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EEVBlog #87: The Electronics Design Merry-Go-Round

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ModernRonin:
I really liked this one. I haven't personally played the optimization / iteration game, but I've known many people who have.

From a technical perspective, I'm curious if a high-side current monitor based on an opamp is possible in this particular application.

I was looking into circuits to sense mains current a while back, and eventually concluded that I'd probably end up having to use an expensive isolation amplifier if I wanted to do high-side current measurement directly. (I assume that some kind of isolation amp is what's inside those expensive high-side current monitor ICs.) But there's got to be a better way, right?

A couple of ideas occurred to me on how to do it. One, is there some way to very accurately level-shift the high-side sense inputs down to lower voltages? (Resistive voltage dividers? Precision resistors are expensive...) Two, is there a way to power the op-amp from a mains-like voltage so the high-side inputs won't be far off common-mode voltage?

I like the EEVBlog because it make me think about stuff like this.

rodneykeeling:
Can someone explain a high-side current monitor? I have a good idea as to what it is, as the name is quite descriptive in itself, but I'm not sure how it would be put into use.

ModernRonin:
You can read the "Applications" section of the data sheet:

http://www.zetex.com/3.0/pdf/ZXCT1009.pdf

Seems to be mostly related to monitoring batteries. My guess is it's mainly about recording how much current they're presently discharging, or monitoring how much current a charger is putting into them.

Zad:
Much appreciation from me for this Vblog. Much of electronic engineering is this "admin" kinda work. It is engineering because you are optimising your components for the given task. You need to know a lot about a lot in order to speed it up much, and they just don't teach it in Universities and Colleges, yet it is the part of the job that has the potential to save employers the most money (excepting extreme circumstances of course).

In the past I have generally used the resistive divider method of high-side current measurement. Looking at the Farnell site, you can get 0.1% for £0:05p. That sort of accuracy is unnecessary in most systems, so if you go to 1% and you find them at 0.4 of a penny. Of course the resistive divider isn't suitable for all systems, but that's engineering for you. "Time, Spec, Budget. Pick 2."

hans:
A lot of read stuff for low and high side current sensing:

http://focus.ti.com/analog/docs/microsite.tsp?sectionId=560&tabId=2180&micrositeId=7

A big advantage of high-side current mode is you aren't measuring against ground, but just a voltage drop on a resistor. You can also wire up the load the way you want, the shunt resistor can be placed anywhere as long as it's in series with the current you want to measure.

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