In this post I have explained 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. I have explained more about the request and the circuit functioning.
You may also want to read how to design a sine wave inverter from the scratch.
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, I have explained them:
As can be witnessed in the given diagram, the IC SG3525 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#1 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 reference internally.
As long as pin#1 is within the specified voltage limit, the PWM correction feature stays inactive, however the moment the voltage at pin#1 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#1 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.
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:
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
Another Design with Automatic Output Feedback Correction
Circuit Design#2:
In the above section I have explained 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 sine wave waveform in its typical format.
Although the modified squarewave or sine wave 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 sine wave inverter output.
Here I have explained a simple method which could be used for enhancing any standard SG3525 inverter circuit into a pure sine wave 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 sine wave inverter in one of my earlier posts, here we apply the same principle for the upgrade.
How the Conversion from Squarewave to Sine wave Happens
You might be curious to know regarding what exactly happens in the process of the conversion which transforms the output into a pure sine wave 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 sine wave, 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 sine wave 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 sine wave waveform is shown below. This design is actually an universal design which may be implemented for upgrading all square wave inverters into sine wave inverters.
As may be in the above diagram, the lower two diodes are connected to a PWM feed or input, which causes the transistors to switch according to the PWM ON/OFF duty cycles.
This in turn rapidly chop the 50Hz pulses at the bases of the BC547/BC557 coming from the SG3525 output pins.
The above operation ultimately force the mosfets also to turn ON and OFF in the same pattern as the SPWM for each of the 50/60Hz cycles. This SPWM is then induced into the transformer primary by the MOSFETs, consequently producing a sine waveform at the output or the secondary side of the transformer.
If an ordinary PWM is used as I have explained below, then its frequency should be 4 times more than the base 50 or 60 Hz 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:
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 sine wave 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 be seen below:
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 manufactured from a standard IC 556 based circuit, wired as an astable and compactor, as shown below:
#UPDATE: The above "slow triangle waves" can be directly acquired from the Ct pin of the IC, that means you can now eliminate or ignore the above IC 555 stage for the slow triangle waves.
As can be seen in the above two images, the fast triangle waves are achieved from an ordinary IC 555 astable.
However, the slow triangle waves are acquired through an IC 555 wired like a "square wave to triangle wave generator".
The square waves or the rectangular waves are acquired from pin#4 of SG3525. This is important as it synchronizes the op amp 741 output perfectly with the 50 Hz frequency of the SG3525 circuit. This in turn creates correctly dimensioned SPWM sets across the two MOSFET channels.
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 sine wave equivalent output from any SG3525 inverter design or may be from any square wave inverter model.
Finalized SG3525 Pure Sine Wave Inverter Circuit
The following diagram shows the finalized design of the pure sine wave inverter using IC SG3525 and SPWM, as per the above explanations.
If you have any doubts regarding the above SG3525 pure sine wave inverter circuit you can feel free to express them through your comments.
UPDATE
A basic example design of a SG3525 oscillator stage can be seen below, this design could be integrated with the above explained PWM sine wave BJT/mosfet stage for getting the required enhanced version of the SG3525 design:
Complete circuit diagram and PCB layout for the proposed SG3525 pure sine wave inverter circuit.
Courtesy: Ainsworth Lynch
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 sine wave design, now let's discuss how a simple 2kva inverter circuit can be constructed using the IC SG3525, which can be easily upgraded to sine wave 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
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 I have 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:
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.
Answer:
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
Capacitors
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.
Kumah Andrews says
Thank you Sir, from the hand drawn circuit I can see you’ve written “shutdown protection” between pin 10 and -VE, indicated 2v. Please am little bit confuse there, can you explain it to me?
Swagatam says
Hi Andrews,
That is the shut-down input pin of the IC. If you configure a feedback system with this pinout, then if the feedback voltage increases above 2V, will cause the IC output to shut down or narrow the PWMs.
Kumah Andrews says
please can I get the gerber file?
