Active, Input Impedance from V and I plots

Maklaka said:

I did, and I tried it just now. That actually worked for a single unit but unfortunately my power supply needed to be cranked to 26 volts to get the second to boot. That being said, I decided to try the CT method. Unfortunately, the CT - by virtue of being a coil - distorts the input sine pretty badly and despite my efforts to make some more windings, does not yield a satisfactory 2V peak to peak signal on the power line.

The gents are screaming about using a voltage follower to try to interface the function generator to the rest of the circuit with a lower impedance. Off to the lab with a hand full of Op-Amps!

Click to expand...

Hi again,


I realize i am a bit removed from the actual system you are testing so there are little things i wont know about, but i dont understand something here that seems rather basic. You say that you can turn all the units on at the same time and the boot, but when you turn one on at a time some wont boot? Sorry that doesnt make sense to me. Maybe what you are dealing with is a soft supply voltage ie input lines that are too long. If that's the case maybe you would add some capacitance across the supply lines before any switch.

What is the nominal input current draw of each unit, and are they all the same or different?
Also, what kind of batteries are you using to power this setup, and how are they connected?

The lines are part of the chain network system and they typically boot with ease from the power supply that I am using. Introducing the function generator in series drops the voltage below the required running voltage for all but the first unit in the chain. They also boot from the 12.5V battery when the function generator is not present. Nominal input current draw per unit is roughly 120mA.

I'm thinking of just going out to rent a power supply that has the ability to place an arbitrary waveform on the line.

Hi,

That would be an interesting way to go yes, but that's all that the units draw each is around 120ma? Seems it shouldnt be too hard to drive that kind of load.

I think that the problem might be that the function generator output is connected to earth ground, I don't think it's floating.

Hi again,


Ok let me know what you end up doing then. I'd be interested to find out how this works out.
As i mentioned, i tested small batteries before just with a resistor in series with the function generator. You dont need to generate a million volts of change just to measure something like this if you can scope down into the tens of millivolts and low amps too.

View attachment 66253

This is the best I could get from a system of one unit using the presumably inadequate equipment on hand. Indeed, not very sinusoidal. The function generator is able to produce a good sinusoid without hooking it up to the system.

Running the formula provided on that data assuming that it is sinusoidal (and it is clearly not) yields a result in the millifarad order of magnitude. This is NOT congruent with previous measurements taken using step-response charge/discharge.

Hi again,

Wow that's nasty. This is so bad that it is not usable. We'd have to get a phase shift which implies a sine phase shift not an arbitrary type phase shift.

Is that frequency the same as the function generator is putting out? I ask because something else could be causing that bad waveform. Normally a sine generator will drive something as a sine unless it is extremely non linear. I guess that is always a possibility though. The only way to measure then would be to step it and use a time domain approach.
Does the generator put out a square wave too, or how about a triangle wave?
Also, what happens if you reduce the gen output to a lower level, any better wave shape?
Sometimes the non linearity goes away with lower plus and minus deviations.

BTW the scope pics are the best way of showing results for these tests.