No Load Detector and Cut-off Circuit for Inverters

The post discusses a relay cut-off circuit which may be included in inverters to ensures that under a no load at the output the condition is quickly detected and the supply cut off, preventing the inverter from operating unnecessarily. The idea was requested by Mr. Rajath.

Technical Specifications

I need to adopt a no load auto cutoff system into my inverter, do you have any suitable design, which could help me. or else can you give any idea on how to achieve ,as i need to shut down the output of the inverter when ever there is no current drawn from it. please help me ,here.

Regards Rajath

The Design

In a few of ay previous posts we have learned how to make overload cut off circuit such as:

Low Battery Cut-off and Overload Protection Circuit.

Motor over current protector circuit

However, the present concept deals with an opposite situation wherein a no load condition is supposed to be detected and cut off for persisting, that is we discuss a circuit for preventing a no load condition for inverters.

As shown in the above figure, a no load detector and cut of procedure can be initiated by incorporating this design in any inverter circuit.

The operational details may be understood with the following explanation:

The circuit comprises two stage, namely the current amplifier and sensor stage using the T3/T4 Darlington pair, and a simple delay ON stage using T1, T2 and the associated components.

As soon SW1 is switched ON, the delay-ON timer counting is initiated through C1 which begins charging via R2 and D5 keeping T1 switched off in the process. With T1 switched T2 is switched ON which in turn switches ON the relay.

The relay enables the positive from the battery to get connected with the inverter so that the inverter is able to start and generate the required AC mains to the intended appliances.

With the presence of a load at the output the battery undergoes a proportionate amount current consumption, and in the course Rx experiences a current flow through it.

This current is transformed into a proportionate amount of voltage across Rx which is sensed by the T3/T4 Darlington pair and it is forced to switch ON.

With T3/T4 switched ON, C1 is instantly inhibited from getting charged, which leads to an immediate disabling of the delay ON timer, making sure that the output of the inverter continues to supply the voltage to the load.

However, suppose the output of the inverter is devoid of any load (no load condition), T3/T4 is then unable to switch ON, which allows C1 to get charged gradually until the potential across it becomes sufficient to trigger T1.

Once T1 is triggered, T2 is cut off and so is the relay. With the relay contacts cut off and shifted from N/O to the N/C contact, the positive to the inverter is also cut off, the system comes to a stand still.