Diode
What’s the purpose of the diode above?
(I know what a diode is for - I just don’t see its purpose here.)
Not so fast!
Don’t be so quick to dimiss jka’s idea. My first 12-channel servo controller needed 4 voltage regulators: One for each group of 4 servos and a seperate one for the microcontroller. Otherwise, the microcontroller browned out.
If a battery or regulator
If a battery or regulator brown out, the diode prevents the cap from discharging back into the source, ie: all the caps stored charge goes to power the the micro, or whatever the device is, hopefully for long enough for a temporary power glitch to be restored. Unfortunately it also drops the input source by 0.7 volts, but nothings perfect.
it´s wasn´t my idea to
it´s wasn´t my idea to dissmiss jka idea. it seemed very correct… and also a bit complicated to me.
i need to take one step at a time 
and this is being far more complicated than i expected!!
Obvious
It’s obvious when you know!! Funny, I don’t think I’ve ever seen that on any of the demo circuits in the facts sheets. They normally just have a cap on the high side and one on the low side. I notice you don’t have a cap on the battery side. What’s the rationale, there?
Maybe I need to introduce the diode to my servo controller board. The Vdrop won’t make any difference: the PIC I’m using is spec’d to something like 4.0-5.5V anyway.
Belive me, it is simple.
Belive me, it is simple. I’ll make a schematic for you tomorrow (it’s 20:17 in Copenhagen right now). It might be, that my way of explaining this is more complicated than it realy is.
Caps and regs
Basic voltage regs internally have a transistor collector emitter junction between the input and the output, hence their requirement for the voltage source to be 2 volts higher then the required regulated output voltage. That’s to get past the 2 diode drops. LDO regs are much better, usng FET devices. Switchers can be even better, but may need caps for different reasons.
Caps on the output of a voltage reg are good, they help stabilize and smooth voltage in much the same way as the diode-cap circuit does, On the input, it is more to help the reg from sources that are far away, or from power-line cycles if converting from AC power. Batteries are like big caps themselves, so there is not a lot of need to smooth that side.
Here is another
Here is another text-explanation, since I don’t have the time to do a drawing right now.
Take a look at http://www.datasheetcatalog.org/datasheets/228/390068_DS.pdf page 21 top drawing. Create two of those. One will be the power supply for the logics, the other for the motors. The input for both circuits is the battery. The ground is common for both, so connect them together.
So, you have + from the battery going to the left side f both circuits. - from the battery going to ground on both circuits and ground on both circuits tied together (which is obvious, since - from the battery goes to both).
Now, take a look at http://www.datasheetcatalog.org/datasheet/texasinstruments/l293d.pdf page 1-4. It might not be totally obvious, but there are two power inputs, VCC1 and VCC2. Now, connect the ground from both regulators to pin 4, 5, 12 and 13. Connect the positive output from regulator 1 to pin 16 (VCC1) and the positive output from regulator 2 to pin 8 (VCC2).
Regulator 1 is our logic power supplay, so we are going to use this for the CPU and other chips as well, so the common ground goes to the other chips GND pin and the positive output from regulator 1 to VCC.
Regulator 2 can be used to power other “noisy” components like servos etc.
Does this make sense at all? It’s just two regulators and 4 capacitors. If you still have problems, you can put capacitors on the CPU and other chips. Ask if you need help with that.
Batteries
Makes sense when you put it like that. So, the cap on the high side is probably used after the rectifier in an AC transformer or a similar application. That’s the sort of technical input we need around here! Thanks.
yes! it doesn´t seem so
yes! it doesn´t seem so complicated at all… a schematic would also be great!!!
i will only be able to get 0.33uf caps on monday!!
it´s working!
My custom motor driver is finally working, i´ve made some changes in the Arduino code.
i´m still using Servotimer1, wich is a nice lybrary for servo control, but has the limitation of only pins 9 and 10 can be used.
Using PWM for the motors is also great because their speed can be changed.
I must give a BIG THANKS to all of you guys!!! your help was precious! and without you i couldn´t make this go through! I learned a lot with this and I feel a bit more confortable to build more sophisticated robots!!
For now I will improve the code and make "farrusco" more reliable and intelligent, and also work on "big br0" … this was the place where I stoped! And now that holidays ended, time is short for robotics projects…
I will post new videos soon
Awesone, great to hear about
Awesone, great to hear about it going! Now maybe a video of it?
here it is
here it is
http://www.youtube.com/watch?v=uoG4_MMlhbA
I also started the walkthrough and will post it first here… as soon as it is finished I will post it in LMR.