I trust that is a typo?
If the current going into the battery was an analog input to the micro, you could dither the pwm, see which gives the greater current, then move the average pwm in that direction.
Amanda
That sure is a typo! But it's only off by thirty ... db.
The micro switches the FET at 72KHz (k_i_l_o_h_e_r_t_z), thanks for pointing that out.
... but the whole idea is to not sample the output current. Turns out that a plot of maximum I*V at different levels of Sun is practically a straight line, at the point where the loaded voltage is somewhere between 70 and 80 percent of the open circuit voltage. Where I could find specs, it looks like the single datum given by panel makers is about 70%. So (as usual) I allow the parameters to be set, changed and stored in EEPROM.
I got the idea from a wikpedia image. Maybe it only implements "pretty good" power point tracking, but with the simplicity I'm hoping that's good enough.
What I'm really jazzed about, though, it the thought of taking the idea to a buck converter. I thinking that a low power converter could handle something like the three panel $200 USD Harbor Freight panels in series (nominally 36 volts) and do MPP feeding to either a 12 or 24 volt system and, while it's at it, doing the charge control as well.
- Ed.[ Parent ]
Fixed the MHZ typo to be KHZ. Just so it was in there correctly for anyone researching and not reading the replies.
Tom
"Education consists mainly of what we have unlearned."--Mark Twain[ Parent ]
What I was getting at with current monitoring, is then you can use the same circuit for your windmill. Unloading the mill really won't work.
I typically use commercial 1 milliohm shunts with an op-amp amplifier. This cct without the analog meter. Scale the gain to suit adc input on micro. http://www.fieldlines.com/story/2004/9/4/193922/2839
Amanda[ Parent ]
Yes, I'm learning that you need the full-boat to control wind. I used a hall effect sensor in my first booster, then used the battery cable as the shunt in the second. The first approach seemed more reliable than the second, at least as far as my abilities could take it. This booster is pretty much a cut and paste from the advice I got building those two, though the "control section" is different in all three.
So far, I haven't used a commercial shunt but one thing that I like from looking at them is the real terminals for attaching real cables. Yes, the Allegro current sensors have really big fat pins but that sticks me with having to implement the part between the cables and the pins and I just don't have that much experience with more than digital power levels. Seeing an experienced engineer like you using a commercial shunt is very persuasive.
For wind, I'm hopeful that the sensing the FET voltage idea suggested elsewhere in this thread works. I'll build a testbed to see if I like it; if not I'll dig out the $10 and get a shunt like yours.
Thanks again, - Ed.[ Parent ]
Removing those will enable track and wire inductance to act as an open circuit for very small time. Hard to explain but here it goes.
Current in an inductor increases with time. At t=0, when mosfet switches off (t=time) the inductor will be an open circuit. This would be very bad for your mosfet as the voltage spike created by the boost converter will have no place to go, thus nothing to clamp that voltage down. That situation might enable that voltage to exceed your mosfet's Vgs.
Another thing is that with such a fast dV/dt (rate of change of voltage) the parasitic capacitor between the drain and the mosfet gate will create a voltage in the gate. This is very very bad as this voltage will turn on the mosfet when it should not be on.
If you make the path from your mosfet drain-diode-C3 very very short you will be running on the safe side as this huge spike's energy will just be stored in C3. C3 will not let the voltage spike so high then.
It is extremely important for the survival of the booster that the output inductance it sees is minimal, and that is achieved by shortening tracks and adding capacitance.
I am not great at explaining things but I hope you get the idea.[ Parent ]
