Introduction: Modified Sine Wave Signal Generator.

Due to a computer crash and lost files I have had to redo this project.

I was inspired to design this circuit by thirteen, two thousand watt, industrial power inverter circuit boards; I bought for ten dollars. I took one circuit board apart to reverse engineer how it worked, and it did not have a signal generator. They are four-bank push pull mosfet driven inverters, beyond the difficulty of building a four bank sine wave driver I don’t think the circuit board can handle a sign wave driver, so I designed a modified sine wave signal generator. The Optocouplers used in the circuits are A3120 and five volts drive the signal in side of the optocouplers, this made logic circuit components ideal for a signal generator.

Step 1: The Signal Circuit

Pure sine wave inverters are best, however they are expensive to buy or build. Modified sine wave inverters will power some equipment square wave inverters will not, and cost about the same as square wave inverters to build or buy.
555 timers generally need a 1kW and a 1MW resistor to achieve a 50% duty cycle and adjust the frequency with the capacitor. An adjustable capacitor, a precise capacitor, or capacitors in series and parallel, can enable you to get the right frequency. However any multivibrator set for a 50% duty cycle will do for the first signal, and since where I live, AC is 60 cycle, in my signal generator the multivibrator is set for 120 cycles. However where you live AC may be 40 or 50 cycles, you may need to set the multivibrator for 80 or 100 cycles respectively. Every thing after the transistors is a compressed version of the circuit board I built the signal generator for. The flip-flop should have a negative transition clock, this is so the And gates are not on when the flip-flop toggles. Optocouplers are used as IGBT/Mosfet gate drives, Industrial inverters, logic inputs, and microprocessor inputs just to name a few appellations. They protect one part of the circuit from the power in the rest of the circuit.

Step 2: Simple Modifide Sine Wave Generator

There is a lot you need to know to completely design an inverter.

Unless you can acquire a custom made transformer your circuit design is ruled by the transformer available to you, if the primary winding is not center taped you are limited to an AB driver. The only transformer you can play with is a center tap primary, if the primary winding is center taped then you can use an A, B, C, and AB inverter driver and it can be supplied with 12 or 24 volts and 6 or 12 volts you can’t play with the watts.

If the primary winding is 1000 watts run max then your inverter is limited to 1000 watts run max and about 1500 watts startup surges.

The transistors and mosfets should be watts doubled, take an IRF640 it is 200 volts 16 amps and 136 watts. That is 200 volts at 0.68 amps to 8.5 volts at 16 amps not 200 volts at 16 amps. An IRF640 will get hot at 12 volts 11 amps constant running doubling the transistors and mosfets will keep them from overheating at full wattage use.

Optoisolators protect the signal circuit from feedback and power surges; they send the signal from the signal generator by light in one direction to the transistor bank.

These three 50 Hz inverter circuit designs are easy to build and the signal circuit is accurate to 1%.

This circuit can be broken down to seven components.

1. Power Supply
2. Voltage Regulator
3. Timer
4. Signal Converter
5. Optoisolation
6. Inverter Transistor Bank
7. Transformer
There can be more components when you add charging and low battery circuits.

These circuits are 24 volts center grounded and AB type inverter; 12 volts center taped primary winding B type inverter, and 12 volt AB type inverter.

Voltage regulator components
3-1N4002 diodes
1-220uF 16v
1-1uF 16v
1-1uF 6v
1-1 kΩ

Timer Components
1-12 kΩ
Use 1-24 kΩ for 60 CY or 1-30 kΩ for 50 CY or 1-39 kΩ for 40 CY
1-0.1 uF
1-10 nF
Any multivibrator with a 240 Hz output for 60 cycles, 200 Hz for 50 cycle, and 160 Hz for 40 cycles will due for the timer circuit. The duty cycle is not important as the first JK flip flop converts the signal to 50% duty cycle and ½ the hurts’.

Signal Converter Components
2 to 4-1.5 kΩ

From the And gate output resistors to the rest of the Inverter circuit the value of the active components depends on the power supply and the inverter transformer.

Step 3: Internal Signals

If you have a digital analyser the internal signals should look like this, however if you don’t have a digital analyser slowing down the frequency and using LEDs can give you the same results.

Step 4: Test Circuit

In this test circuit I set the 555 timer for approximately one-second pulses so I could watch the signal function with LED’s.

Step 5: My Finished Circuit

This final circuit just needs a couple tweaks all four And Gates SN74LS08 are driven by just one of the JK Flip Flops in the SN74LS73AN. I used the first JK Flip Flop to make the 555 signal 50% duty cycle the rest of the additional circuitry is the fan driver and battery monitor.

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