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So I just finished soldering the 16-pin sockets to a piece of perfboard for a bunch of 74HC595's when I realized that I didn't include any decoupling caps in the design.  How likely am I to have problems without them?  And, more importantly, how the do you normally decouple an IC with a 16 pin dip package that's sitting in a socket?  VCC and GND are on pins 8 and 16 at opposite corners of the chip, which leaves me with no idea as to how to place the caps so that they're close enough to the IC to actually work.  I was considering running traces along the two rows of holes between the pins.  Or I could just connect it to one side directly and run a wire to the other side.  I could also just solder the legs of the cap to the pins on the bottom of the board, but just bumping the cap could bend a leg and short it out against another pin.  The second method has the advantage of being insulated, which seems like a good thing.  But it also means putting a short (solid 22 AWG) wire in between the IC and decoupling cap.  So what's the best solution?
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You are quite likely to have problems with lots of 595 chips and no decoupling. The noise from the switching will alter the supply rail and cause the chips to behave strangely.

http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

As close as you practically can, I would put a cap per chip. If you have worries about shorts, use "insulating sleeves" or shrink tubing. Without seeing your layout it is hard to be more specific, but surely you can run a (fairly thick) ground wire down the other side, and tap into that for each Vcc pin?
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Without seeing your layout it is hard to be more specific, but surely you can run a (fairly thick) ground wire down the other side, and tap into that for each Vcc pin?

Sure, that's easy.  So it just needs to connect VCC to any GND, and not specifically the GND pin on the IC?  Another possibility that I noticed was to stuff the cap down in the middle of the socket with the pins sticking out the bottom, but the caps are too big and don't allow the IC to sit all the way down in the socket.  Would that be ok as long as the IC is still pretty secure and can't move/fall out?
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I wouldn't be stuffing things into the sockets myself.

The idea is to connect Vcc to Gnd (via the capacitor of course), not necessarily the pins. I suggested a thick wire so the resistance in the wire doesn't detract from the effect.

I used 4 x 595 chips here. The decoupling caps (the blue things) are nearby, not on the actual pins:

« Last Edit: January 29, 2013, 11:14:42 pm by Nick Gammon » Logged


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How likely am I to have problems without them?

Quite unlikely.
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The idea is to connect Vcc to Gnd, not necessarily the pins.

Thanks, this is the part I was missing.  I was focusing too much on keeping the cap as close as possible to the pins, and not thinking about what it was actually there for.
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How likely am I to have problems without them?

Quite unlikely.
Sorry but dhenry is known here for giving compleatly useless and incorrect information. This pure gem is no exception. I can guarantee that you will have problems without decoupling capacitors. Even one chip needs them. Look at any data sheet and it will say the chip needs them. In my professional career over 80% of why circuits would not work was lack of enough decoupling, so there was decoupling in the circuit but not sufficient.
Even if a circuit appeares to function without one you will find the noise margin is degraded sufficiently such that it is no longer a reliable circuit.
Adding decoupling capacitors has cured about 80% of interment operations reported here.
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Funny thing about *dhenry* is that he runs when challenged by anyone who has more experience.
I just wish he really knew 50% of what he claims expertise in. many times he just simply tells
the truth as his education permits which is frequently either wrong or not applicable to the issues at hand
Probably *dhenry* believes his "Contributions" are useful, Perhaps he does know the failings and is
providing them as 'lessons' to be discovered.. although there is usually a disconnect between the
question and the answer that is vaguely reminiscent of the Jabberwocky's speeches from
"Through The Looking Glass" by Lewis Carrol...

Bob
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The idea is to connect Vcc to Gnd, not necessarily the pins.

Thanks, this is the part I was missing.  I was focusing too much on keeping the cap as close as possible to the pins, and not thinking about what it was actually there for.

Grumpy_Mike and Nick Gammon are right, you need to decoupling capacitors.  With that said, can you "get away" with not using them?  Yes, but you may have bizarre results that are difficult to track down.  I've done several 595 circuits and decoupling caps are always included.

Tim
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Yeah, I figured they'd be needed.  With 8 of them controlled by spi, I figured I should work them in somewhere.  Thanks for all the advice!
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The decoupling capacitors should be placed as close to the chips as reasonably practical. The classic way of arranging power and decoupling for DIP chips whose power pins are on the corners, when a 2-layer PCB is used, is to run Vcc and ground traces lengthwise along the IC between the pins, and place the decoupling capacitor next to the top or bottom end of the chip.
« Last Edit: January 30, 2013, 06:56:26 am by dc42 » Logged

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Just to give you assurance, some one posted that they had never seen a case where decoupling capacitors solved a problem, so I collected some posts where they had. Here they are:-
Example of where adding capacitors solved the problem
http://arduino.cc/forum/index.php/topic,86873.0.html
http://arduino.cc/forum/index.php/topic,88243.0.html
http://arduino.cc/forum/index.php/topic,85247.0.html
http://arduino.cc/forum/index.php/topic,98088.0.html
http://arduino.cc/forum/index.php/topic,94913.15.html
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Just to give you assurance, some one posted that they had never seen a case where decoupling capacitors solved a problem, so I collected some posts where they had. Here they are:-

Oh, the Pesky Facts...
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You can buy sockets with a pre-wired decoupling cap that is connected diagonally across pins 8 and 16.

You could easily do this on a PCB, perfboard or stripboard and site the cap inside the socket and laying flat directly underneath the chip. For SMD it's even easier as the space under the IC is often spare real estate that can sometimes be utilised if you're struggling for room and you are using a socketed DIP.
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