The post explains a 3 powerful yet simple sine wave 12V inverter circuits using a single IC SG 3525. The first circuit is equipped with a low battery detection and cut off feature, and an automatic output voltage regulation feature.

This circuit was requested by one of the interested readers of this blog. Let's learn more about the request and the circuit functioning.

Design#1: Basic Modified Sine

In one of the earlier posts I discussed the pin out functioning of the IC 3525, using the data, I designed the following circuit which is though quite standard in its configuration, includes a low battery shut down feature and also an automatic output regulation enhancement.

The following explanation will walk us through the various stages of the circuit, let's learn them:

As can be witnessed in the given diagram, the ICSG3525 is rigged in its standard PWM generator/oscillator mode where the frequency of oscillation is determined by C1, R2 and P1.

P1 can be adjusted for acquiring accurate frequencies as per the required specs of the application.

The range of P1 is from 100Hz to 500 kHz, here we are interested in the 100 Hz value which ultimately provides a 50Hz across the two outputs at pin#11 and Pin#14.

The above two outputs oscillate alternately in a push pull manner (totem pole), driving the connected mosfets into saturation at the fixed frequency - 50 Hz.

The mosfets in response, "push and Pull the battery voltage/current across the two winding of the transformer which in turn generates the required mains AC at the output winding of the transformer.

The peak voltage generated at the output would be anywhere around 300 Volts which must adjusted to around 220V RMS using a good quality RMS meter and by adjusting P2.

P2 actually adjusts the width of the pulses at pin#11/#14, which helps to provide the required RMS at the output.

This feature facilitates a PWM controlled modified sine waveform at the output.

Automatic Output Voltage Regulation Feature

Since the IC facilitates a PWM control pin-out this pin-out can be exploited for enabling an automatic output regulation of the system.

Pin#2 is the sensing input of the internal built in error Opamp, normally the voltage at this pin (non inv.) should not increase above the 5.1V mark by default, because the inv pin#1 is fixed at 5.1V internally.

As long as pin#2 is within the specified voltage limit, the PWM correction feature stays inactive, however the moment the voltage at pin#2 tends to rise above 5.1V the output pulses are subsequently narrowed down in an attempt to correct and balance the output voltage accordingly.

A small sensing transformer TR2 is used here for acquiring a sample voltage of the output, this voltage is appropriately rectified and fed to pin#2 of the IC1.

P3 is set such that the fed voltage stays well below the 5.1V limit when the output voltage RMS is around 220V. This sets up the auto regulation feature of the circuit.

Now if due to any reason the output voltage tends to rise above the set value, the PWM correction feature activates and the voltage gets reduced.

Ideally P3 should be set such that the output voltage RMS is fixed at 250V.

So if the above voltage drops below 250V, the PWM correction will try to pull it upward, and vice versa,  this will help to acquire a two way regulation of the output,

A careful investigation will show that the inclusion of R3, R4, P2 are meaningless, these may be removed from the circuit. P3 may be solely used for getting the intended PWM control at the output.

Low Battery Cut-of Feature

The other handy feature of this circuit is the low battery cut off ability.

Again this introduction becomes possible due to the in built shut down feature of the IC SG3525.

Pin#10 of the IC will respond to a positive signal and will shut down the output until  the signal is inhibited.

A 741 opamp here functions as the low voltage detector.

P5 should be set such that the output of 741 remains at logic low as long as the battery voltage is above the low voltage threshold, this may be 11.5V. 11V or 10.5 as preferred by the user, ideally it shouldn't be less than 11V.

Once this is set, if the battery voltage tends to go below the low voltage mark, the output of the IC instantly becomes high, activating the shut down feature of  IC1, inhibiting any further loss of battery voltage.

The feedback resistor R9 and P4 makes sure the position stays latched even if the battery voltage tends to rise back to some higher levels after the shut down operation is activated.

SG3525 inverter with output correction control Circuit

Parts List

All resistors are 1/4 watt 1% MFR. unless otherwise stated.

R1, R7 = 22 Ohms
R2, R4, R8, R10 = 1K
R3 = 4K7
R5, R6 = 100 Ohms
R9 = 100K

C1 = 0.1uF/50V MKT
C2, C3, C4, C5 = 100nF

C6, C7 = 4.7uF/25V
P1 = 330K preset

P2---P5 = 10K presets

T1, T2 = IRF540N

D1----D6 = 1N4007

IC1 = SG 3525

IC2 = LM741

TR1 = 8-0-8V.....current as per requirement

TR2 = 0-9V/100mA Battery = 12V/25 to 100 AH

The low battery opamp stage in the above shown schematic could be modified for a better response as given in the following diagram:

SG3525 inverter circuit with low battery cut off

Here we can see that pin3 of the opamp now has it's own reference network using D6 and R11, and does not depend on the reference voltage from the IC 3525 pin16.

