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Low cost high side current measurement

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tylerjbrooks

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I am designing a light dimmer for the US market that has power measurement capabilities. I am looking for a low cost solution. I want to be able to control/measure loads from 0W to 300W in 5W steps (or so) so accuracy is less important than cost.

My first thought was something like I have attached. I have built it on the bench with mixed result. The current sensor is 'touchy' as you might expect. I have also tried a couple traditional instrumentation amplifier configurations with a little better result. I find that in all configurations if I increase the AC voltage much past 120VAC, then the opamps become unstable and slam against the rails. I take this to mean that I am struggling with common mode voltage.

I guess I could use something like the INA148 but it is pretty expensive for our application (>$3 in Q1000). Is there a cheaper alternative? Maybe another approach? Maybe a better (and cheap) opamp that can take higher common mode voltage? Suggestions greatly appreciated.
 

Company EasyMeter GmbH sell power meter chip SPM3D less than 1$, shunt also ~1$, other components (one NPN, 3 diodes, one 300V 0.2uF capasitior, 2...4 LV capacitors, <10 resistors) - ~1$. At the output you can have pulse stream or binary code proportional to input power with accuracy <0.5%. Curent mesurements with shunt could be at any side (high/low).
 

What precludes you from taking your current measurement
off the low side? Presuming that you are not interested in
fault cases, current is the same at both ends.
 

mikersia...
Thanx for the tip about EasyMeter. The chip looks interesting. Where did you get pricing for the chip?

dick_freebird...
I was told that most 'gang' installations (more than one switch in a box) shared a neutral (in the box). I was also told that tying the neutrals together in such a box was standard operating procedure for most electricians. So, if that is the case, then I don't want to complicate the installation by making them find the exact neutral that is going to the load they wish to control/meter. Do you know if this is true? I have seen gang boxes done both ways (neutral bussed together and separate runs).
 

I could be way off on this, but it seems that if you want to measure current, wrap a coil around the hot wire and measure the induced voltage. With a current from 0 to about 2.5 amps you should have no problem measuring a voltage.
 

I am not an expert in this but can't you just use a Hall sensor (from the linkes of Allegro). Is that too expensive.
 

Thank you all for your posts.

Eanema, love_analog...
I was under the impression that current transformers and hall effect sensors were too big for my design (my space is 1.5" by 2.5", double sided). However, I see that they are not huge (especially hall effect sensors) so they might deserve another look.

If I reference my power supply to the high side, then things get pretty simple/cheap. However, it is not clear to me that this will 'pass regulation' so I might still be in the woods. I guess I am going to have to buy the regs and see what they say.

I am posting my high side schematic and spice results. I think this is a pretty cheap result. I haven't put this on the bench yet but I think it is going to work.

Please ignore the 'goofy triac trigger'. I am going to connect all this to an MC13224 and control the dimming remotely.
 

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