shown below:
At points “A” and “B” where the oscillator’s input is applied. A fuse is placed at +Ve rail of high voltage input to avoid short circuit.
Specifications of MOSFETs:
IRF740 N-Channel MOSFET:
- Voltage Drain to Source (Vds): 400V
- Voltage Gate to Sourec (Vgs): +/-20V (Max)
- Continues Drain Current: 10A (Continuous)
IXTP10P50P P-Channel MOSFET:
- Voltage Drain to Source (Vds): -500V
- Voltage Gate to Sourec (Vgs): +/-20V (Max continuous)
- Continues Drain Current: -10A (continuous)
Note: If you couldn’t find exact MOSFETs you can substitute with any equivalent specification.
How to Test and Operate this Circuit:
You need not to purchase the 19 (12V 7Ah) batteries to test the proper operating of this inverter circuit. You can do this in inexpensive way, follow the instructions below:
- Construct the circuit fully with necessary fuse and switches.
- Purchase 28 (fresh 9V batteries) one for the oscillator circuit and 27 for H-bridge.
- Connect the 27 (9V batteries) in series in this manner (it will be a lengthy battery):
- Solder the +Ve and –Ve with thick wires. Now connect the battery terminals to a 40 Watt 230V tungsten bulb. It should glow at full brightness. If it glows, you have connected the batteries properly.
- Now connect the high voltage DC from battery to H-bridge and connect the separate 9V battery to the oscillator and connect the 40 Watt bulb as load at H-bridge.Now, turn ON the oscillator by pushing/sliding “switch 1” first and then turn on the “switch 2” which will power the H-bridge. Reversing the switching sequence may lead to short-circuit.
- As soon as you turn ON the switch 2 the light bulb should glow immediately at full brightness, now your inverter is ready. Now, you can test with other AC loads.
RMS calculations for this Inverter:
Let’s understand what RMS is and why we need to consider in AC current.
Did you know that your 230VAC at your home are not actually 230V peak to peak but, 320V peak to peak? When we say 230VAC, we talk about the average value of the peak to peak amplitude of the AC current or RMS value in short.
Let’s we understand with an example:
If you have a bulb rated at 230V, you are powering it by using 230VDC and sine wave 230VAC peak to peak, which type of current will make the bulb glow brighter, AC or DC?
The right answer is DC with 230V. The AC 230V peak to peak is equivalent to 162V which is the RMS; if you measure the 230V peak to peak with voltmeter it will read 162V. In this case the bulb will glow at the brightness of 162V.
To glow the bulb at same brightness as 230VDC we have to apply peak to peak 320V for sine wave and now the voltmeter will read 230VAC.
RMS for sine wave is calculated by:
RMS = Vpk / square_root(2) where, Vpk is peak to peak voltage.
If it is not pure sine wave, there will be deviations in calculated value. Different waveforms will have different RMS formals.
Now let’s see the same for square wave, and should we apply more than 240 voltage at input to get nominal operating voltage (RMS) of 230/120VAC?
The RMS of square wave is given by:
Vrms = Vpk x square_root (F x T)
Vpk is peak to peak voltage
F is the frequency
T is the time period
For square wave with 50% duty cycle the square_root (F x T) is always = 1.
So the equation becomes Vrms = Vpk (RMS is same as peak to peak value of square wave)
Here we are inputting voltages approx. 240V from batteries, so the RMS at the AC output will also be 240VAC (for square wave). If we were applying pure sine wave to H-Bridge then we have to apply 320VDC at input to get 230VAC output. Similarly different voltage at input if we apply modified sine wave to H-bridge.
Advantage of this inverter circuit:
- Simple circuit design with few components
- High efficiency greater than 90%
- Solar panel compatible
- No need for automatic voltage regulation for most part, as the AC load is directly placed on battery.
Disadvantage of this inverter circuit:
- Square wave is not suitable many medical and sensitive electronics equipments.
- Need huge battery bank / long solar panel array to meet the voltage needs.
- Need separate battery source for oscillator.
- Battery discharge rate affects output AC voltage (240V to 210V).
If you have any question regarding this project, please feel free to express in the comment, you can anticipate a guaranteed reply from us.