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Author Topic: Help with my first PCB design  (Read 2084 times)
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The power dissipated by the pot depends on its resistance and the applied voltage.
P=V^2/R
Work backwards to find the min R  needs to be
R=V^2/P
5*5/.25 =
with 5V, R= 100 ohm or higher.

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Interesting, if I am understanding that correctly, there does need to be a resistor (maybe between +5V and one of the outer pot lugs) of about 100ohms, just to prevent the resistance across the pot from ever being 0. Correct?
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No you are not understanding it correctly!!!

There is NO repeat NO repeat NO need for any external resistor with a pot.
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put the 5v across the built in resistor in the pot and use the wiper to select a voltage between 0 and 5 volts


as long as you have a normal load (or no load) on the output + and - you won't short out the source 5v by moving the wiper.
« Last Edit: June 08, 2011, 05:06:51 pm by dcb » Logged

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Sounds good to me, I will remove the resistors from the pots and save myself some money smiley-grin

How about the Arduino / ATMega328 set up in the schematic? Did I configure the chip so that it will work correctly (once it has been programmed with my Arduino sketch by an external ISP on breadboard)?
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There's a good possibility, the design is pretty straightforward.
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The last couple days I've been looking into getting my PCBs manufactorured someplace, and have come to the realization that I need to make my boards as small as possible to keep costs down. I'm thinking of either going with BatchPCB or Olimex, and I see that BatchPCB charges $2.50/sq inch of board. At my previous board size (2.66x2.0"), that meant spending about $70 bucks (with set up fees) to print just five of my boards! That seemed like a bit much, and would mean that the PCB would cost about as much as the entire enclosure I'm fabricating through Ponoko.

I've gone back and revised my PCB design to be as tight as possible. I downloaded a board file for another device I have laying around (Daisy MP3 Player) and used it as a guide to see how close I could get my components.

Here is my revised board file: can someone with more experience with printing PCBs than me tell me if I need to space things out more, or use different packages, etc?

http://cs.unk.edu/~webb/files/RGB-LED-driver-v10.brd
http://cs.unk.edu/~webb/files/RGB-LED-driver-v10.sch
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Use smaller parts.
For IO holes, go with MA03-1, MA04-1, MA03-2. There's no reason those holes can't be 0.1" spaced, like the pads of the ICs.
Use a smaller resistor for the 10K pullup.  If you stand it up on end,  you can get its two leads in two holes that are 0.1" apart.

The resistors for the LEDs, why are they so big? Those must be 5W packages?
P=I^2 x R, do the math for them, see if you can use something smaller,save some room there too.
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Use the Polygon function, draw a polygon around the perimeter of the board, then use the Rats Nest feature (looks like an X) to fill in the ground plane.

Just getting started with Eagle here as well.  Are there any rules of thumb regarding when and where to use ground planes, and when not to?  Do you tend to use them unless there is a reason not to?
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Alright, I have updated all of the holes to be standard .1" headers (I like them a lot better!), and updated the power resistors packages to be vertical. By re-arranging the parts a little bit, I was able to reduce the board a good amount and get the price per board around $2.30 smiley

The resistors I am using are 2W power resistors, and their datasheets say they are 4mmx12mm - the nearest part in the Eagle libs was R-US0414, which I chose the 'vertical' layout of.

Now I'm just curious, are things TOO close together now? Especially the IC sockets and the crystal in the lower left.

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Took me a while to figure them out. If you have a ground plane(s) it can make the routing easier, as ground is usually one of the more common connections.
I would use all the time if able. Read elsewhere that this could save on etching fluid if making your own boards, that much less copper to remove.  I do it for the noise reduction from higher switching speed lines (course, nothing here is really high speed, look at how well things work on breadboards with wires hanging all over the place!)

I haven't tried this myself, but having one layer be ground and one layer be +5V would also give you in effect a big capacitor also (haven't calculated how much, plugging the 2 plate sizes, distance between them, and maybe the copper thickness, into an excel equation would yield the answer quite handily). I'm not sure I like the idea of having a big exposed area of +5V tho, even with solder mask over it. On a 4 layer board it would be a middle layer and not exposed.

The hardest part I found is getting the polygon closed when you get back to the corner; you get an "invalid polygon error" if not done right. With a 80x100mm board, you can't zoom in to see because you have to be zoomed out to see the other corners. Takes a few tries.

Also took a me a few tries to get rectangles under the tabs of voltage regulators. Add vias to connect the top & bottom layers, then you still have to connect the vias together!  
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consider rotating some of your components to make routing easier
for example
spin R2 180 degrees (look at the air wires!)
same trick with your pots_in header

also worth putting the headers on 0.1" pitch
I know each one is, but they are separated by random amount that are not 0.1"

if you make all the headers touch, or saparated by 0.1" they'll line up better
and you never know, you may want to drop the whole thing onto a breadboard one day

I'd also consider moving R1 to be over next to its friends R2 and R3
that way all the high current stuff is away from the crystal
you can then move the crystal even nearer to its pins
« Last Edit: June 12, 2011, 10:44:51 am by mmcp42 » Logged

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@h4t, looks good! That is about the spacing you would end up with if using 0.1" spaced perf board as well.

As discussed above, add a ground plane polygon.
Check your +5 going out to the pots - it might just  be a rats nest thing, but it looks like all the pins are going to ground.

I would swap the position of R1 and the crystal/caps, you want the crystal traces going to pins 9/10 as short as you can.
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@h4t, looks good! That is about the spacing you would end up with if using 0.1" spaced perf board as well.

As discussed above, add a ground plane polygon.
Check your +5 going out to the pots - it might just  be a rats nest thing, but it looks like all the pins are going to ground.

I would swap the position of R1 and the crystal/caps, you want the crystal traces going to pins 9/10 as short as you can.

snap!
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snap?
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