The original post was to design the inductor for a LPF to allow a 19.2v electric drill charger to operate from a "modified sine wave" 12/120v inverter. I received my 'scope from EBay with only CH2 working but using that I was able to look at the voltage waveforms of my three dc/ac inverters and the input current of the charger when driven from 120v rms line. The two smaller (~90w) inverters have similar output waveforms with ~
2ms dwell at zero and 2microsec rise and fall times. The 140w sample had narrower alternating pulses with ~ 4ms dwell at zero but of course higher peak voltage such that the rms value was still ~120v. Using 0.9 ohm sampling resistor in the neutral return of the charger I measured an alternating pulsed current ~ 3a pk with ~ 3ms zero dwells and narrow leading edge spikes of 4.6a pk. I calculated the average current (Simpson's rule) as 1.35a which agrees with the measurement using an old Avo 8 rectifier multimeter and taking rms/av = 1.11. Crudely sampling this current on paper with a 120v rms sine wave gives 247.6va, a far cry from the 75w on the charger label! No wonder the 90w inverters complained. I tried a surplus Corcom EMC filter with 012 e-3H chokes and 9 e-9F capacitors as someone suggested but the charger wouldn't run even with the 140w inverter. It seems incredible that the charger takes so much current and that with an almost fully charged 19.2v battery pack. If I had both channels available on the 'scope I could see if the phase was such that 247.6va is only 75w. I bought a 7 mFd oil capacitor and a transformer to rewind as a choke but assuming 100va size core area, Perhaps I should re-simulate with a 60 ohm load i/o the 176 ohm load for 75w. Any suggestions? Dick- posted
18 years ago