Swagatam says
I don’t have the gerber files for this project unfortunately…
Daniel says
please I need your guide sir, I made 24v inverter using irfp 260 MOSFET(1pc) when I put on My CRT TV at night the MOSFET doesn’t blow, but another 60v inverter using psm50s20 MOSFET(1pc) it blows at night but not during the sunny day with solar.
although the TV has high starting surge
Swagatam says
Hi Daniel, I could not find the datasheet of the psm50s20 MOSFET, so I cannot figure out why it is blowing up. However it is better to use standard MOSFETs like IRF540, IRFZ44 etc instead of any other untested variants.
Daniel says
Thanks sir, sorted out. Another issue is that on heavy load like freezer, it is not stable, please what can I do?
Swagatam says
That’s good Daniel,
If the load is not stable then you must upgrade the transformer, MOSFETs and the battery according to the load. The output power must be much higher than the initial surge current intake of the freezer.
Kumah Andrews says
Sir, I want to build the Finalized SG3525 Pure Sine Wave Inverter Circuit. what are the voltage for the various capacitors?
Swagatam says
Hello Kumah, all the capacitors can be rated at 25V.
Ronnie says
Hi Swagatam,
Am a big fan of yours. Am a hobbyist ,electronics technician and electrician.
I first built your 3stage high current charger to perfection ,some 7years ago and an inverter using Sg3524 ic
.The circuit is from this site
https://www.homemade-circuits.com/wp-content/uploads/2024/11/250-to-5000-Watts-PWM-DC-AC-220V-Power-Inverter.jpg
.
it worked well for years till recently,it got damaged,I replaced MOSFETs with f1010e,but maintained the buffer transistors.
here is my challenge:
-sg3524 output reads 3v,
-mosfet overheat and blow above 150w ,
Am using 2 MOSFETs per channel.
I have a 7809 for the ic.
f1010e should work well,why the blowing, this inverter works,something is off.
I want to get running then use your 555 ideah to convert to pure sine wave.
it’s a 12v inverter,transformer is 1200va,900watts ,10-0-10 center tapped.
help please!
Swagatam says
Thank you Ronnie,
If you are using 9V supply for the IC, then the peak voltage at the output pins of the IC should also around 9V.
So, you must confirm whether this is happening or not, and you will also need to check the frequency across the output pins, which must be 50Hz or 60Hz.
Without a minimum of 9V at their gates, the MOSFETs will not operate correctly and might keep blowing.
Do you have an oscilloscope, if yes then you can use it to confirm the above details across the IC output pins….
william says
Hola ing. voy a proceder a armar este inversor se onda senoidal, seria necesario poner un regulador 7812 ?
Swagatam says
You can add the 7812 IC after the diode D1 in the following diagram:
https://www.homemade-circuits.com/wp-content/uploads/2016/08/sg3525-sine-wave-inverter-circuit.jpg
william says
How could the design be if I want to increase 2 mosfets?
Swagatam says
You can put any number of MOSFETs in parallel in the existing design without any changes in the circuit.
william says
Thank you, another query would be necessary to add two more diodes 1n4148 at the exit of pin 6 in ic741 if I am going to add god mosphets more or just connect it to the diode 1n4148 x2 that are in the design?
Swagatam says
If you want to add more number of parallel MOSFETs, just connect their gates, drains, and source terminals with each other, in common, in the existing setup, that’s all, no need to change anything else in the circuit design.
Jurandir rabelo says
Hello all right ? about the sine inverter I found cool one thing I did not understand was the fact that the gate resistor is de1k something else and about the output stabilization part is not isolated I found it dangerous what I will suggest and use an optical coupler pc 817 with a TL431 to make the output control making more efficient and accurate.
Swagatam says
Yes, the gate resistors are 1k, the high value is required for reducing loading of the op-amp.
The MOSFET gate discharge will not get affected by the high value 1k because of the reverse 1N4148 diodes.
Sure, you can make the feedback isolated using an opto-coupler.
Seun says
Good day Engineer Swagatam, please what can I do to My efficient modified sine inverter, it could not power a brand of laptop charger, whereas the grid powers it, other brand laptop chargers are working with the inverter.
Swagatam says
Hi Seun,
That specific brand could have been designed to work with pure sine wave AC only, that is why it might not be accepting a modified sine wave AC.
