Hi Mark, If I can only measure and vary the 600 ohm resistive load then all voltages and currents that I can measure are in phase and complex math is useless. This yields Zo=13 ohms but it sounds wrong. I like your 70 ohms above but don't know how to go about measuring it. OT Got to get a new SPICE puter. You were a big help the last time so I may bug you again. Thanks, Harry
Oops. That is treating ro as if it is real, which it isn't. Changing the external load resistance will rotate the phasor, so I over simplified the problem.
Hey Rex, Love your equation but it yields the dreaded 13 ohms. E1=14.81Vac, E2=14.50Vac, R1=600 ohms, R2=300 ohms. Is that the correct answer at 400 Hz? By inspection I say it's about 70 ohms but cannot prove it. The 71 ohm reactance is in parallel with the 600 ohm load resistor. Regards, Harry
Harry, when you change your resistive load you are changing the voltage divider of it and the series capacitor mentioned. So you have changing phase angles. Except you claim it's not in series.
It would have been better if you'd have included a diagram of how you are hooked up and where your measurement point(s) are instead of other people guessing from your verbal description.
when I solve that for Ri (with the help of math software), I get
Ri = 0.567 ohm
Just for fun, I calculated Es at about 14.9 V
Hopefully, I didn't completely screw this up.
So Spehro is right. There is not much there to measure with your setup and Xc dominates.
Just as a sanity check, I removed Ri assuming the circuit is only Xc and Rm. I let Es be 14.9 V and calculated Vm for Rm=600 and Rm=300. I got
14.80 and 14.50, which is almost exactly what you measured. So we can assume Ri is low enough in your circuit to be in the noise with your measurement.
Since your DMM doesn't give you phase, you can't solve for Ro.
...Jim Thompson
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I read in sci.electronics.design that Harry Dellamano wrote (in ) about 'Amp output Z', on Mon, 11 Apr 2005:
I am astonished at all the discussion on this. It actually isn't helped by your loose terminology. You've been using 'output impedance' to mean both the output source impedance and the load impedance. You also seem confused between series and parallel (or perhaps you are mixed up between Thévenin and Norton equivalents).
You DON'T need complex numbers as such, but you DO need to take the reactance of the capacitor into account.
Assuming that the output source resistance is 10 ohms, as shown in the various circuits that have been posted, The voltages you should measure with your DMM are related by:
600/{sqrt(610^2 + 71^2)} and 300/(sqrt{310^2 + 71^2)}
Their ratio is 0.9655. How does that compare with the ratio of the voltages you measure?
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Exactly. Harry does some rather cool and important work, and has a nice lab. Just a glitch.
Probably why I didn't feel like posting multiplication by conjugates and stuff well after midnight and resorted to a bit of hand-waving to describe the results... ;-)
Best regards, Spehro Pefhany
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I read in sci.electronics.design that Harry Dellamano wrote (in ) about 'Amp output Z', on Tue, 12 Apr 2005:
To get 14.81 V out, you need supplies exceeding +/- 21 V, otherwise your output waveform is clipped.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
\'What is a Moebius strip?\'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
You could try measuring the voltage across the cap for starters. Won't be in phase with the load - but it'll tell you the load current !
I'm sorry but you have to be the biggest example I've seen to date of believing simulations over real practice. If you'd learnt Electronics 101 you wouldn't have to ask this kind of stuff.
Yep, I should have reduced it to what the meter actually shows. However I'm pretty sure you can't get the answer you want.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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