First, you can't dim AC with a zero-crossing device! *
Incandescent AC dimmers work by triggering (turning-on) a TRIAC at some point during the AC half-cycle. Once triggered, the TRIAC continues to conduct until the current goes to (nearly) zero at the next zero-crossing.
Assuming the trigger signal is no longer present, the TRIAC turns-off at the zero crossing.
If you trigger at (actually just after) the zero crossing, the TRIAC somes on and stays-on for the full half-cycle and is NOT dimmed.
If you trigger just before the zero crossing, the lamp will come on for a short period of time and it will be very dim.
That means you have to sense the zero crossing or some constant point along AC waveform (with transformer or optical isolation), delay for some part of the half-cycle, and trigger the TRIAC (or TRIAC- based relay). This is something like PWM, but regular PWM will NOT work because it will trigger the TRIAC at some random non-synchronnized point along the waveform, and the TRIAC will remain on for the remainder of the AC half-cycle.
(PWM signal - 1 = 60VAC, 255 = 240VAC), but when I connect a load (lamp) it's 1 = 240VAC and 255 = 240VAC.
A digital multimeter will not reliably measure the average of a voltage that's switching on & off or jumping around unless it has a "ture RMS" feature. An old analog "mechanical" meter will work better, and since there is often some leakage, you'll get better readings with a lamp connected.
- You can make a simple high - low - off dimmer that turns lamp on for only the positive (or negative) half cycle. But, you can't do full-range dimming with a zero-crossing device.