Can someone please explain grounding on a pcb to me, specifically perfboard?

rwgast_logicdesign

Ok Ive been getting a little bit of help from some really knowledgeable people around her and I thank you big time those of you who have sent me pm's you know who you are.

Most of my electrical experience up until now has been AC systems for commercial buildings and Industrial work like wind/solar farms and power plants, or even DC electrical systems in cars. Working on circut boards with fragil components in the millia and micro amp range is completely different, or maybe not so much as I would excpect. Right now I am reading a decent book on electronics enginerring to bring me up to speed. My problem is im not sure how grounds are done on a perfboard and what the best practices are. I tryed googling this but most articles were talking about ground planes on a multi layer pcb.

So when reading schematics do all the grounds go to the same place? Ie do you interconnect every vss point somehow? And if so would you do this even between diffrent voltage i.e. ground 5 and 3 volt circuts togather? In my head im thinking you would just make a patch of solder or add a small copper sheet to the perfboard and jumper wire every vss wire to it, which if all the vss points need to touch seems like the cleanest way to ground.

Mike Green

The whole question about how to connect things depends on the circumstances. It depends on the voltages involved, the currents that have to be carried, and the frequencies involved, not just the repetition rates of the signals, but the rise and fall times needed. When currents get high, the resistance of the interconnections becomes significant. Similarly, when pulses start getting around a microsecond and shorter, wires start acting like transmission lines. On the other hand, when currents and frequencies are lower and pulse edges are slower, you can do typical perf-board point-to-point wiring. For grounding, you need to have everything tied together as a single ground. After all, voltages are always relative to something and that's usually the device ground.

Rules of thumb ... you're probably ok with most currents on the order of a couple of hundred milliAmps or less ... you're probably ok with most signals down to maybe 100ns ... if your connections get to be longer than a couple of inches, their resistance is starting to become significant and the connections are starting to act like antennas and transmission lines, so start thinking about heavier wires and making coarse grids for a ground plane. Make sure to use 0.1uF ceramic capacitors between Vdd and Vss close to each IC. For simple ICs, you can probably share a 0.1uF capacitor between pairs of ICs. For complex ICs, you may need more than one capacitor for each IC.

Look up descriptions of "ground loops" on the internet. When you have high intermittent currents going through one section of a circuit, you will have a voltage drop across that area. If you have different portions of the control logic using different ground points, there may be a voltage difference between the ground points as a motor say draws peak current. That introduces noise into that portion of the control logic which you don't want. The answer is to have all the control logic grounds connect together at one point not subject to intermittent high currents.

rwgast_logicdesign

Thank you for clarifying for me. Now basically for a simple board it basically connecting all the vss points with jumper wire will work. Now is it overkill to mount them all to a small copper plate or something along those lines? I know in most cases a better smooth ground always makes things run smoother. I would like to use a system like the on a perfboard for OCing the propeller, Im just trying to make sure the power source is as clean as it can be. Is there some sort of standard copper piece sold sold by suppliers for this purpose? Looking at perfboard on digi i may need to do some more research but the ones with ground planes are a bit outrageous..

average joe

I do a lot of work with perfboard and have had mixed experiences. Planning is the key. Decent bypassing *.1uf ceramic caps close to the ic* will get you a long way. I try to use star grounding when working with mixed signal design. Otherwise I use a large gauge wire to jump all vss's together as you described. I have had a fair amount of success with complex digital-only designs. When I work with audio, grounding becomes WAY more critical. Digital ground ALWAYS leaves the board separately from Analog ground. This pertains to analog design, but you might find it helpful.
http://www.aikenamps.com/StarGround.html
attached are some of my ugly boards

cavelamb

I'll add a couple too.

I mostly work with wire wrap wire for this kind of circuit.
That's 22 to 24 gauge. Might look a little small to you!
But it works fine here.

I use a modified technique.
A short wrap on one end, and a hook soldered at the other end.
The wrap takes a special tool - but they are not expensive.
Get one with a built-in stripper. (WIRE stripper!)
That's the little section in the middle of the gold tool.
Those guys are over 30 years old!

The other pics - my Proto board SD card socket.
It is lightly super-glued to the back of the connector brick.
Bottom shows the wiring.

Be as neat as humanly possible...
It pays off in reliability and troubleshooting troubles.

The brown board is a GP interface set-up for a Gadget Gangster Platform.
I'm using wire wrap type male pin header - inserted from above and soldered below!
Then a VERY LIGHT bit of super-glue under the the plastic part of the header to
attach it solidly to the circuit board. Otherwise they wobble (not a problem but it
feels wrong). Soldered in parts sucks when you find a problem!
Sockets are worth more than their weight in gold.
Lie the LED and resistor on this board - just a dual row header cut to length.

As for grounding, yes, a single point ground is prefered.
But that really inly becomes an issue when you have multiple power supplies.
Ground them all at one pint and avoid the hassles.
For starters though, just stilc to single power sources until you feel froggy.

average joe

@cavelamb
Those are WAY better examples of what pcb's should look like.

Dr_Acula

Now is it overkill to mount them all to a small copper plate or something along those lines?

If all your chips use a similar amount of current (ie a few mA each or less) then you can daisy chain grounds in any sort of pattern you like. Eg a propeller, eeprom and some support A to D and D to A chips.

However, if one device uses a lot more current, eg a relay, or there are back EMF voltages (again a relay) or you are using a motor/motor driver, solenoid driver, or (ballpark) any device that might use more than 50mA, then I would probably do what you are suggesting and have a common physical ground location.

On a small board you probably don't save much wire daisy chaining vs bringing grounds to a common point, so it may well be just as easy to bring things to a common point.

Having said that, you can get away with not bringing everything to a common point by giving every IC its own local short term energy store. Like Mike Green says, these are called "bypass" capacitors and a really common capacitor is a 0.1uF capacitor mounted next to every IC across the power supply pins. I used to try to save costs by leaving some out, but they are so cheap when you get them in bulk I now just mindlessly give every chip its own 0.1uF cap. The propeller is a bit of a special case as it needs two 0.1uF caps. I also given higher current components their own larger caps - eg an SD card also gets a 22uF tantalum across its supply pins.

@cavelamb - nice soldering!