Power Supply

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I am new to Eagle and the whole PCB thing here so bear with me. I have gotten my schematic done and I am now trying to construct the board from this. I was advised earlier to make a large ground polygon on both the top and bottom of the PCB to lower noise, which I did. Now I am having trouble adding a power supply. I will be using an external device here in the lab here at my school to provide +15 and -15 volts. I don’t know whether I should jsut make a hole in the board to solder in a wire or if I should dedicate an entire layer to it. Also I this sysytem will be operating at 25 MHz, is that high enough that I should be worrying about impedance mismatching and reflections at the frequency? haha, I feel so lost. Any help would be appreciated.

Thanks

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I often use screw terminals like:

http://www.sparkfun.com/commerce/images … minals.jpg

I don’t solder directly unless its a fix installation and will never be taking it out of the enclosure.

25 mhz isn’t that high but you should still use good techniques to avoid cross talk, noise, etc. I make all my bends 45 deg.

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Thanks for replying. I have a number of compants that need a 5 volts supply. Would It be wise to have them all connect through signal wires to the 15 volt supply or owuld it be a better idea to dedicate one entire layer to the 15 volts and then connect the power supplies to that layer directly, similar to a ground plane.

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Projktone:
I am new to Eagle and the whole PCB thing here so bear with me. I have gotten my schematic done and I am now trying to construct the board from this. I was advised earlier to make a large ground polygon on both the top and bottom of the PCB to lower noise, which I did. Now I am having trouble adding a power supply. I will be using an external device here in the lab here at my school to provide +15 and -15 volts. I don’t know whether I should jsut make a hole in the board to solder in a wire or if I should dedicate an entire layer to it. Also I this sysytem will be operating at 25 MHz, is that high enough that I should be worrying about impedance mismatching and reflections at the frequency? haha, I feel so lost. Any help would be appreciated.

Thanks

At that speed it shouldn't be a problem. If you have a 25Mhz crzstal onboard you should keep the traces between it and whatever as short and straight as possible. Otherwise keep all high speed lines away from noisier lines (ie power) and you should be fine. 25Mhz just isn't fast enough, in my experience, to cause any problems.
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Projktone:
Thanks for replying. I have a number of compants that need a 5 volts supply. Would It be wise to have them all connect through signal wires to the 15 volt supply or owuld it be a better idea to dedicate one entire layer to the 15 volts and then connect the power supplies to that layer directly, similar to a ground plane.

Projktone,

In EAGLE components, such as digital ICs and op amps, that require one or more power connections, have automatic hook up in the schematic and board editors.

For example, let’s start a new design in EAGLE.

(1) Open a new schematic

(2) From the library “40xx” pick a logic gate 4011 and package type 4011D (An ordinary 14DIL)

(3) Drop one gate on your schematic and notice it’s called IC1A

(4) Drop another - IC1B

(5) Drop four more. The last one will be IC2B

Don’t don anything else except switch to the board view.

Notice EAGLE has placed two packages, it knows there are four of these gates per package, and six gates on your schematic equals two packages.

Now pick up one package and place it within the PCB outline. Notice there are “airwires” in yellow?

Use the eye tool to inspect the airwires. These are Signal:VDD and Signal:VSS. In other words EAGLE has automagically generated two nets for power and ground, called VDD and VSS for these particular gate packages.

Depending upon your circuit there will be at a minimum two nets, one for ground and one more for a supply. A complex design could have many supplies and grounds, the point is they are each a net in EAGLE, and even if not show with wires in the schematic they are connected in the board.

Next, Planes and Ground Planes. If you add planes to any board it will drive the cost up, so only use them if you must. A good solution is to fill the unused areas of the bottom layer (and/or top layer if double sided) with copper which makes it a ground plan and gives all the advantages noted in prior posts here.

In EAGLE this is called flooding, using Polygons. I prefer to do this last after routing all the other signals, but it’s just my work habit and not critical to a good PCB design session.

We can’t see you design or your work habits, so please try my little experiment above and tell us that you did it (takes five to ten minutes).

From that you may have new questions…

Comments Welcome!

Peter