I tried the modification on my Kill-A-Watt this afternoon. I don't really have the right setup to accurately assess the results, but they look promising-ish.
However, there are some caveats I learned.
For one, not all Kill-A-Watts are the same even if they look the same on the outside. I assumed that mine would be like The Electrician's, but actually, my two KAWs are quite different from each other, and different from his.
First, one of them is obviously highly cost reduced. It has single sided boards, just cheaper construction all around. I didn't even bother to measure the shunt because I didn't have a 4-wire setup that could do it properly, so I just removed it and put in the 0.2 ohm wire resistor I had bought. Result was that the meter read approximately 2x what it used to. Deduction: the shunt was not 0.002 ohm, but 0.1 ohm. Also, in the process of removing the shunt with my underpowered iron, I used brute force, and actually delaminated the trace around one side of the shunt. Joy. Anyway, I put it together again and it is about as good as it ever was, plus a ginormous blob of solder. Measures about 5% low all the time, which is what it always did. About half of that is from the voltage error, and half from current, as near as I can tell. Oh well.
Undeterred, I decided to apply the hack to my other KAW. I removed the shunt trying to use a little less brute force this time. There was so much solder on the connection, though, and I didn't want devote my whole roll of desoldering braid to the project, so I pushed pretty hard again. Similar delamination result, but less severe this time. Anyway, got the new resistor in there, and now the meter appears to read about 55x what it used to. More deduction: That old shunt wasn't 0.002 ohm, but 3.6 mohm or thereabouts.
Which leaves you wondering if they just grab any old piece of wire, shove it in there, and then trim as necessary. Sadly, the trimming must be done by flashing some register, since there are no pots in these devices at all.
Anyway, I have limited gear to calibrate. I used a Fluke 87V, a variac, and a 43W incandescent bulb to calibrate the current at various settings. It was very linear with respect to the ammeter, so I feel pretty good about the current measurement, at least for a sinusoidal signal. The variac introduces some inductance, so I got a none unity pf, but with a more or less constant VA addition (so the variac was consuming constant VA but the bulb wasn't) but without a power meter to compare, I have no idea if the unit got that right.
I also tried driving a 47k resistor. This came up as unity power factor (good) and based on the current measurement from the 87V and a mains measurement from same, the power estimate was pretty good (relative to the 55x adjustment). It is a few percent high. Anyway, I have reasonable confidence that this thing basically works correctly for nice loads. Because it uses sampling and multiplication in a DSP I hope there is no reason to think it would not work well for less nice loads.
One more trap for young players:
- to get a bit more precision out of my 87V, I was using the mA setting. No problem with the static current, but I zapped the fuse with the inrush current turning on the variac. :-( Goddam, it looks like an expensive fuse, too. And I knew that would happen. For some reason I thought that the fuse would not respond so fast. Silly me. Of course it will
OK, I think that's all for now. Overall, I think I'll leave the mod in. Pics attached.
Regards,
Dave J
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