An inverter provides power backup for mains-based appliances in the event of a power failure. Most of the inverters available in the market have complicated circuit design and are not very economical. Some of them produce a square-wave output, which is undesirable for inductive loads.
Here we present a simple inverter circuit that produces 50Hz quasi sine-wave output using a single IC CD4047 and some discrete components, which makes it a very cost-effective solution.
Fig. 1 shows the circuit of the MOSFET-based 50Hz inverter. It comprises a CD4047 multivibrator (IC1), IRF250 MOSFETs (T1 through T8), transistors and a few discrete components.
IC CD4047 has built-in facilities for astable and bistable multivibrators. The inverter application requires two outputs that are 180 degrees out of phase. Therefore IC1 is wired to produce two square-wave output signals at pins 10 and 11 with 50Hz frequency, 50 per cent duty cycle and 180-degree phase-shift. The oscillating frequency is decided by external preset VR1 and capacitor C1.
These two signals drive the two MOSFET banks (bank-1 and bank-2) alternatively. When pin 10 of IC1 is high and pin 11 low, MOSFETs of bank-1 (T1 through T4) conduct, while MOSFETs of bank-2 (T5 through T8) remain in non-conducting state. Therefore a large swing of current flows through the first half of the primary winding of inverter transformer X1 and 230V AC develops across the secondary winding.
During the next half cycle, the voltage at pin 10 of IC1 goes low, while the voltage at pin 11 is high. Thus MOSFETs of bank-2 conduct, while the MOSFETs of bank-1 remain non-conducting. Therefore current flows through the other half of the primary winding and 230V AC develops across the secondary winding.
This way an alternating output voltage is obtained across the secondary winding.
The sinewave output is obtained by forming a tank circuit with the secondary winding of the inverter transformer in parallel with capacitors C5 through C7. Two 2.2µF capacitors are connected to the gates of the MOSFETs in both the banks with respect to the ground if proper sinewave is not produced. Natural frequency of the tank circuit is adjusted to 50 Hz. Current consumption with no load is only 500 mA due to 50 per cent duty cycle of the square-wave signal. As load is increased, current consumption increases.
Supply voltage to IC1 is limited to 5.1 volts by using zener ZD1 and resistor R4 with the external battery as shown in Fig. 1.
Low-battery indicator. The low-battery indication circuit consists of transistor T9, preset VR2, zener diode ZD2, resistors R5, R6 and R7, LED2 and capacitor C2. The 12V supply voltage from BATT.1 is applied to the low-battery indicator circuit with full load (not more than 1000 watts) connected to the inverter output. The voltage across the load is 230V AC. At this instant, adjust preset VR2 such that zener diode ZD2 and transistor T9 conduct to drop the collector voltage to 0.7 volt keeping LED2 ‘off.’
If supply voltage goes below 10.5 volts, the voltage across the load decreases from 230V AC to 210V AC. At this instant, zener diode ZD2 and transistor T9 do not conduct and hence the collector voltage increases to about 10.5 volts and LED2 glows to indicate low voltage of the battery. At the same time, piezobuzzer PZ1 produces an audio tone indicating low battery.
Low-battery cut-off. If the battery is discharged to zero volt repeatedly, the battery life will decrease. The low-battery cut-off circuit consists of transistor T10, preset VR3, zener diode ZD4, resistors R8 and R9, capacitor C3 and diode D1.
Adjust preset VR3 such that when the voltage across the load is above 200 volts, zener diode ZD4 and transistor T10 conduct. The collector voltage of T10 is about 0.7 volt in this case and hence the SCR (SCR1) will not conduct.
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But if the voltage across the load goes below 200 volts, zener diode ZD4 and transistor T10 will not conduct and the collector voltage of T10 will increase, causing the SCR to conduct.
Once the SCR conducts, the supply voltage to IC1 (CD4047) will be 0.7 volt, due to which IC1 will be unable to produce the voltage pulses at output pins 10 and 11 and the inverter will turn off automatically. During this state, the SCR remains conducting.
Low cut-off of the inverter can be set at the load voltage of 170 volts for tubelight, fan, etc. So the tubelight and fan will not be switched off until the voltage goes below 170 volts.
No-load cut-off. If there is no load connected at the output of the inverter, the output voltage is 270 to 290 volts. This voltage is sensed by the 0-12V tap at the secondary winding of inverter transformer X1, which is connected to the no-load cut-off circuit comprising zener diode ZD5, transistor T11, preset VR4, resistors R12 and R11, and capacitor C4.
When no load is connected, the voltage at 12V tap will also increase. This voltage is rectified by the full-wave bridge rectifier comprising diodes D3 through D6, filtered by capacitor C4 and given to transistor T11.
Adjust preset VR4 such that if the inverter voltage goes above 250 volts, zener diode ZD5 and transistor T11 conduct. This increases the emitter voltage, hence the SCR fires to switch the inverter ‘off.’ When proper load is connected, the inverter will automatically turn on.
An actual-size, single-side PCB for the inverter is shown in Fig. 2 and its component layout in Fig. 3. Suitable connector CON1 is provided on the PCB to connect the MOSFET banks and the transformer externally. Connector CON1 pins A through F are also marked on schematic. Assemble the circuit on a PCB as it saves time and minimises assembly errors. Carefully assemble the components and double-check for any overlooked error. MOSFETs should be mounted over heat-sinks using mica spacers as the insulators between them.
Connect the 24V supply terminal directly to the centre tap of the primary winding of the inverter transformer, which carries maximum current of more than 50 amperes with 1000 watts. Current depends on the load applied. There is no need to add a switch in the high-current path to make the inverter turn on and off. The inverter can be switched on and off by low-current switch S1.
The author is an associate professor and head of Department of Physics, Kisan Veer Mahavidyalaya, Wai, District Satara, Maharashtra