Discrepancy In Wattage, Voltage And Current Readings

Hello Sidban,

Could someone please explain why this big difference ?

The individual Meter inaccuracies are multiplied, to get the total inaccuracy.

Has it anything to do with the type of power supply (smps) ?

Yes, but refer to my earlier answer too.

For further help .....

Could someone please explain why this big difference ?

I don't consider that a 'big' difference.

Del

PS...Maybe try the same measurements when driving a more resistive load...say a couple of incandescent light bulbs to give about the same total wattage?

Del

The errors in the current and voltage measurements come to about 4.5%?

I would suspect the waveform accounts for any small difference you see.

There will be small losses in the form of heat.

Further hysteresis iron losses.

Add to the above, the total combined inaccuracy of the meters, and that is where the "missing energy", (your 'big difference'), has gone.

If you used Certified Laboratory Grade Meters, your results may show a smaller "big difference" than you have shown.

Remember also that each time you measure something, you affect its original state, thus the actual meter readings are always different from what the unmetered situation really was ...

Had to Google smps to be sure what it was.

I've had the same problem and realised that the instruments were only accurate for sine waves.

By their function smps take current in a non sinusoidal form,so your current reading will be in error.Similarly the wattmeter will also be in error for non sine wave current. Volts will be correct.

Have a look at the current waveform on your 'scope with a current transformer,you will find it a highly chopped sine wave.

Lets not get into how to connect your 'scope to the mains for voltage waveforms.

Thanks a lot for your reply and for the replies of all the others. I stand enlightened on the matter.