How to adapt power regulation schematic for 24v?

[corrado33]

So when I'm designing my power supply, I know it's good to have a large capacitor somewhere between your battery and your fuse.  Like in the H-Bridge tutorial on this site.  Yes I have also read the power regulation tutorial on this site.  And I've also read Admin's opinions that 24v shouldn't be regulated down to 5v, and it's better to just use another smaller supply.  However, I have absolutely NO ROOM in my design. 

So according to this schematic...


I don't think i'd need a switching regulator because my motors are 24v, as well as my battery supply, so I'd just need a LARGE capacitor before the H-Bridges for the motors(wired in parallel), then a transistor (most likely a 7805 regulator (with LARGE heatsink)) that'll provide power for the microcontroller, tiny servo and other things etc.  However, I'd need another SMALLER capacitor before the microcontroller and things right? 

If I put the capacitors here, do I need to include them in my H-Bridges?  (Not that I'm complaining I just don't want to have them if I don't need them)  I'll try to get a schematic made up and post it here sometime soon.

[Soeren]

Hi,

So according to this schematic...

A switcher should have a capacitor in front of it too and it should be a good quality low ESD type.
Further, it's a little odd to use the term mF, as it's interpreted different in the U.S. and Europe. 1 mF is 1,000 µF, but if the schematic is from the U.S. one will never know if they use mF when it should have been µF.
I would use higher values than 10 µF in that position, but certainly not 10,000 µF - and the output cap should be at least 22µF.

I don't think i'd need a switching regulator because my motors are 24v, as well as my battery supply, so I'd just need a LARGE capacitor before the H-Bridges for the motors(wired in parallel), then a transistor (most likely a 7805 regulator (with LARGE heatsink)) that'll provide power for the microcontroller, tiny servo and other things etc.

7805 is NOT a transistor, it's an IC with several transistors inside!

How much power are your 6V gizmos gonna draw?

Lets say you need at least 1A at 5V.
A "24V" Lead Acid is 25,2V when it has rested say 8 hours after a full charge (more if you don't wait, up to nearly 29V).
That means you have a drop of ~20V in the 7805. That equates to 20W (to get a usefull 5W), which the 7805 needs to dissipate, and you claim that you don't have room for a switcher... How are you gonna fit a huge (large won't do) heatsink with forced cooling then?

Go with experience, use a switcher, it's really not that hard.

[corrado33]

Thanks Soeren.  Anyway, here is my schematic for my power regulation.  I've designed it with your ideas and the tutorial's ideas in mind.




Now I will walk you through it.  First on the left is y battery (obviously).  Next you see my fuse and switch.  The first cross path is my battery monitoring circuit.  This was basically taken from the battery monitoring circuit provided on this site using the equation 50000/(29/(24-1))  50000/(Battery Voltage/(Desired Voltage - 1)).  The unconnected wire goes to my microcontroller. 

Next you will see my LED to tell me when the robot is getting power.

Next you will see C1 (Capacitor 1)  According to soeren, this cap should be between 10 uF and 10000 uF (well, at least greater than 10 uF). 

Next you'll see a 24v regulator.  First question... Do I need a 24v regulator?  Also, I don't think I'll be able to put one there, because ALL of the current going to all 4 motors will go through it, and that COULD be up to 11.2 A if all four motors were stalled at the same time.  So if I need a 24v regulator, I'll have to put them before each motor, right?

Next you'll see C2.  According to Soeren, this should be at least 22 uF (Where did Soeren get that number?).

Next you'll see my output to my motors.

Next you'll see my 5v switching regulator.  Will this one work?  http://dimensionengineering.com/DE-SW050.htm

Next you'll see C3.  I have NO idea what value this one should have. 

Finally you'll see my 5v output to my microcontroller and other 5v things (At most a tiny servo or two)

So I have a few questions.  What should the values for C1, C2, and C3 be, and why?  Do I also need capacitors on my H-Bridges?  (As seen in the H-Bridge tutorial)  Do I need a 5v regulator AFTER my 5v switching regulator?  For some reason I'm thinking that a switching regulator is more like coarse sandpaper while a linear regulator is like fine sandpaper. (If you're sanding voltage that is.) 

[Soeren]

Hi,

Next you'll see a 24v regulator.  First question... Do I need a 24v regulator?

No.

Next you'll see C2.  According to Soeren, this should be at least 22 uF (Where did Soeren get that number?).

He got it from decades of experience

Next you'll see my 5v switching regulator.  Will this one work?  http://dimensionengineering.com/DE-SW050.htm

Next you'll see C3.  I have NO idea what value this one should have. 

Yes, it will work if you use it like this:
http://That.Homepage.dk/PDF/24V+5V_PSU.pdf

Finally you'll see my 5v output to my microcontroller and other 5v things (At most a tiny servo or two)

Servos, controller, sensors, it all ads up.

Do I also need capacitors on my H-Bridges?  (As seen in the H-Bridge tutorial) 

Yes, especially if your leads are more than a few inches.

Do I need a 5v regulator AFTER my 5v switching regulator?[/b]  For some reason I'm thinking that a switching regulator is more like coarse sandpaper while a linear regulator is like fine sandpaper. (If you're sanding voltage that is.) 

You cannot place a 5V linear regulator where you have only 5V and you won't need it.
Load regulation alone can be up to 100mV in an 7805 and that's not the only source of error, while the DE-SW050 has a max. ripple of 100mV. Add the 470µF and it will be lower. (C1 and C2 values in my schematic is minimum values - and their marked voltages is minimum as well larger is OK).

[corrado33]

Thanks Soeren.  I really appreciate it.  

So C1 > 10uF, C2 > 22uF, and C3 is whatever is on that schematic you gave me.  (Actually I believe there is C4 there as well).

One question though.  In the diagram you gave me, why use an inductor instead of a capacitor?  Is it because (if I'm remembering from physics 1 correctly) capacitors don't allow current flow when fully charged?  Is it personal preference?  Any benefits to either one?

And what inductor would you suggest (As I'm a newbie to inductors)

[Soeren]

So C1 > 10uF, C2 > 22uF, and C3 is whatever is on that schematic you gave me.  (Actually I believe there is C4 there as well).

Not in the schematic I made.

One question though.  In the diagram you gave me, why use an inductor instead of a capacitor?

Because I don't use a capacitor when an inductor is needed - those are as good as opposites and certainly not interchangeable.

 Is it because (if I'm remembering from physics 1 correctly) capacitors don't allow current flow when fully charged?  Is it personal preference?

No, it's because it's an LC-filter, which will keep motor transients from the logics supply.

Any benefits to either one?

This does not compute.

And what inductor would you suggest (As I'm a newbie to inductors)

Strip one from an old PC power supply (Dumpster Diving is a Gentlemens Sport you know) - the ring cores which are yellow are powdered iron (=> low frequency) and will be good - grab one with as many windings as possible - the more the merrier.

[corrado33]

Once I thought about it, could you perhaps suggest an inductor to buy?  I kinda want to use my old psu to make a lab top +5 and +12 v power supply....

If you could maybe suggest a digikey part number, or even explain what to look for when buying inductors I'd appreciate it.  Thanks!