I've designed circuits for 50 years. You know what, I'm tired of it. Tweeking resistors,
adjusting pots, compensating for variability of parts, it used to be fun. Now just about
anything you could build is available dirt cheap fron China in some sub assembly. Anything
really custom can be built with these little micros. It's the size of my thumb to my knuckle.
One runs my entire house, refrigerator two PWM water heaters with temp controls, pump,
transfer relay, and two MPPT charge controllers. The charge controllers are just a FET,
diode, inductor,capacitor, and opto isolator driver. Run an entire house for $2.50. These
boards have a built in power regulator, USB communications port and the assembly can be used
as a breadboard, just solder resistors right onto it. If you have a laptop and can find a
USB cable the total investment is under $3. If you can load it in, winner winner chicken
dinner. That costs nothing.
Take a look at what you want to do. The battery is read in and converted to a voltage in
three lines of code. Set the PWM frequency to something slow like 20Hz that the block FET can
handle. That voltage is fed into one line of code called MAP. At 14.2V and under it outputs
a PWM of 0%. At 14.6V it outputs a PWM of 100%. Anywhere in between the it is proportional.
And you didn't need to know nuttin to make that calculation. Now I don't like short pulses.
They just cause heating. Another two lines of code makes any PWM under 10 a zero and over
245 100%. Don't like how it runs, change a number or two. Once one is running, you'll never
go back to the old way of doing things.
Anyone who makes a cad drawing is tech savvy enough. I need some help on this board. For seven
months of the year I have no way to do any real world tests on this stuff. Just think you would
really enjoy working with these and the projects would have really sophisticated features. A
couple boards is the price of a good cup of coffee.
http://www.ebay.com/itm/1PC-MINI-USB-Nano-V3-0-ATmega328P-CH340G-5V-16M-Micro-controller-board-ArduinoO9-/302163571894?hash=item465a5a34b6:g:VzgAAOSwM4xXaeioWith the three phase project, triggering didn't need to be sophisticated. Any error was quickly
averaged out. The FET with DC can't take an analog signal. That will just cause heating. Something
has to pulse it on and off. A TL494 switching regulator chip could do that. It even has a built
in reference and can be set to a low frequency. A 100A FET only has the power dissipation of about
25W even with a good size heatsink due to thermal resistance. Multiple FET can be placed in parallel
if precautions are used to make them share current. An analog step function could drive multiple
FET with individual resistors. That all gets into a lot of parts, the reason why PWM is so popular.
The SCR blocks are a nifty device that is easy to implement in AC. I just haven't found anything
useful that can be done with the FET counterpart.
On further contemplation I think your circuit needs to oscillate. This can be done with the addition
of a single resistor in the common line of the opto in the original . Power the opto with a resistor in series and determine what value will produce 10-20mv. This will produce positive feedback in the reference divider of the 431 making the turn off voltage lower. That will insure the FET turns on for a period of time. Temporarily using a pot will help determine the best amount of feedback. Resistor is likely 4.7 to
10 ohms.
Christmas is coming up. If you don't have one I suggest buying yourself a USB scope. A client
purchased one for a project a few years ago that was later abandoned. Small, portable and
electrically isolated when used with a laptop. For $65 it comes with two probes, suggest buying
a 100X probe as it is easy to overload the input at moderate voltages. It prevents you from accidentally switching from 10X to 1X. Anytime you see a perfect square wave, the scope is likely overloaded. With this you can get time and voltages easily of a waveform. This willl allow you to share data with others by creating jpg files. I love using mine at the camp.