Automatic Inverter Output Voltage Correction Circuit

The common problem with many low cost inverters is their incapability of adjusting the output voltage with respect to the load conditions. With such inverters the output voltage tends to increase with lower loads and falls with increasing loads. The circuit explained here can be added to any ordinary inverter for compensating its varying output voltage conditions in response to varying loads.

Design#1: Introduction

The circuit was requested to me by one of my friends Mr.Sam, whose constant reminders prompted me to design this very useful concept for inverter applications.

The load independent/output corrected or output compensated inverter circuit explained here is quite on a concept level only and has not been practically tested by me, however the idea looks feasible because of its simple design.

Circuit Operation

If we look at the figure we see that the entire design is basically a simple PWM generator circuit built around the IC 555.

We know that in this standard 555 PWM design, the PWM pulses can be optimized by changing the ratio of R1/R2.

This fact has been appropriately exploited here for the load voltage correction application of an inverter.
An opto-coupler made by sealing an LED/LDR arrangement has been used, where the LDR of the opto- becomes one of the resistors in the PWM "arm" of the circuit.

The LED of the opto coupler is illuminated through the voltage from the inverter output or the load connections.

The mains voltage is suitably dropped using C3 and the associated components for feeding the opto LED.

After integrating the circuit to an inverter, when the system is powered (with suitable load connected), the RMS value may be measured at the output and the preset P1 may be adjusted to make the output voltage just suitable enough for the load.

How to Set Up

This setting is probably all that would be needed.

Now suppose if the load is increased, the voltage will tend to fall at the output which in turn will make the opto LED intensity decrease.

The decrease in the intensity of the LED will prompt the IC to optimize its PWM pulses such that the RMS of the output voltage rises, making the voltage level also rise up to the required mark, this initiation will also affect the intensity of the LED which will now go bright and thus finally reach an automatically optimized level which will correctly balance the system load voltage conditions at the output.

Here the mark ratio is primarily intended for controlling the required parameter, therefore the opto should be placed appropriately either to the left or the right arm of the shown PWM control section of the IC.

The circuit can be tried with the inverter design shown in this 500 watt inverter circuit

Parts List

• R1 = 330K
• R2 = 100K
• R3, R4 = 100 Ohms
• D1, D2 = 1N4148,
• D3, D4 = 1N4007,
• P1 = 22K
• C1, C2 = 0.01uF
• C3 = 0.33uF/400V
• OptoCoupler = Homemade, by sealing an LED/LDR face to face inside a light proof container.

Design#2: Automatic RMS Correction using PWM

The above explained load correction circuit looks good but it is not too impressive because of two reasons, first the making of an LED/LDR opto coupler is convenient a could be cumbersome for many. Second reason is the control will not be 0 to 100%, rather it would either 0 to 50% or 50 to 100%.

The next design can be considered perhaps an ideal approach of implementing a load independent auto output correction using PWM from a IC 555.

The circuit shown above can be effectively used as an automatic load triggered RMS converter and could be applied in any ordinary inverter for the intended purpose.

The IC 741 works like a voltage follower and acts like a buffer between the inverter output feedback voltage and the PWM controller circuit.

The resistors connected with pin#3of the IC 741 is configured like a voltage divider, which appropriately scales down the high AC output from the mains into a proportionately lower potential varying between 6 and 12V depending upon the output status of the inverter.

The two IC 555 circuit are configured to work like modulated PWM controller. The modulated input is applied at pin#5 of the IC2, which compares the signal with the triangle waves at its pin#6.

This results in the generation of the PWM output at its pin#3 which varies its duty cycle in response to the modulating signal at the pin#5 of the IC.

A rising potential at this pin#5 results in the generation wide PWMs or PWMs with higher duty cycles, and vice versa.

This implies that when the opamp 741 responds with a rising potential due to a rising output from the inverter causes the output of IC2 555 to widen its PWM pulses, while when the inverter output drops, the PWM proportionately narrows at pin#3 of IC2.

Configuring the PWM with Mosfets.

When the above auto correcting PWMs is integrated with the mosfet gates of any inverter will enable the inverter to control its RMS value automatically in response to the load conditions.

If the load exceeds the PWM the inverter output wil tend to go low, causing the PWMs to widen which will in turn cause the mosfet to turn ON harder and drive the transformer with more current, thereby compensating the excess current draw from the load

Design#3: Using opamp and Transistor

The next idea discusses an opamp version which can added with ordinary inverters for achieving an automatic output voltage regulation in response to varying loads or battery voltage.

The idea is simple, as soon as the output voltage crosses a predetermined danger threshold, a corresponding circuit is triggered which in turn switches OFF the inverter power devices in a consistent manner thereby resulting a controlled output voltage within that particular threshold.

The drawback behind using a transistor could be the involved hysteresis issue which could make the switching fairly over a wider cross section resulting in a not so accurate voltage regulation.

Opamps on the other hand can be immensely accurate as these would switch the output regulation within a very narrow margin keeping the correction level tight and accurate.

The simple inverter automatic load voltage correction circuit presented below could be effectively used for the proposed application and for regulating the output of an inverter within any desired limit.

The proposed inverter voltage correction circuit can be understood with the help of the following points:

A single opamp performs the function of a comparator and a voltage level detector.

Circuit Operation

The high voltage AC from the transformer output is stepped down using a potential divider network to about 14V.

This voltage becomes the operating voltage as well as the sensing voltage for the circuit.

The stepped down voltage using a potential divider corresponds proportionately in response to the varying voltage at the output.

Pin3 of the opamp is set to an equivalent DC voltage corresponding to the limit which needs to be controlled.

This is done by feeding the desired maximum limit voltage to the circuit and then adjusting 10k preset until the output just goes high and triggers the NPN transistor.

Once the above setting is done the circuit becomes ready to be integrated with the inverter for the intended corrections.

As can be see the collector of the NPN needs to be connected with the gates of the mosfets of the inverter which are responsible for powering the inverter transformer.

This integration ensures that whenever the output voltage tends to cross the set limit, the NPN triggers grounding the gates of the mosfets and thereby restricting any further rise in the voltage, the ON/OFF triggering continues infinitely as long as the output voltage hovers around the danger zone.

It must be noted that the NPN integration would be compatible only with N-channel mosfets, if the inverter carries P-channel mosfets, the circuit configuration would need a complete reversal of the transistor and the input pinouts of the opamp.

Also the circuit ground should be made common with the battery negative of the inverter.

CAUTION: THE PROPOSED DESIGN IS NOT ISOLATED FROM INVERTER MAINS VOLTAGE, EXERCISE EXTREME CAUTION DURING THE TESTING AND SETTING UP PROCEDURES.