Quote from: poynt99 on May 15, 2012, 12:50:39 PM
No, I think you are confusing what I'm saying. Yes, the SPICE simulation indicates that if we add the currents as shown on the diagram, all makes sense. BUT this is only for the case when Q2 is present.

I stated two (2) distinct Q1 bias conditions, one when Q2 is present, and one when Q2 is absent. You stated that we should be able to add the two currents to produce the total load current, even when we change the biasing by removing Q2. I am trying to explain that this is NOT the case. The biasing is different in each case, therefore Q1 is NOT biased to the same current. The two conditions are not equivalent.

Let's look at your example again:

With Q2 present, you have a load current of 1.74A, and a bias current of 90mA, correct? That adds to 1.83A. If I am understanding your assertion, you expect the load current to be 1.83A when Q2 is pulled, correct?

With Q2 pulled, the actual measured load current in your case was 1.94A, not 1.83A, correct? I am saying that they should be different, because Q1's biasing is different in each case.

There is nothing that strange going on, I assure you. The circuit is obeying KCL (Kirchhoff's Current Law), and if you measured the Q1 Drain current as I did in the simulation, you too will see that KCL holds true.

.99,

In both situations (Q2 pulled or not) the bias voltage on the Q1 gate is ABOVE the linear area and will make
the MOSFET go into full saturation. With Q2 pulled you have +12 Volt on the gate. With Q2 NOT pulled you have +7,5 Volt
on the gate. What makes so difficult for you to understand this fact? So for both cases the MOSFET is fully ON
conducting current.

The ONLY thing that is different in both cases is that we put our 90mA through the Drain Source of the MOSFET.

Now explain why the there is a LOWER current going through the MOSFET when we put a 90mA current
through the Drain Source.

Next question, what is the total resistance the 12 Volt bias must "see" to be 90mA through the circuit?

Last question, what is the total resistance the main current from the 24 Volt must "see" to be 1,74 Amp?

Sorry for all the questions, but I really like to know what is going on in the circuit.

GL.

PLEASE NOTE: As .99 pointed out in the comments, in that last video, the low figure that I gave for the wire spool is implausible and of course wrong. I MISREAD the digital meter ! The meter only reads down to one microHenry anyway. That's what I get for not sleeping.   

The spool of wire actually measures 28 microHenry. I've edited the video description to reflect this, noted how I got the error and so on. The meter is actually agreeing with other measurements of known inductors as long as they aren't "resistive" inductors... it even warns about this in the manual, oh well.

So thanks again for noticing my 3-orders of magnitude Texas sized "tad" and I'll go with 28 microHenry for that spool of wire.

OK... carry on..... sorry to interrupt....

Groundloop:

I explained why the current changes, you just have to go back a few pages.  I made three or four postings about it.

MileHigh

Rosemary:

Re: Request for documenting the test runs for the dim bulb testing.

It would not be desirable to see a dozen or more DSO captures in support of the dim bulb testing.  I would say that a maximum of four would be fine.  It goes without saying that the configuration of the circuit should be exactly the same for each capture.

For each DSO capture in support of the dim bulb testing we would like to know the details about the setup.  How many batteries, things like that.  We would like to see pictures of the setup.  We would like you to list the the DSO negative power measurement for each capture.  Therefore if you are going to use a 0.25 ohm current sensing resistor then you have to do the calculations to show us the measured negative wattage.  We also want to know what the function generator voltage is without it being connected to the circuit, for each capture.  So you measure it on the scope or with a multimeter, your choice, as long as you make the measurement.  We also would like to know what the current measurement in the running circuit is when checked with a digital multimeter.  If the digital multimeter is going crazy than use an analog multimeter.  We expect a few hundred milliamps of current flow so you ideally you would have an analog current meter with a full-scale deflection of zero to 500 milliamps, or a full-scale deflection of zero to one amp.

So you can imagine four captures with a text file that records the configuration and data for each capture.  If you want to be really nice you can put the whole thing into a spreadsheet.  There should be no resizing of the scope capture images.  You could also include the results of the dim bulb testing in the spreadsheet.  Then we can simply download the spreadsheet.

We want no ambiguities in the data because that causes doubt.  This is what open source is supposed to be all about.

I have just given you a few suggestions about documenting the "COP infinity" power measurements on the setup while the dim bulb test is running.  If you can think of any more things to add then please add them.  You know that you have been haunted by setup and circuit ambiguities in the past.  This is your chance to get it right and document yourself properly.

I would like to see you post acknowledging my posting and sharing your thoughts with us.

When do you expect to start the testing?

MileHigh

GL,

I would encourage you to measure your actual VGS, as I have a hard time believing that it could be 7.5V.

All my sims show no more than about 4V max when Q2 is installed.