[JoshK]

Thanks Bob.  I had found a neat site last night that displayed how to calculate the shunt resistor needed.   I didn't get what CTR stood for but I figured what it mean in context.   The full scale is 5 amps and the CTR is 80/5 (display is in amps), so you are transforming 5 amps into 80 amps. 

Except I am not sure I want to draw my entire system through a resistor and resulting voltage drop.  I will probably just add a switch to switch it in for measuring and then switch it out again. 

[ctviggen]

Oh, that makes sense.  Yeah, do you really want to waste that much power for no reason?  I wonder how my Monster Power Center does this, as it constantly measures current and voltage?  They must use some bleed-off type of system to minimize the power loss. 

[Audiovista]

Here are some lossless methods to measure AC current:

Current transformers:

http://rocky.digikey.com/WebLib/Amveco-Talema/Web%20Data/AC1015.pdf

(www.digikey.com, search for TE1015-ND, there are many different ratings).

Hall-effect sensors (can measure AC and DC):

http://dkc3.digikey.com/PDF/T071/P2008.pdf

(link to a Digi-Key catalog page).

Boris

[JoshK]

Thanks Boris!  I should have thought to use a tx.  That makes a lot of sense (pun?).   I still need to work out the details to how that works.   I am assuming the mains go through the primary and the secondary has the transformed voltage/current, then I need to work out the multiplier off that V/I to get up to my scale.

[Audiovista]

Glad I could help

With a CT you just pull one mains wire through the center of the CT and secondary current will be reduced by the turns ratio. You need to close secondary with a resistor (I to V conversion and scaling - just make sure to operate within CT's linear range). If you need a DC signal, make sure to put diodes before the resistor, to eliminate the diodes' voltage drop. Then, you may want to eliminate ripple by filtering the signal (active filters are probably better than simple capacitors, but it depends on how fast response do you need.

In the end, it may happen that a hall-effect sensors are simpler to use, even though they are little more expensive.

Measuring power is more complex, but I think there are ICs out there that will do the job for you. I'll be glad to try helping if you get stuck.

Boris

[JoshK]

OK, I was going to split this thread as I have completely hijacked the original topic, but I think the orig poster got his question sufficiently answered so I will keep it here for continuity.

I looked at those CT and I am not sure they are appropriate for my application since they are 1000:1 step down.  I think I am ideally shooting for 16:1 but anything in the 1:1 to 16:1 range could work for me.  To be clear, I am trying to use the ammeter I have in the picture that has a full scale of 5amps but reads full scale at 80amp (which is 16:1).  I like the idea of a CT because it is minimal loss but the 1000:1 stepdown wouldn't work as far as I can see as it steps down the current much too far.