voltage divider pintching off current! need some help...

Discussion in 'General Electronics Chat' started by jaydag77, May 28, 2007.

  1. jaydag77

    Thread Starter Member

    May 28, 2007
    17
    0
    I am trying to build a small v-reg module for a bot I'm building. The reg board needs to provide 12V @~1A for a H-bridge, 5V~.5A for the steering h-bridge and 5V @~150mA for the micro and other digital. I have sort of made up a battery pack consisting of rechargeable nicad AA's (1.2V @600mA) wired in series to make up ~14V and in parallel to double the current. I am not by any means a formally trained designer, so I have been going through alot of crap trying to figure out the best way to implement this spec. So far I have changed the design at least 5 times- now its just getting stupid...I seem to be over-thinking every part of it, which is good in a sense because I really do need to be able to do the calculations and proper design techniques, so all of the 'floundering' has not been a total waste....

    What I have at his point is a small 20x53mm pcb, 1 sided with a couple of jumpers on the other side. The battery pack leads run into the pcb through a 4001 diode and a filtering cap (~470uF 25V) onto two v-regs. The first is 12V 1A and the second is the 5V 150mA. Each have respective 0.1uF caps attached to both in and outputs and also an indicator LED run through resistor and to gnd. The voltage drop for the additional 5V @~500mA will be provided by a divider from a parallel trace from the 12V vreg output. I have been having alot of problems finding the right resistors to make up the required spec- I can find the voltage fine but the current gets pintched out so much the motor doesn't move at all and I get no reading/minute results on the multi-meter. I'm wondering is it better to do a voltage divide using diodes rather than resisitors? what is the difference? does one regulate current better than another? Finally I am wondering if I am doing something dead wrong here.

    I would post a schematic but I didn't draw one up on the CAD- just paper scribbles., only have the attempted layouts on the CAD and I'm too 'ashamed' to post em,.--One particular aspect which has been causing me lots of concern is the gnd network. The overall design will be modular- in that each 'functional ' block will be on its own small PCB. I've tried to break them down as simply as possible- the batt charge module runs into the reg module which splits off to respective H-bridge PCB's, motor(s) feedback PCB, uC mainboard and sensor modules. Some of these boards are plainly analog while others may be a mix and some just digital. I have tried to keep the analog portions all running from the 12V reg outputs and the digital only from the 5V 150mA reg. The problem I am having is that they all share a common ground due to the config of the regulator module--and I am wondering if this is a setup to failure, what some things I can do to help this possible situation without a complete re-do, etc.

    Please any advice is more than welcomed, heckling, tips etc....thanks in advance, jd

    PS- I had to completely re-register to post today,, I forgot my password, tried to get a temp issued but the server didn't ever send one! I tried a few times too! geez....
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Chances are that you're too concerned about regulation. Your big H bridge will probably do fine running off the battery pack. Your other regulators may simply attach to the battery pack, too. I wouldn't worry about dropping voltage into the 7805 - it's output current is low enough that a small heat sink will keep it cool enough.

    You have a common battery pack, so the analog & digital grounds have to be common, too. Run them separately to a point near the battery - terminal. Don't worry about current limiting resistors in the outputs of the regulators. Their loads will only use the current that they need.
     
  3. jaydag77

    Thread Starter Member

    May 28, 2007
    17
    0
    Thanks for the response!! I decided after reading the reply to post the latest layout I had made yesterday--please note that it was only intended to show routing possibilities, so there is errors and things which are not going to make sense like the missing LED indicators (because they are attatched on the botton side but are through hole packages and run out to the edge of the PCB.) The two traces on each length edge are gnd's,, they are there because the pcb modules attach to eachother by 'posts' which are in each corner, so the modules are stacked.,, so just keep those in mind if viewing the layout pdf!

    In response to your replies,,

    "Chances are that you're too concerned about regulation. Your big H bridge will probably do fine running off the battery pack."
    - I would like to think so, I know the big H will run fine direct from the pack,, and it might just be the way- my thoughts are that I could reduce the battery pack initially, say take out the parallel cells, so 14V @ 600mA,, then use that line to feed all analog circuits by using a current booster circuit of some sort to bring it back up to ~1A: so the big H get direct line, the small H gets a reduced voltage (divider) and current line from original source(600mA max), and rest like feedback, etc also get similar treatment but at lower voltage yet derived from the divided voltage output (small H +line). Back up at the battery-pack there is also a split to feed the 5V 150mA regulator so as to feed the digital portions of the design. Thus, just one reg on this approach (5V 150mA LDO) and discrete parts to regulate the analogs? What concerns me about this though is noise coming back down the lines from the motors and/or whatever else and interfering with the uC operations, causing resets/brownouts, whatever? Again I may be dead wrong but just my guess.

    "Your other regulators may simply attach to the battery pack, too. I wouldn't worry about dropping voltage into the 7805 - it's output current is low enough that a small heat sink will keep it cool enough."
    - I think this part may have been misunderstood?,, as seen in crappy layout posted, both regs are fed by a split from the battery's input just after the filtering caps(in parallel), so both regs are getting ~14.4 @1.2A ideally- well within the specs of both parts. The 12V reg was going to get a sink, I was going to leave the 5 alone but you may be right about it requiring a sink! I've never designed anything needing a micro at these specs, usually I work with lower voltages (1.2-5V) in the mA region.

    "You have a common battery pack, so the analog & digital grounds have to be common, too. Run them separately to a point near the battery - terminal." THANKU- this particular tidbit is what I was seeking above all,, if I understand correctly, it means that if I was going to seperate the analog and digital I would need to use two separate sources (two battery packs in this case?)? and would keep everything separate in that regard within the layout? Thats how I interpret it anyways....

    "Don't worry about current limiting resistors in the outputs of the regulators. Their loads will only use the current that they need."--This is a good one too, if it conveys what I think. The problem with it in this design though is that the 12V reg has more than one load requiring more than one voltage and current,,so although there is no limiting on the big H trace, the small H and other analog circuit would not handle the current supplied by the unlimited trace,, if that makes any sense at all?? so would therefor require some sort of current limiting prior to reaching the target component, no? I guess another part of that is whether or not current limiting should be done at the target component or on the reg board, or does that even matter?

    I guess if I took more time to read more than I do then I might better know if I'm on the right track...but I think your advice will take me farther than I have been so far today, so thanks again!!
     
Loading...