feedback and stability

Michael Robinson · 2009-12-20T20:32:04+00:00

I built a voltage regulator for an old truck I own. The circuit uses a voltage controlled oscillator with a square wave output to drive the gate of a mosfet that controls the alternator's field. The voltage regulation feedback loop uses a voltage divider from the alternator output/vehicle battery, connected to the reference pin of a TL431. The TL431 controls the vco.Now, for a bit of a digression. Years ago I built a boost converter with a similar feedback loop, the major difference being that it used a bjt instead of a TL431. But it worked on the same principle, with a voltage divider from the output to the base of the bjt, which controlled a blocking oscillator. The output voltage wandered up and down and was very unstable, so I put a capacitor from the collector to the base of the bjt. That fixed the problem and made the output voltage stable.So, having had this experience, I put a cap from the cathode to the reference of the TL431 in my truck regulator. When I installed it in the truck, the alternator output regulation was unstable; the voltage jumped up and down like crazy. After much head scratching, on a hunch I took wire clippers and removed that cap from the circuit board. Then the regulator worked GREAT. System voltage perfectly stable, good load regulation.Perhaps I could be forgiven for thinking the boost converter and the vehicle voltage regulator would operate on the same principle and that a cap would have the same effect, in view of the fact that in both circuits the cap provides negative feedback in the controlling element of the feedback loop. But the two circuits have opposite behavior, and adding a cap to the controlling element in the feedback loop has exactly the opposite effect in each circuit. The boost regulator was unstable without a capacitor, and stable with one. Contrarily, the alternator regulator was stable without a capacitor, and unstable with one.At best I have some intuitive sense that the presence of a big mechanical device that...

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Jim Thompson 16 years ago

Take a look at this...

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The snag is that the OpAmp IS compensated for unity feedback, for who-knows-what resistive and capacitive load.

So some caution is advised. You can, for instance, apply the usual feed-forward/feed-around schemes to isolate the load capacitance form the feedback loop.

I have, in the past, used these device as OpAmps, driving matched photo-couplers, to get a quite linear _analog_ photo-link ;-) ...Jim Thompson

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ehsjr 16 years ago

duty

alternator.

Thanks!

Ed

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Jim Thompson 16 years ago

reference pin

the duty

alternator.

You are quite welcome! ...Jim Thompson

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Fred Abse 16 years ago

Same as shown in the TI data sheet

-- "Electricity is of two kinds, positive and negative. The difference is, I presume, that one comes a little more expensive, but is more durable; the other is a cheaper thing, but the moths get into it." (Stephen Leacock)

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Jim Thompson 16 years ago

Sheeeesh! I guess that must mean I'm right ;-) ...Jim Thompson

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| 1962 | Help save the environment! Please dispose of socialism responsibly!

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Michael A. Terrell 16 years ago

It was bound to happen, some time! ;-)

Greed is the root of all eBay.

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JosephKK 16 years ago

variable

Please reread my question. Please answer it. Did or did not someone=20 discuss wobbling it around the corner of the knee?

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Jim Thompson 16 years ago

Maybe you did. I don't remember it. It's dumb. That sort of "solution" doesn't stick in my brain ;-) ...Jim Thompson

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JosephKK 16 years ago

variable

Not much of answer, but it will do. You just heard it from JT, regulating by wobbling around the zener knee = is dumb.

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Jim Thompson 16 years ago

[snip]

Yep, it's dumb.

Wobbling is definitely DUMB! ...Jim Thompson

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feedback and stability

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