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small (3A) AC current measurement

Hi guys,

I'm trying to measure resistance, current, and voltage of a resistor (heater) connected to a 0-24V @ 3A transformer.

The heater's consumption is actually 2A max., and I'd be controlling it with a triac driven with an 8-bit micro, which I'd also use to get the above readings and send them to the host.

This is my initial layout:

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As the transformer only has 0-24V output, and I'm using the usual op- amps + shunt configuration, my main worry is that what I've done is actually electrically safe.

Also, any other comments are also very welcome, including suggestions as per the op-amps (part) to use, about the layou itself, etc.

This is the "pseudo-code" I more or less have in mind for it:

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Cheers,

Reply to
Luis Perez
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If "N" means the neutral wire and not protective earth then no, it is not safe. In pretty much all jurisdictions I've dealt with it isn't allowed unless the whole thing is fully isolated per their requirements. It says "Earth Ground" at the transformers so it must be connected to PE, not N.

Also, the schematic doesn't say whether the two kinds of ground are connected and where. So the dividers at the opamp inputs can be floating which wouldn't be cool.

Put some values on the resistors so we can all see whether it's ballpark accurate. Usually large values for the dividers and then a low-ohms shunt won't be so stellar for accuracy.

Can you ground one side of T16's output? That would make life a ton easier.

The triac might need a snubber. Depends on the type.

I'll leave that to the code gurus :-)

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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The way you have your opamps they will operate with no feedback and thus work more like voltage comparators with their outputs saturated at either VCC or GND. Not so useful for feeding into an A/D converter.

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Michael Karas
Carousel Design Solutions
http://www.carousel-design.com
Reply to
Michael Karas

=3D0Bz...

If there's a safety issue I'm not seeing it, as long as you relabel 'N' as 'E'. Your 9v transformer gives about 13v dc, leaving Vreg to drop 8v. You might as well run them off the 24v psu. The whole arrangement of multiple opamps seems odd. Starting with a differential stage might make more sense, with both inputs divided down to lower v.

NT

Reply to
NT

=3D0Bz...

Thanks, done.

The maximum voltage the lm7805 accepts (as per it's spec) is 18V, but even if it worked, wouldn't it get too hot?

Reply to
Luis Perez

d=3D0Bz...

You dont need regulation for the opamp.

Reply to
NT

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