EMRFD Message Archive 3740

3740 2009-11-05 13:10:50 ha5rxz Power Taps & Converting RMS Volts to Watts Message Date From Subject I have built a large dummy load and the power tap from W7ZOI's power meter project. Rather than build the power meter as well I am using my oscilloscope with a fifty ohm load on the input. Everything is working but I am having problems with the math: Power in watts = (Vrms)^2 / 50 as I am using a 50 ohm load BUT The power tap has a total resistance of 3000 ohms and a 50 ohm resistor with a 3000 ohm resistor in parallel is 49.18 ohms. So, should I use (Vrms)^2 / 50 or (Vrms)^2 / 49.18 to compute the power? HA5RXZ Hi Much simpler is Watts equals Vpk squared divided by 100. And Vpk is easy to take off the Oscilloscope screen grid, its half V peak to peak. Why? The aritmetic is RMS equals Vpk multiplied by 0.707, and 0.707 squared is 0.5. 73 Michael Mi5mtc Unless the power you are measuring is greater than 100 or 200 watts, you could measure the voltage on the scope directly using a 10:1 probe. Calculate the resulting voltage for the power you expect, and insure it won't overload the scope or probe ratings. If you use the divider, 49.18 might be the theoretical resistance the transmitter sees, but there are enough sources of measurement uncertainty that the resulting 2% error won't matter much. Stray capacitive coupling, scope measurement error, etc. Don't forget the divider expects to be terminated in a 50 ohm load at the scope also, and you have to include the division ratio of the power tap into the calculations. John KK6IL At 01:10 PM 11/5/2009, you wrote: >I have built a large dummy load and the power tap from W7ZOI's power >meter project. Rather than build the power meter as well I am using >my oscilloscope with a fifty ohm load on the input. Everything is >working but I am having problems with the math: > >Power in watts = (Vrms)^2 / 50 as I am using a 50 ohm load > >BUT > >The power tap has a total resistance of 3000 ohms and a 50 ohm >resistor with a 3000 ohm resistor in parallel is 49.18 ohms. > >So, should I use (Vrms)^2 / 50 or (Vrms)^2 / 49.18 to compute the power? > >HA5RXZ Thank you to both of you for the reply. I made a small error in my previous post, the resistance of the power tap is 2500 ohms rather than 3000 ohms, this makes the change in the impedance very close to 2% (49.2 ohms). I will therefore use this figure in my calculations. As for the accuracy involved I would like to get the measurements as close as possible. My oscilloscope has already been calibrated to better than 1% and by purchasing additional resistors I managed to find a combination which gave the needed value to within one ohm (measured on a lab grade instrument). The same accuracy was used in the selection of 50 ohm resistors for the power tap terminati just so that confusion stays... the peak to rms approximation works only for a sine wave function. it is assumed that the measurements are for a sine wave. - farhan Hi Ashhar The voltage measurement, peak to peak, that I suggested uses an Oscilloscope. So it is easy to check that it is a sine wave. 73 Michael Mi5mtc