Pin6 of the opamp employs a zener diode in order to stop any leakages that might disturb pin10 of the SG3525 during its normal operations.

R11 = 10K
D6, D7 = zener diodes, 3.3V, 1/2 watt

Circuit Design#2:

In the above section we learned the basic version of IC SG3525 designed to produce a modified sine wave output when used in an inverter topology, and this basic design cannot be enhanced to produce a pure sinewave waveform in its typical format.

Although the modified squarewave or sinewave output could be OK with its RMS property and reasonably suitable for powering most electronic equipment, it can never match the quality of a pure sinewave inverter output.

Here we are going to learn a simple method which could be used for enhancing any standard SG3525 inverter circuit into a pure sinewave counterpart.

For the proposed enhancement the basic SG3525 inverter could be any standard SG3525 inverter design configured to produce an modified PWM output. This section is not crucial and any preferred variant could be selected (you can find plenty online with minor differences).

I have discussed a comprehensive article regarding how to convert a square wave inverter to a sinewave inverter in one of my earlier posts, here we apply the same principle for the upgrade.

How the Conversion from Squarewave to Sinewave Happens

You might be curious to know regarding what exactly happens in the process of the conversion which transforms the output into a  pure sinewave suitable for all sensitive electronic loads.

It is basically done by optimizing the sharp rising and falling square wave pulses into a gently rising and falling waveform. This is executed by chopping or breaking the exiting square waves into number of uniform pieces.

In the actual sinewave, the waveform is created through an exponential rise and fall pattern where the sinusoidal wave gradually ascend and descend in the course of its cycles.

In the proposed idea, the waveform is not executed in an exponential, rather the square waves are chopped into pieces which ultimately takes the shape of a sinewave after some filtration.

The "chopping" is done by feeding a calculated PWM to the gates of the FET via a BJT buffer stage.

A typical circuit design for converting the SG3525 waveform into a pure sinewave waveform is shown below. This design is actually an universal design which may be implemented for upgrading all square wave inverters into sinewave inverters.

adding a chopper circuit to SG3525 inverter

As may be in the above diagram, the lower two BC547 transistors are triggered by a PWM feed or input, which causes them to switch according to the PWM ON/OFF duty cycles.

This in turn rapidly switch the 50Hz pulses of the BC547/BC557 coming from the SG3525 output pins.

The above operation ultimately force the mosfets also to turn ON and OFF number of times for each of the 50/60Hz cycles and consequently produce a similar waveform at the output of the connected transformer.

Preferably, the PWM input frequency should be 4 times more than the base 50 or 60Hz frequency. so that each 50/60Hz cycles are broken into 4 or 5 pieces and not more than this, which could otherwise give rise to unwanted harmonics and mosfet heating.

PWM Circuit

The PWM input feed for the above explained design can be acquired by using any standard IC 555 astable design as shown below:

IC555 pwm with SG3525 inverter

This IC 555 based PWM circuit can be used for feeding an optimized PWM to the bases of the BC547 transistors in the first design such that the output from the SG3525 inverter circuit acquires an RMS value close to mains pure sinewave waveform RMS value.

Using an SPWM

Although the above explained concept would greatly improve the square wave modified output of a typical SG3525 inverter circuit, an even better approach could be to go for an SPWM generator circuit.

In this concept the "chopping" of each of the square wave pulses is implemented through a proportionately varying PWM duty cycles rather than a fixed duty cycle.

I have already discussed how to generate SPWM using opamp, the same theory may be used for feeding the driver stage of any square wave inverter.

A simple circuit for generating SPWM can eb seen below:

adding SPWM to SG3525

Using IC 741 for Processing SPWM

In this design we see a standard IC 741 opamp whose input pins are configured with a couple of triangle wave sources, one being much faster in frequency than the other.

The triangle waves could be possibly extracted from a standard IC 556 based circuit, wired as an astable, as shown below:

IC 556 triangle wave generator

This forces the opamp to compare the two coinciding peaks of the waveforms and generate SPWM or sinewave PWM consisting of a proportionately widening and narrowing PWMs in each cycle.

