Trying to understand RMS
Brian Carpenter
Posts: 728
I have purchased 2 of the amp 25.· If you click on·the picture, it will take you to the pdf. I have 5 volts from a bench power supply feeding the sensor and i have my oscilloscope connected to the signal wire. The spec sheet says that at 5volts and and no load, i shuld be reading an output voltage of 2.5 volts.· At 5volts and 25 amps (ac)·i should be getting a sine wave that is 3.425 at the high·and 1.575 at the low.· this is .925 volts either side of 2.5 volts.· The reading is linear between the range.· So my question is, knowing all the above, how do i determine amperage at points in between.· is it as simple as
2.5v···· = 0· amps
2.685v· = 5· amps
2.87v··· = 10 amps
3.055v· = 15 amps
3.24v··· = 20 amps
3.425v· = 25 amps
So i connected my 800 watt toaster(thats whats stamped on the bottom) to the device and i get .22v RMS
so that is an output of 2.72v.· which puts me at 5.95 amps (pretty close).·
Here is my problem.· My scope is doing the RMS math.· How do i make a uC do it? prop or sx
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
It's Only A Stupid Question If You Have Not Googled It First!!
2.5v···· = 0· amps
2.685v· = 5· amps
2.87v··· = 10 amps
3.055v· = 15 amps
3.24v··· = 20 amps
3.425v· = 25 amps
So i connected my 800 watt toaster(thats whats stamped on the bottom) to the device and i get .22v RMS
so that is an output of 2.72v.· which puts me at 5.95 amps (pretty close).·
Here is my problem.· My scope is doing the RMS math.· How do i make a uC do it? prop or sx
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
It's Only A Stupid Question If You Have Not Googled It First!!
Comments
800 watts? Perhaps you should take your toaster to this house.
Check out this link; it has formulas for just about everything.
http://www.calculatoredge.com/#electrical
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
This post is a work of art. Variations in spelling and grammar are intentional, artistic endeavors that add value to all of mankind.
· For a pure sine wave, RMS voltage = peak-to-peak voltage * 0.3535
· Or in your case you could use (peak voltage - 2.5) * 0.707
Bean.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
"It is our choices, Harry, that show what we truly are, far more than our abilities."·Dumbledore from Harry Potter
www.iElectronicDesigns.com
Post Edited (Bean (Hitt Consulting)) : 10/22/2008 12:34:34 PM GMT
if my peak voltage for the toaster is 3.05volts. I take (3.05 - 2.5 = .55) if i then take that and (.55 * .707 = 0.38885 ) What does this number associate with? sorry i am confused.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
It's Only A Stupid Question If You Have Not Googled It First!!
The RMS value is the effective value of a varying voltage or current and is only true for sine waves. It is·equivalent to the steady DC (constant) value which will give the same effect.· So in other words.... the 0.38885V·would be the equivalent steady DC voltage that would have the same effect as the 0.55V peak AC supply.
Reference:
http://www.kpsec.freeuk.com/acdc.htm
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 10/22/2008 4:37:02 PM GMT