[SOLVED] OPAMP simple circuit problem

[pplus]

Does anybody have idea what could be done to maximize stability only (all other considerations, such as speed, or the load voltage accuracy can be treated as irrelevant)? In other words, is there anything that would guarantee ONLY stability (to take this to the extreme, even at the cost of reducing total gain to some ridiculously low value) regardless of all other parameters variations (i.e. type of opamp, mosfet and load)?

Can I ask why you decided to use exactly the shunt regulator? I almost did not see the real devices to it. Its only virtue - inherent short-circuit protection.

 

[seeker13]

Hi pplus - of course, but AFAIK, this is not shunt regulator it's a series one - the mosfet channel is in series with the load; the load voltage is regulated by varying mosfet channel resistance (in accordance with opamp output).

 

[ZekeR]

Regarding the power supply, as I mentioned - its a simple transformer+Greatz bridge+filter (4x2200 uF and ceramic 100nF in paralell) circuit - do you know what could cause problems here

Well, considering that you're drawing 2A (12V/6Ohm), the power supply will have 120Hz ripple of approximate magnitude:
\[\Delta{}V=\frac{I\Delta{}t}{C}=\frac{2(1/120)}{4*2200\rm{\mu{}F}}=1.9\rm{V}\]

Your power supply voltage has a ~1.9V ripple voltage (in roughly a sawtooth waveform). This means the reference voltage is rippling up and down ~0.9V. The circuit has a limited bandwidth, so although its output voltage will reflect the reference, it will be smoother.

Who knows what happens at the input of your multimeter when you set it to measure DC voltage? It might have some nonlinearity at its input which rectifies this ripple into a DC voltage, and therefore a slightly more jagged ripple will appear as a different voltage from a slightly less jagged ripple. It might be a good idea to place a low-frequency LPF ahead of the multimeter in an attempt to prevent any rectification, perhaps an RC with values R=100kOhm, C=10uF.

All indications from simulation show that the circuit should be stable with good margins, and all conclusions drawn from intuition and analysis (thus far) indicate the circuit should work. IMO, the missing piece is trustworthy data. For that, we really should be using an oscilloscope.

Hopefully you have a friend who has an oscilloscope you can use. Or, you can go to a flea market and pick up an old analog oscilloscope--those can go for pretty cheap sometimes, although they often have problems that need fixing. I got a Tektronix 7603 for $20 once, and half (2) of the channels don't work quite right but it's still usable (plus, the non-functioning channels can be swapped out by buying a new module). If you want something that's new and digital, Rigol sells some pretty reasonable oscilloscopes at reasonable prices, such as the DS1052 ($400). Picoscope also sells some pretty affordable PC-based 'scopes. And if you absolutely refuse to buy anything, you can have some success with using your computer's sound input as an oscilloscope--there is some freely downloadable software which will plot the waveform being fed in--as long as you understand its limitations and are careful not to break it.

 

[seeker13]

Hello everybody, I'd like to thank you all for participating in solving this puzzle - I finally got it nailed down, and I'm a bit sorry to end the thread in anticlimax, but here goes...

So, in post #37 I detailed all possible categories of reasons I could think of that could cause this behavior. But I forgot about one other category - measurement errors. What could go wrong with simple voltage measurement, you ask? Normally, not much, but when you measure millivolts, and circuit has few amps flowing through, its a different story. In short, I measured various voltages by treating everything connected to the same wire (disregarding the physical dimensions of wire) as the same point, as I usually do. But usually I don't deal with several amps of current, and when I do, typically don't care for voltages that are fractions of Volt in amplitude.
As mr. Murphy would predicted, it turns out that I measured the voltage at probably worst possible physical point - I connected multimeter + probe at the load itself (its a hefty resistor with a nice thick terminals), and kept it there (since +V is my reference point), while probing various potentials by moving negative probe around. So I was always measuring (not negligible) voltage drop along the wires and all the connections. To make things worse, the load was not very firmly fixed to the rest of the circuit, but enough to make a galvanic contact. The breadboard itself seems to have quite thin wire internally connecting the contacts. The combination of these factors conspired to produce up to 1V of voltage drop (so, it all added about 0.3-0.5 ohms overall resistance if current is 2-3 A). This is the reason that I got such a varying results - it was due to the physical point I chosen to pick for measurement, and not related to the various other solutions you proposed and I tried. The primary criteria for choosing the point of measurement was the ease of access (since the mosfet was getting hot fast, I wanted to complete the measurement as quickly as possible). The voltage drop along the wires and contacts due to the large current was the last thing on my mind, but there you go - live and learn...
Once I realized this, I reverted to the original solution (the before post #26) and measured the voltage difference directly at the pins of opamp inputs - the voltage was just a few millivolts, irrespective of the load I tried, exactly as you expected it should be.

Once again, I'm sorry for wasting everybody's time, but I'm glad that, with your help, I got it solved and learned quite a bit thanks to you, so thanks again!

 

[ZekeR]

This is the reason that I got such a varying results - it was due to the physical point I chosen to pick for measurement, and not related to the various other solutions you proposed and I tried.

Ahh, you won't see that in simulation! Unless, of course, you were able to see that far ahead and add the parasitic resistances into your model. Well, I'm glad you were able to figure it out.

 

[Audioguru]

Nearly EVERY time a circuit has problems it was built on a solderless breadboard and the breadboard caused the problems.