Diy mppt?

[fabieville]

i have been studying the circuit and i was looking at the diode (SF52 superfast diode) that is across the output going to each of the motor rail and i was wondering if that diode would have to change if i was going to operate the circuit at a higher current say for example the 30A that i mention earlier?

[Xringer]

That diode is there to protect the transistor from spikes that originate from the motor.

Anytime you apply power to an inductor, (like a motor coil), it creates a electromagnetic field.
If the power is switched off, the EM field is no longer supported (no current flow).
It collapses backwards, right into the coil. It was stored energy.

Since it's now moving through the coil windings in the opposite direction,
(back in, instead of out) it generates a sharp power pulse (or spike),
in the opposite polarity..

The spike voltage is the reverse of the applied voltage, that's why the diode is in the circuit backwards.
It's not used, until the backwards spike appears.

So, the rating of the diode you pick, depends on the size of the coils in your motor.
A big honkin motor might need a higher amperage diode.

The diode needs to be fast (a fast switching speed), so it can intercept
that spike (turn it into heat), before it can harm the MOSFET..

If your load isn't a motor or other inductor, you might not even need
a special diode. Just a plain old 1A silicon general purpose switching diode might work fine.
Since you only need it for the weak spike that comes from the EMF around the power wires to the load.

Most designers include protection diodes for the collapsing EMF..

[fabieville]

so like how i am planning to use a battery at the load instead of a motor then i can always option for the 1amp diode right?

[Xringer]

Yeah, the battery and the output capacitor is going to help keep the inductive effects
of the long lines to a minimum. The 1A should work okay.
One of those 0.1uf disc caps across the line will help too.

SO! You are building a battery charger! That's interesting, since there are tons of
pretty good Solar battery chargers on the market for cheap money..
However, those with full MPPT are a good deal more expensive.
I own three little 12V 8A chargers (PWM type I think) that I use on my 3 little 10w PVs.
They work real well, since the loads on the batts are pretty low.
The old Honda CRV batt is only used once in a while on my Ham Radio receiver/scanner.



Using the pump-control circuit should work out pretty well. It's going to zap pulses
of sweet spot power into battery, kinda like a PWM charger.

But, you need to be careful not to overcharge your battery.


As you can see, by adjusting the pot so the transistor fires at the 17V MPP,
you are going to get a fast, high current charge on your batteries.

But, once they are charged up, they need to be in 'float' mode and
that voltage is typically much lower than 17v..

I skipped any over-charge problems by using these..

They shut off when the battery is fully charged.


Cut-off is at 14.2 volts..

[fabieville]

yes thanks for the info. I really appreciate your help in explaining the whole schematics to me. I know a little about electronics and i am kinda have it as a hobby. I am going to run the output of the mini maximizer to my diversion charge controller which in turn is connected to my battery and whenever the battery reaches 14.8v the charge controller will send over the input current to damp load. What is the prefer setting of the mini maximizer to let the PV operate at its peak value? My PV panel VOC is 21V and rated voltage is 16.8V. What setting do you recommend that i set the mini maximizer to operate?

[Xringer]

Okay, if your pulsed output going right into a DCC, you should set the pump-controller pot to fire the MOSFET
at about 17.6v or what every your panels have listed for 'Voltage at Pmax'..
AKA the Sweet Spot..

You might be able to adjust it using an amp meter, during a slightly overcast overcast day,
when your batteries are not already at full charge..

Please keep us posted on your results.

I would really like to work on my MPPT stuff, but I'm up to my neck
in all kinds of stuff this winter..

Cheers,
Rich

[fabieville]

ok thanks. Also currently I have a large capacitor 12000uf 63V connecting to the output of my PV and going to my charge controller which has improve my charging system greatly so far. Should I remove it if I am going to connect the mini maximizer to the PV? what are the disadvantages/advantages of letting it remain to the input of the mini maximizer?

[Xringer]

You can try it both ways. If you leave it on the PV, it should be near the the mini-max board.
Because, with a cap that large, there will be some serious current pulled out, when the transistor fires.

If you have the cap out on the panels and mini-max indoors, the pulse will cause a big EMF surge all down the line.
Put them close together, and the EMF field will be much smaller.

My guess is, AM radio and shortwave reception will be affected,
if the input PV lines are seeing big current spikes..

If the cap is indoors, next to the mini-max, it will pull power out of cap
at a good rate, and the PV will replenish the cap at more even rate,
(slower rise time), depending on sunlight conditions.
The cap will probably act as a big noise filter.

[fabieville]

in regards to what you stated If I put the cap out on the panels and mini-max indoors, the pulse will cause a big EMF surge all down the lines. What exactly is this? Is this a voltage spike or voltage & current spike? And is this good or bad for the mini maximizer/or the battery?

[Xringer]

Originally Posted by fabieville

in regards to what you stated If I put the cap out on the panels and mini-max indoors, the pulse will cause a big EMF surge all down the lines. What exactly is this? Is this a voltage spike or voltage & current spike? And is this good or bad for the mini maximizer/or the battery?

Yeah, voltage and current. It's a power spike. A charged cap can supply
current almost instantaneously to the mini-max.
The cap is a quick-dump device. It there is a sudden sharp jump
in current flow, the inductance* of the line will make the line look
like a high resistance for an instant. And then, as the current flows,
the magnetic field will rapidly expand outwards..
When that field hits a nearby AM radio, it's going to jam the tunes with a burst of noise.
Noise that repeats with every new pulse.


*note:
XL or inductive reactance is normally a factor in AC circuits,
but it also affects DC switching circuits..
Here's a tutorial. Inductive Reactance

As you can see, XL is measured in ohms, like a resistor.
And if F (frequency) is increased, XL will go up..

You aren't using AC, but a square-wave with a real fast rise time,
is kinda like using a very high frequency AC..

So, there will be an instant resistance to current flow in the long lines.
It will impede sooth operation of your rig. Slowing it down a bit.
Since the panels will likely have a bit slower rise time (recovery time),
they shouldn't pulse the line so hard.(Like a loaded cap).

So, put cap indoors, right next to the mini-max..