how to draw the pcb stuff?

Discussion in 'General Electronics Chat' started by zero_coke, Jan 27, 2013.

  1. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Im new to the whole pcb stuff, and I wanna take my schematic I have on Multisim and turn it into a pcb. How do I do this?

    I basically wanna build my circuit on a chip so I've been using breadboards till now and I need to go to chip mode :)


    EDIT: I see some examples of PCB boards, but some of my components are giant like a palm-sized transformer. I don't think the PCB software takes this into account and it kinda jumbles everything into one small area...what do I do about this?
     
    Last edited: Jan 27, 2013
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    One of the most often used PCB packages is Eagle from Cadsoft, and there are lots of tutorials for it on the net, one especially good is from Sparkfun - this and the next two sections. First you will have to transfer your schematic from multisim, and I think you will have to do this more or less manually.

    I am not quite sure what you meant in the edit, the PCB software basically doesn´t take anything into account, you need to take care of all the details. You usually can choose from lots of standard components like resistors, diodes, ICs etc., but for special things like a transformers or other less common components you will have to make your own library parts - schematic symbol and footprint for the PCB. This is also covered in the sparkfun tutorial.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Multisim directly imports to the Ultiboard layout program if you can obtain that.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    You have to manually move and rotate the components into appropriate position. You try to minimize trace lengths and crossovers by observing the "rat's nest" wiring connections (available on most layout software) as you move the components.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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  6. gootee

    Senior Member

    Apr 24, 2007
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    Don't expect good results from an autorouter, for any non-trivial analog circuit. And you probably don't need anything that imports a schematic, unless this is in a professional setting. The schematic-aware PCB CAD packages were historically mostly only so you wouldn't make a change to one and forget to add it to the other.

    PCB CAD packages will enable you to lay out and draw the separate layers of your PCB, such as top copper, bottom copper, top artwork, bottom artwork, holes, board outline, and inner layers if you are lucky-enough to have those available.

    And when you are ready to have a board professionally manufactured for you, the CAD software will produce Gerber files and an Excellon drill file that you can send to someplace like 4pcb.com, where they have a free design checker that will accept your files by email and automatically generate a hi-res PDF of each layer, and flag any un-manufacturable areas, and build an interactive price quote, and automatically email all of that to you.

    But there is MUCH more to it than just getting everything connected with the correct topology. I am not an expert but I have learned a few things the hard way:

    A schematic almost never shows the true or desired nature of the ground connections, for example. And if you are not using a separate full ground plane, the grounding scheme can be critically important, even at low frequencies. e.g. Don't run quiet grounds and noisy grounds through the same conductor, on their way to the star ground or power supply cap ground. Actually, it's best not to "daisy chain" DC power traces, either. Ground-conductor currents will induce voltages across the parasitic inductances (and resistances) of the conductors and the voltages will appear back at the non-ground ends of all of the conductors that share any length with them. If one of those ground locations is the ground reference point for an amplifier input, for example, the induced ground voltages will be arithmetically summed with the amplifier's input voltage! That would be "a BAD thing". Use a whole separate trace to connect sensitive grounds to the power supply ground.

    The schematic will also usually not make you realize that you need to minimize or eliminate all enclosed geometric area for every conductive loop, unless you are wanting to make antennas. Wires running to and from the PCBs have the same problem. So either use shielded twisted pairs with the shield connected to chassis ground on one end only, or tightly twist (4 turns per inch or so) each pair. That is especially important for big transmitters and sensitive receivers, by which I am referring to things like the AC input pair and tranformer secondary pairs (big transmitters) and low-level signal input and signal ground pairs (sensitive receivers). If they are wires, twist them tightly, ALL the way to each end. If one AC Mains input wire has to go a switch, take them BOTH there, tightly twisted, all the way to and from the switch, and have one hug the switch as it goes around to be tightly twisted for the return trip to the transformer.

    You also may not realize that chips, such as opamps, often behave badly if their pins are connected to PCB traces of any significant length, i.e. components that connect to a chip pin should generally be placed right at the pin, not a centimeter or two away.

    Schematics also often don't show the decoupling capacitances that are required for the supply pins of active devices. And they almost never give much clue as to how to best place them or route their connections. Make them short! Place the smaller caps right AT the pins. Consider using multiple smaller-value electrolytics in parallel, in place of any electrolytics, to minimize the impedance.

    Schematics also don't usually tell you how WIDE or narrow to make the PCB traces.

    One good thing to learn, sooner rather than later, is how to add a new part to the library of parts, in a PCB CAD package, probably after creating your own library so you can leave the original ones alone, and so your parts won't be lost of you download a new version of the built-in parts libraries.
     
    Last edited: Jan 27, 2013
    zero_coke likes this.
  7. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
    1
    Alright thanks guys :)
     
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