PCB layout help

Discussion in 'General Electronics Chat' started by vijaybala85, Jul 23, 2010.

  1. vijaybala85

    Thread Starter Member

    Jan 7, 2010
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    0
    Can someone experienced in PCB layout take a look at this file and tell me whats going wrong? Can we have a single source 24V supplying voltage to the entire board or should components like a solenoid that has 10ohm resistance and draws large currents be connected and directly powered from power supply?

    Can we have same "power supply" tracks that power the relay output as well as the control circuit (op amps here) which power the relay inputs? I feel that this layout is over burdening the power supply. Is that true?

    Any help would be grately appreciated. Thanks

    V
     
  2. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    Without a schematic, it's hard to follow, however here are some comments:
    - You can power everything from one supply if it can supply enough current. You may have to add additional filtering so high current loads don't glitch the rest of the circuit.
    - You need to be careful with power supply routing. I'd run separate traces from the power connector to the high current loads, and separate traces to the opamps so the high current loads don't modulate the opamp supplies
    - You need to make the power and ground traces MUCH wider. A 30-mil trace will act like a fuse at 12A. If you are really handling that kind of current, I would use large pours instead.
    - You need to add both bulk and decoupling caps to the board.
    - Traces should not make 90-degree bends; use two 45-degree bends instead
    - Don't let the unused inputs of either the opamps or the inverter float.
    - You need flyback diodes across the relay coils to protect the opamps. Depending on the relays, you probably need driver transistors too.
    - Do you need any mounting holes for the "24V actuator board"?
    - Connector labels are all going to be hidden under the connector - I'd move them so they will be visible
    - You may want to add LEDs to the 24v and 5v rails
    - I like to add a few pads tied to ground so you have somewhere convenient to hang ground clips from scope probes or meters.

    /mike
     
  3. vijaybala85

    Thread Starter Member

    Jan 7, 2010
    92
    0
    thank you so much n1ist for all your comments. I really needed help and I appreciate it.

    I was concerned about the power supply traces going from the top. BUt, didnt pay close attention. The relays draw upto 2A each and routing 24V from there to the opamps was not a good idea. But, I got the boards done. So I am thinking of cutting the trace out and adding jumper wires or conductor pen to add trace from supply to th eopamp. the power supply I use is 24V 12A supply, so it can source. Just that the route to the opamps is wrong. Do have any resources from where I can learn to change traces?

    what do you mean by 30 mil trace acting like a 12A fuse? Do you mean it can handle only upto 12A?

    Where do I add bulk and decoupling caps??

    I do have flyback diodes on the solenoids.

    I like the idea of ground pads. Will implement it in next version of the board.

    There are no holes on the actuator card. I am using a glue gun to stick it on the rectangle space.

    Changed the label positions.

    What is the use of LED's for rails? How to implement them?

    Final question, how to tie unused circuits in LM324 and IC 7404?


    Thanks a bunch! You have no idea. You just saved my day. :)

    V
     
  4. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    For a prototype, you can cut the traces with a sharp knife and jumper around using insulated wire. It's best to solder the wire to a via or component lead.

    30 mil trace is good for about 2A. At 12A, it would burn out like a fuse. There's a chart in ExpressPCB's design tips page, or you can use the trace width calculator over at
    http://www.desmith.net/NMdS/Electronics/TraceWidth.html to calculate the correct trace width for a given current and thickness.

    Decoupling caps are about 100nF, and located at each IC, from the power pin(s) to ground. For parts with multiple supply pins, use one per pin. They handle high-frequency noise, and provide a local reservoir for the chips to draw from when switching.

    Bulk caps (electrolytic or tantalum, say 47-220uF) are located where the power supply enters the boards, and sometimes after regulators (check the regulator data sheet), These provide bulk filtering.

    I like having LEDs on power supply rails, especially for prototypes, so I know that the power is on. Just an LED and series resistor from the rail to ground. For production, you can just leave them off.

    The question about the unused inputs was answered in your earlier post at
    http://forum.allaboutcircuits.com/showthread.php?t=37694

    How much current do the relays draw? If it is more than 10-20mA, you will have a problem there...

    /mike
     
  5. vijaybala85

    Thread Starter Member

    Jan 7, 2010
    92
    0
    Thanks! For now, I have cut traces and placed jumper wires from supply to each relay. THe solenoid that is driven by the relay has a resistance of 10ohm. So it will draw upto 2A each. I didn't realize that when I was testing each relay at a time. But, combined testing proved that the op-amps were not getting enough supply from the trace.

    by 30 mil, u mean 30 mil inches??? Please let me know.
     
  6. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    A mil is a thousandth of an inch.
    /mike
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It's a good idea to use "copper fill" or "copper pour" to create ground and supply planes. An infinite copper plane will have zero inductance; where long and narrow copper tracks will have a great deal of inductance. It is difficult to compensate for this un-planned for inductance if your board is operating at even moderate speeds.

    Sprinkle 0.1uF/100nF bypass capacitors liberally around the board. 1 per square inch and one per IC is usually good. Try to arrange your components so that they have short signal paths, and use the smallest amount of board space possible.

    If you have unused opamp channels, connect the inverting input to the output directly or via a 0-Ohm jumper, and connect the non-inverting input to a reference point somewhere between the power rails; or optionally to the output of another opamp that is in use. This will keep the opamp from oscillating. Make sure the opamp is unity-gain stable. If it is not unity-gain stable, things get a bit more complicated, but you can use a similar scheme. Just make certain to select feedback resistors that will keep the opamp within a stable range.

    Comparators and CMOS IC's can cause similar problems (oscillations if inputs unconnected). All unused INPUTS must have a current path to +V and/or ground. Unused OUTPUTS can be left floating.
     
  8. vijaybala85

    Thread Starter Member

    Jan 7, 2010
    92
    0
    Thanks sgtwookie! :)
    ________________________________________________
    Just make certain to select feedback resistors that will keep the opamp within a stable range.
    ______________________________________________________

    How do you do that?

    Also, which manufacturer makes really durable opamps? LM324 only does so much... Can you specify model number?

    Thanks!
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    It depends on the opamp. You have to read the datasheet of the opamp in question. If you use resistance values that are too low, your opamp will have it's output range reduced. If you use values that are excessively high, you may induce a significant amount of noise in the output.

    LM324's are very old and very slow opamps. They can sense inputs right down to the negative rail, but are limited on the high side.

    You might take a look at an LMC6484 as a replacement. It's a quad CMOS opamp that has rail-to-rail inputs and outputs. It's not really fast, but it has a 1MHz bw, which beats the LM324.

    There are so many thousands of opamps out there it's hard to make a recommendation unless you have a specific application in mind.

    LM6132/4, LM6142/4, LM6152/4, LM6171/2 LMH6642/3/4 - take a look at those. They're similar, but go from fairly slow to really pretty fast. You pay for the speed. If you don't need the bandwidth, use the slower models.
     
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