Seun says
Thanks engineer, your guide article was amazing, I got a cleaner wave, but this pure sine wave consumes much more current than modified wave inverter, why is this so, sir.
Swagatam says
Thanks Seun,
PWM sine wave should consume less current because it uses PWM technology, if it is consuming more current, then you must investigate why this is happening…
ANIL RAJPUROHIT says
SIR, WHAT TO DO FOR PROTECTION OF IC3525 IN PARTICULAR CIRCUIT
Swagatam says
Anil, you can supply 12V DC to the IC through a 7812 IC, and also the arrangement of C6, C7, R7, as shown in the first design.
Azhar says
Hi sir
I want to control duty cycle of dsp30f2010 plz help me also I have sent an email about project
Swagatam says
Hi Azhar, sorry, i have not yet investigated this specific IC, so currently i have no idea regarding its working specifications…possibly I will try to study it and try to figure out your problem.
Hillary says
is there any other way to filter a square wave to sine wave using RC filter
Swagatam says
Here’s an example of a sine wave inverter using RC filter:
https://www.homemade-circuits.com/wp-content/uploads/2023/10/simplified-4047-pure-sine-wave-inverter-circuit.jpg
Seun says
Thanks for this, please what kind of capacitors are CX, cy, and please give a values to experiment with.
Swagatam says
In the finalized sine wave design, CX must be adjusted to a value which generates triangles waves of around 300 Hz, you can try any online 555 calculator to find the value of this CX….where is CY, i cannot find it?
Seun says
please what type of capacitors are they electrolytic or ceramic
Swagatam says
It cannot be electrolytic because the value could be in nF or pF. So, you can use ceramic disc or PPC…
Seun says
please sir, help out, will CX, cy be same values, please I can’t find any online calculator to calculate to get 300hz, the ones online are only astable.
Swagatam says
Here’s the calculator you can use to find CX:
ohmslawcalculator.com/555-astable-calculator
I cannot find where is CY??
AJTH says
You’ve asked in Oct 14, 2024 about 2 experimental items (CX & CY) you drew on a picture https://www.homemade-circuits.com/wp-content/uploads/2023/10/simplified-4047-pure-sine-wave-inverter-circuit.jpg
Swagatam says
I was referring to this diagram:
https://www.homemade-circuits.com/wp-content/uploads/2016/08/sg3525-sine-wave-inverter-circuit-1200×596.jpg
AJTH says
Thanks, I checked it out.
However, I read you asked for the same item twice (above) when a person asked [Seun October 14, 2024 please sir, help out, will CX, CY be same values, (…) ], but that man never answered, and I felt compelled to fill in that blank, since you currently NEVER left and open question like that (unless a lazy person wanted you to do all the jobs).
I will visit that site often. That’s an electronic library, sir! I hope it never pass away (like me).
Swagatam says
Thanks AJTH, for your kind words. I appreciate it very much…
If you are referring to the 4047 circuit, it is actually an RC series integrator circuit.
The best way to get the values of CX and CY is to find it through some trial and error and practical experimentation, because calculating it can be a lot difficult.
Daniel says
Hello sir, please can I use sg 3525 inverter circuit to invert 48v DC to 12v ac
Swagatam says
Hi Daniel, yes, you can convert any voltage to any level as you like, using an inverter IC like SG3525 or any other similar IC. It simply depends on the transformer winding ratings…
Daniel says
please how can I embed WiFi network to monitor and automate sg3525 inverter design.
Swagatam says
Daniel, Currently I do not have any circuit for monitoring an SG3525 inverter through Wifi network…
Daniel says
I discovered with 12v AC output, the transformer saturates causing damage. Please how will I do it?
Swagatam says
Please explain your problem in detail, I cannot understand it.
Daniel says
I noticed that pure sine wave strained MOSFETs than modified sine wave, is this correct, sir?
Swagatam says
MOSFETs are built to handle high frequencies in the MHz range, so an SPWM with a few 100 Hz frequency can have no adverse impact on the MOSFETs, according to me…