I am still, in fact, working on this project. In my last project log I was trying to find a way to enable/disable the hacked RT8299 buck regulator to give me a open-circuit voltage (Voc) reading, which is then used as the maximum power point target until the next sample is taken.

I've played with a variety of different oscillators, and eventually landed on a single Op-Amp multivibrator. It's an attractive solution thanks to the low parts count: offering a potential for significant cost reductions. The basic circuits looks like this (image courtesy of this site):

The principle is quite simple. The OpAmp acts as a comparator, comparing the voltage on a capacitor to a reference voltage. The reference is generated right off the OpAmp's output (R1 & R2), and resistor R is used to limit the charge/discharge rate of the capacitor.

This basic multivibrator circuit generates 50% duty cycle square waves, but for this MPPT application, I need something closer to 99% duty cycle (sample VOC briefly, turn on RT8299 and track MPPT the rest of the time). This can be easily done by adding a diode in parallel with resistor R to provide a fast discharge path. To control the discharge rate, another resistor is used in series with the diode.

The modified circuit ends up looking like the image below (found from this site, which contains enough interesting material to keep folks such as myself busy for entirely too long):

The only difference between this image and the actual circuit I build was that the values of R1 and R2 are flipped, effectively inverting the output duty cycle.

This circuit works well and integrates nicely with my current MPPT implementation, with a caveat. In order to get long periods (over 10sec), I need a large RC time constant. To keep cost down, I chose the largest R in my arsenal in an attempt to make the C reasonable.

Bad idea!

With a sufficiently large R, not enough current enters the input of the OpAmp (they have a required minimum, usually referred to as the "Input Offset Current"). When not enough current enters the OpAmp, the circuit no longer oscillates.

This left me with the uncomfortable position of having to chose a large C, which is more expensive.

Right off the bat, this doesn't sound like a deal breaker, until you add up the costs. After adding the cost of the OpAmp multivibrator, the large timing capacitor, the reference voltage used to regulate the output, the output comparator (an OpAmp "can" be used with sub-par results), the feedback override OpAmp (see this post for an explanation) and the Voc sampling circuit (see this post), it ends up being cheaper to use a PIC.

Somehow the universe decreed PICs to be amazingly cheap.

All this to say, I'm no better-off than I was prior to this nonsense science.

This is just part of the news. I'll post another entry with my other experiments.