Rectifier

put a full wave rectifier (optionally)

Put a proper low pass filter to filter off the pulsation.

A half wave rectified signal varies in amplitude from zero up to a maximum voltage.

With your multimeter set to ready a steady dc current (or voltage) the internal Analogue to digital converter sees a varying signal, which it doesn't like.

Most multimeters have the A/D clock set to cancel out mains frequencies as much as possible, but what you are doing is using it on dc to measure a ½ ac waveform, so its confused and will give invalid readings.

Look up the form factor of the half wave rectified signal and you will see its a miracle it reads anything sensible!!!

It sounds like you're using an inverter without a power-regulation IC (integrated circuit) right after it.

A half-wave rectifier will enable most of a half-cycle of current to be fed into a capacitor, which will be downstream of the diode that will allow the current through. The capacitor charges up if the line voltage minus the voltage drop across the diode and the resistor between the capacitor and the diode is greater than the voltage across the capacitor.

Once the capacitor starts to charge, it'll keep doing so until the crest of the line voltage will be reached. Past that point, the capacitor starts to discharge into the load, thus decreasing the voltage across the capacitor leads. If the load is a pure resistor (no inductance, no capacitance), then the voltage across it will drop as the capacitor's drops, the current passing through it will drop in proportion, both until the capacitor starts to recharge during the next half-wave. That's why your current reading bounces up and down ... your current's magnitude really DOES oscillate.

The way to eliminate these oscillations is to stick a voltage-regulation IC (integrated circuit) just downstream of your rectifier's capacitor (so between it and your load). National Semiconductors and a whole lot of other manufacturers make these.

Cheers!
DZ

Using a full wave rectifier would improve the situation. Adding a big filter capacitor (big electrolytic) is also a good idea. The reason for the fluctuation is that the half wave rectified current is pulsating. Your digital meter samples the current periodically and the sampling times occur at different parts of the waveform and give you the fluctuation.

Google: Pi filter and build one and put on output of the rectifier.As mentioned, a full-wave bridge is more efficient.A half-wave rectfier will only give 30 hz pulses, a full wave will give 60 cycle pulses,and a full wave bridge will give 120 hz pulses.Much smoother and easier to filter.The capacitors in the Pi filter stabilize the voltage, and the chokes stabilize the current.

Good luck with your project.

HTRN