Converting DC Ammeter to AC Ammeter

DC AMMETER CONVERSION

Many of us prefer analog meters over digital meters for our control panels. Suitable AC meters seem to be in short supply. For this reason I purchased two Radio Shack DC analog meters, on closeout sale, planning to convert them to AC. Ezak's post, "Panel Meters" prompted me to show my conversion of the DC Voltmeter to AC. See:
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The AC Ammeter was posing a challenge. I did not want to build an AC to DC instrument amplifier for it, because that would require an additional power source for the amplifier. The neat thing about analog meters is that they can be powered by the voltage or current that they are measuring. Then Lionelsoni suggested using a bridge rectifier wired to supply AC current to the load and DC current to the meter. Figure 1 shows how I imagined implementing the idea.

Figure 1

One of the problems when using half or full wave rectified AC on DC meters is that the meter needle tends to follow the ripple, evidenced by the needle vibrating at 60 or 120 Hz. Lionelsoni said, in part, "...the shunt might provide enough damping to make the meter readable." In other words, the shunt would dynamically brake the meter like dynamic brakes on a train engine brake the train. See < for more information. Photo 1 shows my experiment.

Photo 1

I am happy to report that the meter needle is dead still.

Table 1 shows actual current, DC without rectifier, AC with rectifier, when meter is reading cardinal points.

Table 1
FS V = 0.09437 VDC
R = 0.09437 Ohm
0 - 1000 mA DC Ammeter
Meter____Actual__Actual
Reading_DC amp_AC amp
200_____186____209
400_____380____428
600_____583____654
800_____776____870
1000____971___1090

I had imagined a small glitch at the zero crossing point of the load waveform due to the nonconductive region in a diode's voltage curve. Puzzlingly, the voltage across the load looks like a smooth sine wave. [?]

Photo 2