Peer review request: First PCB design

Discussion in 'The Projects Forum' started by spinnaker, Feb 20, 2010.

  1. spinnaker

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    Well I got the ferric chloride in. I got my laserjet printer yesterday. The copper PCBs just came in today (that was fast). I am off to buy some Scotch Brite and some acetone.

    I am pretty much ready to go but I was wondering if someone could give my design a look over.

    Sarge Wookie took a look at the schematic but if someone wants to take another look then that is fine.

    I have included the copper, silkscreen and schematic.

    I will post the DMM part of the circuit at a later time.
     
  2. JimG

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    Why is the source for the 5V regulator not going through the full wave rectifier? (Not suggesting it should be changed -- I am just trying to learn).

    Jim
     
  3. spinnaker

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    At suggestion of the Sarge. My full wave bridge was supplying 35V once filtered. This was too much for the 7805. I am using a center tap from my transformer.

    The 5V supply will be for my PIC DMM supply only.
     
  4. JimG

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    Makes sense. Thanks.

    Jim
     
  5. SgtWookie

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    Tell me - is the secondary of the transformer center-tapped, or is it actually two separate windings?

    If the latter, you could make an advantageous change to cut the filter voltage in half yet increase current capacity for when you need lower output voltages. It will also reduce power dissipation in your regulators.

    Also, you may find that you will need a clamping circuit to prevent the filter voltage from getting higher than about 40v. You don't want to exceed the input-to-output limits of the LM317.

    There seems to be a lot of space between your components. Do you really want the board to be so large? To a certain extent, smaller is better.

    Why is D8 off by it's lonesome at the end of a long trace?

    You have parts in the middle of traces in various places, like the lower side of D1 and D3, upper side of C4 and C6, C9, C10. Those can cause problems on hobbyist boards - I mentioned this before.

    I see you haven't used the copper fill. Using the copper fill will save a good bit on etchant, along with speeding up your etching time. No point in taking off copper that would be better left there.
     
    Last edited: Feb 20, 2010
  6. spinnaker

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    I have 2 transformers both have several wires on the secondary. I have no datasheet on either of these. I assume I could just ohm it out and see what is connected to what? I will do this and report back.


    I thought about this after your enlightening advice on the 7805. But I am not sure how to proceed. How would I clamp it to 40V? Can you recommend something?


    I thought I did a pretty decent job. :) I did not want to get myself too crowded for my first project but I will try to tightened things up.

    Is the general placement of components OK? I really do not want to get in to a whole redesign just to save an inch or two of board but I don't mind tightening things up a bit.


    Did not no how to handle that. My thought there was AC on one side of the diode and dc on the other. I did not want a long AC run. But I guess it is really half wave rectification anyway. Maybe after I tighten things up I will have some for the other +5v components.

    Sorry missed that in your last post. But I am not sure what you mean. Are you saying if I have one long trace (like a bus), if I want to to connect a component to it, the component should have it's own trace, leading of the main trace?

    Yeah I gave that a try. I found it a little hard to use so I used the rectangles instead. But I will give it another try.


    So the fill does not really need to be connected to anything?? Just as long as it is masking open space? Do you have a sample of waht that might look like?
     
  7. SgtWookie

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    You can't just measure the resistance and figure it out. The secondaries will have very low impedance. You will need to connect the primaries to their rated voltage, but unfortunately you do not know what that is, as you don't have a datasheet.

    Randomly connecting wires to mains voltage/power is a very un-healthy idea.

    Were you planning on just using one transformer, or both of them?
     
  8. spinnaker

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    I was planning on using just one. I at least know something about the transformers.

    The first is very well marked right on the transformer. Everything is color coded. Unfortunately, I just looked at it again and it only has one secondary, the additional leads were for the primary to connect to 240vac.
    I was using this supply for my bread boarding because it had it's own circuit breaker.

    But this one is kind of large and odd shaped. I'd rather not use it though it does seem to be the better build of the two. It has nice sturdy wires for the secondary, while the one below, looks like they just exposed the winding wire.

    I am a little concerned that I may have problems with wire breakage in the one below but it is the only one of thetwo with a center tap.




    The second was salvaged from a very old power supply that was used by a friend of mine (not a PC supply much older than that :) ).

    So at least I know where the primary is. Plus it is marked 115V.

    No I would not randomly hook up wires to the main. :)

    This second transformer has several wires on the secondary.

    There is one pair of wires where I get 10VAC. On the other wires I get 33 VAC. I am concerned that once I rectify and filter that, I will be approaching the limits of that lm317.


    How would I clamp it?
     
  9. spinnaker

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    The Shack has a 25VAC 2 AMP transformer with center tap. If that second one I have does not work, I might just get the one from the Shack.

    Also thinking about putting the PIC / DMM supply on it's own PCB. It would be nice a simple an would make a good first board I think.
     
  10. SgtWookie

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    Something like this would work for a clamp:

    [​IMG]

    Q3 is a power Darlington, something like a TIP135-TIP137 or TIP145-TIP147.

    V2 and V3 are the outputs from your two regulators; see the yellow and green traces on the simulated O-scope.
    Q1 and Q2 are voltage followers.
    Whichever regulator output has the lower setting will be on the emitters of both Q1 and Q2, plus about 0.7v; the aqua trace on the simulated O-scope.

