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Building a power meter application with an 8-bit MCU

5/15/2011 05:10 AM EDT
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B.V.Rao
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re: Building a power meter application with an 8-bit MCU
B.V.Rao   5/20/2011 5:53:28 AM
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I agree with about ENOB not quite capturing the meter performance. I also fail to understand how averaging can be done on samples, without reducing effective sampling rate, where phase relation and measurement of at least 3rd harmonic is very important. Flash life is not mentioned in the article, will it replace EEPROM where load survey information need to be logged? I have designed meters with controllers from 3 different manufacturers and meeting (qualified to) the IS standards (very close to IEC, in some places more stringent). My experience is as follows: 1. Even a 10bit converter with appropriate front end analog circuitry will meet the 1% requirement, needs multi-point calibration and time. 2. when using a multiplexed ADC the phase correction algo is vital for accuracies in lead/lag 3. Some (Most?) of the SOCs are found lacking the required code/data space to fully implement various comm. protocols and event logging. For this Vendors do provide solutions that are difficult to maintain. While surveying the market I have even found some meters with non-linearity (still meeting the spec. as the spec demands testing at fixed points) In my view a 16bit (min), differential input, 500mV, 500usec(minimum) ADC is the best bet for metering application. Any GP micro with h/w multiplier and good memory space, LCD driver, serial ports etc., would do for rest. Finally, it is BOM cost and not technology any more in this segment. You choice should depend on how much you can bargain from your chip vendor.

UnderboatBoy
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re: Building a power meter application with an 8-bit MCU
UnderboatBoy   5/19/2011 4:15:58 PM
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PDF it or forget it... Sorry

grehgrth
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re: Building a power meter application with an 8-bit MCU
grehgrth   5/19/2011 10:05:44 AM
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kendallcp
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re: Building a power meter application with an 8-bit MCU
kendallcp   5/18/2011 5:12:19 PM
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The relationship between meter reading error and input power for any given ADC is very subtle, and a broad-brush statement of an ENOB requirement completely fails to capture it. If the ADC's specific linearity behaviour is not good enough, averaging will also not help. You just get the same wrong number with less variability. When trying to persuade meter manufacturers to accept a general purpose micro rather than using a dedicated design that's characterized for this specific role, you need to show a great deal of proof that the impairments in the signal path produce predictable and bounded errors in the power measurements. Most of these errors could be calibrated out, but no meter manufacturer wants a calibration process that takes many minutes to run. Single-point is the way to go, and that can't compensate for any gain linearity errors, either in the ADC or in the current sensing element.

apummer945
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re: Building a power meter application with an 8-bit MCU
apummer945   5/18/2011 9:39:40 AM
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it would be very practical to have the e-mail address of Radomir Kozub around that article

Laxman.Karandikar
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re: Building a power meter application with an 8-bit MCU
Laxman.Karandikar   5/18/2011 8:57:05 AM
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The equation of ENOB does not give stated result.

Efried
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re: Building a power meter application with an 8-bit MCU
Efried   5/18/2011 8:13:26 AM
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I don't know why WLAN routers/switches don't have embedded web servers with S0 and 1wire interface. Would ease home automation so much.

Jerry.Brittingham
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re: Building a power meter application with an 8-bit MCU
Jerry.Brittingham   5/18/2011 2:03:46 AM
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Where is the schematic?

Haldor
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re: Building a power meter application with an 8-bit MCU
Haldor   5/17/2011 8:37:32 PM
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Figure 1 doesn't make much sense. Drawing power from the mains neutral? And isolating the high side voltage and current measurement from mains power is also not a trivial task. Did the author actually build this power meter? How about a link to a schematic?

Haldor
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re: Building a power meter application with an 8-bit MCU
Haldor   5/17/2011 8:34:31 PM
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Figure 2 is a mess (rendered with IE V7). Does anyone else see it correctly (not all jumbled up)?

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