Low cost high side current measurement

[tylerjbrooks]

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.

 

[mikersia]

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).

 

[dick_freebird]

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.

 

[tylerjbrooks]

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).

 

[eanema]

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.

 

[love_analog]

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.

 

[tylerjbrooks]

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.