Author Topic: EEVblog #409 - EDMI Smart Meter Teardown  (Read 21626 times)

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Offline Pilot3514

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Re: EEVblog #409 - EDMI Smart Meter Teardown
« Reply #50 on: March 24, 2013, 02:08:01 pm »
Just think about what you mean by 'negative'. Negative relative to what? Relative to the VSS pin of the micro? Not necessarily...
Obviously I meant negative relative to the VSS of the micro.  I was talking about the ADC, the ADC is part of the micro.  The input to the ADC must be between the ADC's ground and reference voltages and must also be between the ground and supply voltages of the micro.

If you make a potential divider between the ...
This pretty much describes the second schematic that I posted previously.
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unlike those wasteful Scotch.
 

Offline lewis

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Re: EEVblog #409 - EDMI Smart Meter Teardown
« Reply #51 on: March 24, 2013, 03:19:03 pm »
Obviously I meant negative relative to the VSS of the micro.  I was talking about the ADC, the ADC is part of the micro.  The input to the ADC must be between the ADC's ground and reference voltages and must also be between the ground and supply voltages of the micro.
Yes, I understood what you meant. I was trying to get you to reaffirm your concept of 'ground'.

This pretty much describes the second schematic that I posted previously.

I didn't look at that diagram. Yes, you've got it right!
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Offline Pilot3514

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Re: EEVblog #409 - EDMI Smart Meter Teardown
« Reply #52 on: March 24, 2013, 05:33:44 pm »
Yes, I understood what you meant. I was trying to get you to reaffirm your concept of 'ground'.
No offense taken.  I sometimes get frustrated when I am looking for intelligent dialog on a specific issue and get general comments or restating the question as a statement instead of adding to the discussion.  I would much rather have someone tell me I'm a fool, and then tell me why, than tell me the problem is the same if looked at it from a different angle. 

One solution to the problem was not to try to reference the voltage from ground but from a point half way between the supply and ground.  With that answer and a voltage divider I have a workable solution.

As always, there are other solutions, but this one is simple and should work.  I need to look at what will happen if my voltage still goes negative or above the supply voltage.  It could go over +/- 2.5 V, in the example, if the current exceeds 2.5 A.  Do I want to clamp that with production diodes?  Which is more forgiving, the input to the micro or the input to the OpAmp/Buffer.  Maybe one or both have some level of input protection built in.

Do I loose my protection or my range?  I don't think I want to incur the expense of a rail to rail OpAmp to get that last little bit of range.  Which, to me, means protect the front-end of my micro with a buffer or get the full range by reading the voltage directly.

I would like to think that the solution is not so obvious that I missing it or that I am the only one thinking about such things.

I didn't look at that diagram. Yes, you've got it right!
I'm glad you saw it.  It took me some time to put that together.  My CAD is not a good as "Dave CAD".
I'm not cheap, I'm frugal
unlike those wasteful Scotch.
 

Offline sakujo7

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Re: EEVblog #409 - EDMI Smart Meter Teardown
« Reply #53 on: November 23, 2015, 09:38:49 am »
Here is were the problems begin.  My little micro controller (insert your favorite) has a built in ADC but it reads volts, not amps.  So I need to convert volts to amps.  This is done all the time, I need a resistor.  So if a put a 1K resistor across the transformer, I get 1 volt per amp in the primary.  This looks promising.

I'm not sure if anyone is still interested in this, but a 1k resistor is probably too high and will cause linearity errors and such. The data sheet for the particular CT should specify a nominal burden. If the output is too low, use an op amp to amplify it up to something more usable.

If you want a safe way to sorta measure voltage, an unregulated voltage transformer (eg. 240V to 12V AC) with a voltage divider will do the job. Unlike a CT burden which should be as low as possible, you instead want higher resistors here, such as 10k and 100k to get a ~1V output without overloading the transformer. You'll likely have a big phase error which will need to be compensated for if you want real power measurements.
 


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