When this optimized PWM is fed to the first circuit design causes the output from the transformer to produce a further improved and gentle sine waveform having properties much identical to a standard AC mains sine waveform.

However even for an SPWM, the RMS value will need to be correctly set initially in order to produce the correct voltage output at the output of the transformer.

Once implemented one can expect a real sinewave equivalent output from any SG3525 inverter design or may be from any square wave inverter model.

If you have more doubts regarding SG3525 pure sinewave inverter circuit you can feel free to express them through your comments.


A basic example design of a SG3525 oscillator stage can be seen below, this design could be integrated with the above explained PWM sinewave BJT/mosfet stage for getting the required enhanced version of the SG3525 design:

basic SG3525 inverter design for SPWM integration

Complete circuit diagram and PCB layout for the proposed SG3525 pure sine wave inverter circuit.

Courtesy: Ainsworth Lynch

SG3525 with IC 555 PWM chopper

SG3525 inverter PCB component layout

SG3525 PCB track side layout

Design#3: 3kva Inverter circuit using the IC SG3525

In the previous paragraphs we have comprehensively discussed regarding how an SG3525 design could be converted into an efficient sinewave design, now let's discuss how a simple 2kva inverter circuit can be constructed using the IC SG3525, which can be easily upgraded to sinewave 10kva by increasing the battery, mosfet and the transformer specs.

The basic circuit is as per the design submitted by Mr. Anas Ahmad.

The explanation regarding the proposed SG3525 2kva inverter circuit can be understood from the following discussion:

hello swagatam, i constructed the following 3kva 24V inverter modified sine wave (i used 20 mosfet with resistor attached to each, moreover i used center tap transformer and i used SG3525 for oscillator).. now i want to convert it to pure sine wave, please how can i do that?

Basic Schematic

SG3525 Pure Sinewave Inverter 2000 watts

My Reply:

Hello Anas,

first try the basic set up as explained in this SG3525 inverter article, if everything goes well, after that you can try connecting more mosfets in parallel.....

the inverter shown in the above daigram is a basic square wave design, in order to convert it to sine wave you must follow the steps explained below The mosfet gate/resistor ends must be configured with a BJT stage and the 555 IC PWM should be connected as indicated in the following diagram:

SG3525 RMS controlled Inverter Circuit

Regarding Connecting parallel mosfets

ok, i have 20 mosfet(10 on lead A, 10 on lead B), so i must attached 2 BJT to each mosfet, that's 40 BJT, and likewise i must connect only 2 BJT coming out from PWM in parallel to the 40 BJT? Sorry am novice just trying to pick up.

No, each emitter junction of the respective BJT pair will hold 10 mosfets...therefore you will need only 4 BJTs in all....

Using BJTs as Buffers

1. ok if i may get you right, since you said 4 BJTs, 2 on lead A, 2 on lead B, THEN another 2 BJT from the output of PWM, right?
2. am using 24 volt battery hope no any modification to the BJT collector terminal to the battery?
3. i have to use variable resistor From oscillator to control the input voltage to the mosfet, but i don't know how i will go about the voltage that will go to the base of the BJT in this case, what will i do so that i want end up blow up the BJT?

Yes, NPN/PNP BJTs for the buffer stage, and two NPN with the PWM driver.
24V will not harm the BJT buffers, but make sure to use a 7812 for stepping it down to 12V for the SG3525 and the IC 555 stages.

You can use the IC 555 pot for adjusting the output voltage from the trafo and set it to 220V. remember your transformer must be rated lower than the battery voltage for getting optimum voltage at the output. if your battery is 24V you can use an 18-0-18V trafo.

Parts List

IC SG3525 Circuit 
all resistors 1/4 watt 5% CFR unless otherwise specified
10K - 6nos
150K - 1no
470 ohm - 1no
presets 22K - 1no
preset 47K - 1no
0.1uF Ceramic - 1no
IC = SG3525
Mosfet/BJT Stage 
All mosfets - IRF540 or any equivalent Gate resistors - 10 Ohms 1/4 watt (recommended)
All NPN BJTs are = BC547
All PNP BJTs are = BC557
Base Resistors are all 10K - 4nos
IC 555 PWM Stage 
1K = 1no 100K pot - 1no
1N4148 Diode = 2nos
Capacitors 0.1uF Ceramic - 1no
10nF Ceramic - 1no
Miscellaneous IC 7812 - 1no
Battery - 12V 0r 24V 100AH Transformer as per specs.

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