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Electronics => Electronics => Topic started by: madsci1016 on October 20, 2009, 09:17:26 PM

Title: Shunt Resistor or Current transformer
Post by: madsci1016 on October 20, 2009, 09:17:26 PM

I would like to build a electrical monitoring system for my house similar to this:

http://www.picobay.com/projects/2009/01/real-time-web-based-power-charting.html (http://www.picobay.com/projects/2009/01/real-time-web-based-power-charting.html)

Does anyone know where i could get an inexpensive current transformer probe or reasonable shunt resistor to monitor current on AC mains? I have been googling for days but have't found a current transfomer probe less then $100 (I would like it around 20)

Thanks

Title: Re: Shunt Resistor or Current transformer
Post by: waltr on October 21, 2009, 08:29:35 AM

The current probe pictured in the link will be in the $100+ range.
There are board mounted current sensors of various types that are cheaper. These will require additional circuits to make a measurement.

Do NOT use a shunt resistor when measuring AC mains. The current transformer and hall sensors provide isolation of the measurement circuits from the mains.

Here are a few links:

http://www.allegromicro.com/en/Products/Categories/Sensors/currentsensor.asp (http://www.allegromicro.com/en/Products/Categories/Sensors/currentsensor.asp)

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=582-1033-ND (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=582-1033-ND)

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=TE1010-ND (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=TE1010-ND)

These devices do require you to build a board that is then electrically connected the the mains wiring.
 PLEASE BE CAREFUL as there is enough voltage and current in the mains to kill.

Title: Re: Shunt Resistor or Current transformer
Post by: madsci1016 on October 21, 2009, 06:12:30 PM

I am well aware of the danger, and have thousands of hours working with electrical power that makes the breaker box in my house seem like a 9V battery. Thanks for the warning though (I'm really not being sarcastic, I appreciate anyone who checks before giving advice. )

Actually, the probe pictured costs $15 dollars new.

http://200.78.236.216/catalogo/interior3Shop.asp?pdto=PUN-285 (http://200.78.236.216/catalogo/interior3Shop.asp?pdto=PUN-285)

They just won't have them in stock for months. It seems odd to me the are so cheap. I just don't want to what that long.

Isolation from AC mains and Shunt Resistors can be achieved through opto-isolators. Is that not enough, or am i missing something?

Thanks for the links, i will research them when i get a chance to sit down and read through spec sheets.

Title: Re: Shunt Resistor or Current transformer
Post by: billhowl on October 21, 2009, 07:34:07 PM

If you like to use shunt resistor to monitor current on AC mains, you can use this Avago Technologies HCPL-7520 linear optoisolator.
The HCPL-7520 isolation amlifier / isolated linear current sensing IC is designed for current sensing.
http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2008/cj72_xg37/cj72_xg37/index.html (http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2008/cj72_xg37/cj72_xg37/index.html)
http://www.avagotech.com/pages/en/optocouplers_plastic/plastic_miniature_isolation_amplifier/hcpl-7520/ (http://www.avagotech.com/pages/en/optocouplers_plastic/plastic_miniature_isolation_amplifier/hcpl-7520/)

Title: Re: Shunt Resistor or Current transformer
Post by: waltr on October 21, 2009, 07:55:37 PM

Bill,
 Nice project write up in the link. thanks.

madsci1016,
 Right, there are a lot of newbees on this site so felt the mains warning as necessary. I've done a lot of electrical work but I am a bit cautious in the breaker box.
Good luck with the project.

Just remembered that Microchip has a few app note on power meters and power PICs directly off the AC mains. There may be some info in these app notes that you can use.