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| Fixture to safely measure mains AC amperage |
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| alm:
--- Quote from: bdunham7 on June 06, 2022, 06:03:48 pm ---If you are wanting a PF near 1.0, you need to use a variac and not a dimmer. A phase-control (dimmer) circuit is likely going to result in some pretty poor results from current clamps and probably your IOT power plugs. A P3 KillAWatt or the (direct) amp measurement of a good DMM with TRMS will be much better. --- End quote --- What is the advantage of a TRMS DMM over a TRMS current clamp here that makes the DMM so much better? Crest factor? Bandwidth? Comparing the earlier mentioned Fluke 375 to Fluke 175 doesn't show much difference: 500 Hz bandwidth, crest factor of up to 3 for the clamp, compared to 1 kHz bandwidth for ACI, crest factor of up to 3 for the DMM. Obviously measuring current is not a good indicator of power as soon as there's any phase shift between voltage and current, like with a capacitive or inductive load. A DMM is no better than a clamp meter there. For this you either need to do digital or analog multiplication of instantaneous voltage and current, like decent plug-in power meters do. |
| bdunham7:
--- Quote from: alm on June 06, 2022, 09:14:56 pm ---What is the advantage of a TRMS DMM over a TRMS current clamp here that makes the DMM so much better? Crest factor? Bandwidth? Comparing the earlier mentioned Fluke 375 to Fluke 175 doesn't show much difference: 500 Hz bandwidth, crest factor of up to 3 for the clamp, compared to 1 kHz bandwidth for ACI, crest factor of up to 3 for the DMM. --- End quote --- Yes, mostly bandwidth...and I wasn't thinking of the 175 as a 'good' DMM in this context. Nothing against it, but as you point out, it isn't any better than a decent clamp for this. Many DMMs have more suitable ACI BW of 10kHz and more. --- Quote ---Obviously measuring current is not a good indicator of power as soon as there's any phase shift between voltage and current, like with a capacitive or inductive load. A DMM is no better than a clamp meter there. For this you either need to do digital or analog multiplication of instantaneous voltage and current, like decent plug-in power meters do. --- End quote --- Yes, power is different, but the EEVBlog 121GW and the P3 KillAWatt, among others, can manage at a reasonable price. And there's obviously more than phase shift to worry about as even linear PSUs with transformers can get very spiky. But the OP asked about current. |
| BeBuLamar:
The 175 has slightly higher accuracy 1.5% vs 2% and 2.5% for the 375. Also the 375 has resolution to 0.1A while the 175 has the resolution of 0.01A. Also the 175 is less expensive than the 375. But since you said "Safely" I would much rather use the 375 or other clamp meter. |
| alm:
--- Quote from: bdunham7 on June 06, 2022, 10:21:36 pm ---Yes, power is different, but the EEVBlog 121GW and the P3 KillAWatt, among others, can manage at a reasonable price. And there's obviously more than phase shift to worry about as even linear PSUs with transformers can get very spiky. But the OP asked about current. --- End quote --- That's true, but a TRMS meter (clamp or otherwise) with sufficient bandwidth and crest factor should be able to manage a spiky current waveform. But no amount of fancy measuring of the current will help if you try to measure power and the voltage and current waveforms are substantially different (either in phase or shape). And since the goal of this is apparently to calibrate a power-measuring device, I think it's good to make the point that just measuring the current may not be sufficient for non-resistive loads, like a small SMPS without power factor correction. |
| bdunham7:
--- Quote from: alm on June 07, 2022, 08:54:51 pm ---And since the goal of this is apparently to calibrate a power-measuring device, I think it's good to make the point that just measuring the current may not be sufficient for non-resistive loads, like a small SMPS without power factor correction. --- End quote --- Rereading the first post, I think you may be right that the OP has not considered this properly. |
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