With this tester, you can assess the capacity of Inverter to handle the load. It gives a warning beep, if the inverter load exceeds above its load handling capacity. Inverters are available in different ‘Size ratings’ from 50 VA up to 50000 VA. VA indicates the ‘Size’ (capacity) of the inverter which represents its maximum load capacity. Commonly available inverters are 500 VA, 800 VA, 1000 VA, 1500 VA etc. An inverter needs Peak or Surge power and Typical or Average (Usual) power to handle the load. Peak power is the maximum power that an inverter can supply usually for short time. If the load increases above the rated capacity of the Inverter, it will over load the inverter that may leads to its permanent damage.
The inverter capacity tester circuit senses the current passing through the Ampere Meter connected in series with the Battery Negative and Inverter Negative. If the load is within the handling capacity of the inverter, say 400 watts for a 500 VA inverter, current flowing through the meter is low and if the load increases, there is a corresponding current flow through the meter and the circuit senses this increased current to activate the alarm.
To compare the current through the meter, a Differential Amplifier built around IC1is used. Since the current flowing through the meter is too low, the voltage developed will also be very low. The differential amplifier amplifies this minute voltage to give a high output around 2 volts (as set by VR1).This is used to control the Schmitt trigger built around IC2. When the voltage level in the Trigger input of IC2 (Pin 2) is less than 3 volts (as set by VR2), output of IC1 remains high which lights the Green LED and inhibits the buzzer and Red LED. This indicates that the Inverter load is not high. When the load increases, the current flowing through the meter also increases and the output of IC1 gives more than 3 volts which changes the output of IC2 to low. Red LED turns on along with beeps from the buzzer. This indicates that the inverter is overloaded.
How to use it to measure inverter capacity?
In order to assess the inverter capacity, it is necessary to calibrate the circuit with a known load. No inverter will handle the load power as specified in its VA. Volt Ampere is used to indicates the maximum load capacity of the inverter. VA = W x Inverter loss. Inverter loss is typical around 1.15. If the total load connected to the inverter is 400 watts then the minimum inverter size should be 400 x 1.15. That is 460 VA. A 500 VA is suitable for the load.
We can use 100 watts bulbs as load to calibrate the circuit. Connect points A and B to the 12 volt battery using Crocodile clips. Connect points C and D to the inverter using crocodile clips. Always observe polarity while connecting. Then connect four 100 watts bulbs (for a 500 VA inverter) to the inverter out and switch on the inverter. So the load is 400 watts. Slowly adjust VR2 till buzzer stops beeping and Red LED turns off. At this time, Green LED should turn on. This indicates that the load is within the limit. Mark the Wiper position of VR2. Then connect one more 100 watt bulb. At this time Buzzer should beep. If not, adjust VR1 and VR2 till buzzer beeps. Mark that position also. This indicates that the inverter is handling 500 watts which is an overloaded condition of the 500 VA inverter.
How it measures load capacity
The circuit is now calibrated for the maximum load capacity. In VR2, now there are two makings. One for 400 watts and another for 500 watts. Turn the POT VR2 to the 400 W position and connect it to the 500 VA inverter with 400 W load. If the buzzer beeps, it indicates that the 500 VA inverter is capable of handling only less than 400 watts. The same procedure can be used to calibrate the circuit for 800 VA inverter with Seven 100 watts bulbs. Use a preset for VR1 and 10K Lin POT for VR2.