    D1 is a 33v 1W Zener diode. R3 sources about 15mA to D1. Power dissipation in D1 is nearly constant at about 500mW. If the current through R3 exceeds 15mA, Q3 starts turning on and shunts power through R4, R5, and R6. These are non-inductive power resistors actually available at Radio Shack.

    Q3 will need a heat sink.
     
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  11. spinnaker

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    Thanks for the circuit!!!! :)

    But I am wondering if I am getting over my head for my first project. After reading some other posts of the LM317 on difference in input voltage over output voltage and how it limits the current capability, I might be best off to just have a 0-12V supply.

    It would solve a lot of problems. I would not need to worry about over driving the LM317 and I could go back to a single regulator for the LM317s and the 7805.

    I will post the new PCB shortly. I think I did a pretty good job of decreasing the size.
     
  12. spinnaker

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    BTW The only reason I wanted 0-30 was to simulate the output voltage of a solar panel for an up coming project. But I might be able to just hack something together temporarily to do that.
     
  13. SgtWookie

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    Using linear regulators for a wide-range supply is definitely problematic.

    If you really want a wide voltage range, you need to look at building a switching supply. Otherwise, you wind up with a room heater due to the power dissipation in the regulators.

    An ATX PC power supply converted to a bench supply makes for a very good and quick-build general use bench supply; and if you need a variable source, you can add a linear regulator circuit from the +12v side. You can even buy a brand new ATX supply for under $20.

    The ATX output has more noise on it than a linear supply, but you could always add more filtering if so desired.

    I have a little 250W Compaq ATX supply I converted to a bench supply.
    +5V @ 25A
    +5v @ 1A standby (always on)
    +3.3V @ 14A
    +12v @ 8A
    -5V @ 0.5A
    -12v @ 0.8A
    With that little supply, I can do most of what I need to do. It's not my only supply, but it's the only supply that stays on my bench all the time - simply because it's so useful.
     
  14. spinnaker

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    Yeah that is on my list. This first project was mainly for a learning experience. I did not plan to use it long term.
     
  15. spinnaker

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    I have updated the schematic. I switched to a 12.6 VAC transformer.

    I then connected the PIC regulator back to the main bridge.

    Enlosed is the new PCB and schematic.

    I added the filled planes. I did not go too crazy with them, I wanted to be sure I was on the right track.



    When I cut the board, should I cut it a bit wider than the template, to make it easy to peal off the template? Should I mask the extra space?


    Oops forgot to add the PDFs. :eek:
     
    Last edited: Feb 21, 2010
  16. SgtWookie

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    I don't see anything attached to your new post. Too large, or did you forget?

    I usually leave the board just a bit larger than the transfer. I use magazine print, and it doesn't peel off; you scrub it off with a worn, soft tooth brush.
     
  17. spinnaker

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    You must have posted this right when I was fixing it. :)


    Should I mask the extra bit somehow?
     
  18. SgtWookie

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    Did you know that D2 should be between the upper side of C4 and the regulator output, cathode to the regulator output? This is to drain C4 when the supply shuts down. As shown in your schematic right now, it serves no useful purpose.

    If you're going to use the Radio Shack 12.6v xformer, it has a center tap, and you can power the 7805 from the center tap.

    [eta]
    As far as the extra board - I don't worry about it.

    You should use the copper pour over the entire board. Otherwise, you'll just use up your etchant more quickly.

    You should try to make your traces oriented vertically, horizontally, or on multiples of 45°. Running traces at odd angles can result in problems. You have a couple of oddly-angled traces where you're getting mighty close with clearance. You also still have some areas where you have pads in the middle of traces, which I've said a few times now is not a good idea for homemade PCB's; you risk making problems for yourself when you try soldering on the components. If you "lift" the middle of a trace off a board by overheating, how do you plan on fixing it?

    I didn't bother editing all of it, but I've shown some suggested changes in blue:

    [​IMG]
     
    Last edited: Feb 21, 2010
  19. spinnaker

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    It is based on a design I found elsewhere. I think it say it was for short protection or some such thing. But doesn't the LM317 already have that?

    I will make the change at your recommendation.

    They are only 10uf caps. Would they store that much of a charge?

    Can I just leave D1 and D2 (and their brothers) out? I don't think I have seen an LM317 regulator with them.

    What drains the larger filter caps? That always seemed to be the bigger issue with this supply. The original designer suggested a permenent load.



    I will go ahead and add the center tap back in. One diode rectifier, just like before?
     
  20. spinnaker

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    Sorry do not understand. Do you mean it does not matter which way I go? If so I think I want to go with a separate board again. It will make a nice simple first PCB.



    Yes I know, I wanted to make sure I was doing it correctly in the small areas. Do they look ok? Just cover the whole board now?

    OK I will fix those. But it creates problem for my DMM board but we can discuss that when the time comes.

    As far as clearance, that is why I had things spread out before. Maybe I got a bit too crazy trying to save space. I will spread things out a bti.

    I thought I got all of the pads in the middle of runs. Maybe I am not understanding. Is it one of the ones you marked above? Or are my traces going to the pads too big? Or do I need a longer trace to the component coming from the main trace?

    So for being dense, I know you told me but I don't think I fully understood.

    BTW Thank you for using a blue pen and not a red pen, where you might risk lowering my self esteem. :)
     
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