19-10-2017, 10:36 PM
Without a look inside, it's hard to be sure, but it appears to be a switched mode power supply minus output rectification. So the AC is converted to DC with a bridge and capacitor, then chopped up ready to be pushed through the internal transformer. The chosen frequency will be a design decision - the higher the frequency, the smaller the transformer, but too high a frequency and other problems occur. I'm not aware of 22kHz being a standard, and if anything, that does seem to be a bit on the low side for the physical size of the unit.
In theory, the switching frequency shouldn't affect the current draw from the mains because of the initial rectification. In practice, there will be an amount of the switching frequency leaking out, but I don't think it should significantly affect a moving iron current meter.
Digging deeper, there are two more interesting clues: a near-unity power factor, and it's dimmable.
Both these features imply the presence of power-factor correction. This is an additional boost converter that sits between the bridge rectifier and the main switching converter, and its job is to ensure that the current draw from the mains is as sinusoidal as possible (as opposed to the "gulps" of current that are drawn when you have a simple rectifier and capacitor). To do this, the IC controlling the boost converter monitors the input voltage, and it's easy to use this as an input for the purposes of dimming.
If this is the case, then there's even less reason to suspect that a moving iron ammeter will be grossly inaccurate.
Further investigation is required. I'd start by measuring the output voltage and current delivered to the load. Then, remembering that these units aren't 100% efficient (perhaps 90%?), that gives me a better idea of what the input current should be (rather than making assumptions based on the bulbs used). But, measuring the output is non-trivial, given the high-frequency nature of the output waveform. I'd want to look at the AC input current on a 'scope that is also showing the applied input voltage to see how sinusoidal and how in-phase it is.
Nothing is simple
In theory, the switching frequency shouldn't affect the current draw from the mains because of the initial rectification. In practice, there will be an amount of the switching frequency leaking out, but I don't think it should significantly affect a moving iron current meter.
Digging deeper, there are two more interesting clues: a near-unity power factor, and it's dimmable.
Both these features imply the presence of power-factor correction. This is an additional boost converter that sits between the bridge rectifier and the main switching converter, and its job is to ensure that the current draw from the mains is as sinusoidal as possible (as opposed to the "gulps" of current that are drawn when you have a simple rectifier and capacitor). To do this, the IC controlling the boost converter monitors the input voltage, and it's easy to use this as an input for the purposes of dimming.
If this is the case, then there's even less reason to suspect that a moving iron ammeter will be grossly inaccurate.
Further investigation is required. I'd start by measuring the output voltage and current delivered to the load. Then, remembering that these units aren't 100% efficient (perhaps 90%?), that gives me a better idea of what the input current should be (rather than making assumptions based on the bulbs used). But, measuring the output is non-trivial, given the high-frequency nature of the output waveform. I'd want to look at the AC input current on a 'scope that is also showing the applied input voltage to see how sinusoidal and how in-phase it is.
Nothing is simple
