Long length switched ground

As nothing more than a life long electronics enthusiast without a degree ot her than popular electronics magazine and a distaste for math, I have the f ollowing embarrassing question for the design wizards.

My design skills often times are "if it works for a couple of days and does n't get hot we're good to go".

So there's something that actually I've done for a long time without proble ms but my guess is there probably is one somewhere.

Say you have a circuit on a pcb and it does this and that and your power su pply is ample with a simple LM317 regulator or whatever.

Ok -- now to interact with your circuit you have possible long runs (say 10

0' max) of good gauge wire that simply open/close the ground circuit to an optoisolator(s) (which is located on the pcb). And not just 1 but several ( think burglar alarm etc). ground closes -- opto triggers otherwise it's idl e.

Now I need an education on "possible" flaws in doing this (although I've ne ver had a problem). I'm aware of any induced voltage through a lightning st rike down the block etc but what are you going to do about that? -- shit ha ppens.

Are there any nuances I'm missing with this? With the ground circuit basica lly being floated around everywhere? The same ground I'm using on the pcb? Should there maybe be some isolation (diode or something?) or also, would p roblems depend on the complexity of the circuit onboard?

Noise causing something? (.1uf is my favorite part so always use plenty of them). -- anyway hope this makes sense

Reply to
mkr5000
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It would take a lot of noise to make that circuit misbehave. I'd probably split the LED's current limiting resistor in half, put both halves in series with the ground, and put a TVS zener and a pretty big cap (like 10 uF ceramic) from the junction to board ground. Extra points for uprating the outboard resistor to something physically bigger that has good pulse handling.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Actually Phil, I have the LED resistor on board going to the anode side. An y advantage to having it in the ground path instead? Also one thing I notic ed about these opto isolators I use (and others), it's amazing how little c urrent it takes for them to trigger on the NPN output to a decent low resis tance. I actually could use a much higher value of resistance than what I'm using. But I also want a good solid "hit" when it triggers so I kept the v alue low. Like I say, never had a problem but I always like hearing you guy s.

Can you put what you said in baby steps? Not following exactly.

Reply to
mkr5000

Generally it's a good idea not to have an off-board connection going straight to a semiconductor, because most are easily damaged. Optocouplers are a bit of a special case, of course.

Your circuit sounds like this:

+VDD 0 | R R R R | ... stuff | | --- |/ cruel \ / ~~> | world --- |V | | 0-----------* ... stuff

which realistically will probably work fine, but leaves you no good way to protect the LED from sufficiently-nasty transients, especially positive-going ones that can damage the LED by zenering it. Since we're splitting hairs and suchlike, what I'm suggesting is this:

+VDD 0 | stuff | ... | | --- |/ \ / ~~> | cruel --- |V world | | 0----R1R1---*-------*--R2R2----* ... | | / stuff CCC --- CCC / A | | GND GND

where R1 = R2 = R/2 or thereabouts, C is something like 10 uF 25V ceramic, the diode is something like a P6KE6.8AG

and R1 is something reasonably beasty, such as a 1W wirewound. The cheapest on Digikey is actually 3W:

For extra paranoia points, add a Schottky rectifier anti-parallel with the LED, and another beefy zener from VDD to ground (to prevent the supply from getting dragged up too high by a big transient).

But as in computer security, the right answer depends on your threat model. None of these things will protect against nuclear EMP or a direct lightning strike.

And for hobby/home purposes, as I said your original version is probably fine.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Do you really need optoisolators?

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

Well it's interfacing with a PIC microcontroller (but maybe doing a PI SBC in it's place) so rather be safe than sorry. And unlike you guys, where $$ is an important consideration, the extra cost is not an issue.

Thanks -- sounds like my "assumption" that all is well is ok. But I'll study those improvements and possibly breadboard it.

Reply to
mkr5000

I need to clarify maybe, that I'm using the same power supply for everything. The internal circuit and the optos which can have those long cable runs.

It would be best to have 2 separate power supplies I'm sure, one just devoted to the opto isolators led's and another for the circuit. But like I say, never had a problem with just one supply but had concerns about it -- hence my question.

Reply to
mkr5000

The reason that I ask is that I've seen lots of optos used when isolation wasn't needed, as if they are some sort of magical noise filters. They aren't.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

mply open/close the ground circuit to an optoisolator(s) (which is located on the pcb). And not just 1 but several (think burglar alarm etc). ground c loses -- opto triggers otherwise it's idle.

Well, if there's an attack potential, you might consider that someone might find a pair of wires, and apply a high voltage to it. No problem at the sensor (it's an o pen switch), but it could blow your optoisolator's LED, after which the node is permanen tly stuck 'open'.

An input circuit that can be checked for continuity and sensitivity under s oftware control would be possible, where cut or shorted wiring would be detectable events.

Reply to
whit3rd

Since we're

+1 Mark
Reply to
makolber

Is there a possibility of noise getting into the ground of the electronics through the ground wire? I'm assuming there is a ground wire. Is ground only through earth ground? I would think handling of the ground is every bit as important as the rest of the circuit.

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Rick C
Reply to
rickman

As others have said, how you protect the circuit depends on what you want to protect against. I have seen electronics get very upset when the ground is not properly handled. In essence a ground loop is created so that noise can enter on the ground wire and pass through the ground of the circuit exiting at another grounded point. The voltages involved mess up the circuit operation and in one case of a static discharge - damage circuitry.

If you don't know what you wish to protect against saying your circuit seems to work is a bit like an elephant charm. You haven't seen any elephants so the charm must be keeping them away.

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Rick C
Reply to
rickman

Good point.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Let's assume a normal environment, not industrial but an office building maybe. And forget about lightning induced noise or "rare" incidents.

(And Phil I have yet to look closer at what you drew out).

But here may be some bad news --

I don't have an earth ground. Just wanted to use a 2 prong plug so it's all floating. Probably what I may do like I said is use a separate supply just for the optoisolators. That would "help" keep the outside world out of the internal circuit.

Reply to
mkr5000

The main problem with the external ground is ground loop or other stray currents in the ground path that affect your circuit. If you use a star topology to connect the various ground connections you should have no real problem since your remote device is not grounded other than through your cable. Keep the ground path from the cable to the supply common as short as possible and share as little of the conductive path as possible with the electronics ground path. Use separate paths on the circuit board only connecting commons at the power supply.

I don't really thing two supplies are required in this case since the power supply and remote switch are both isolated from earth ground.

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Rick C
Reply to
rickman

Ok -- just thinking that for what it's worth the optos "supposedly" have a KV or something of isolation?

And the common (ground) to all the outboard switched, relays etc could be u p to 100ft or possibly longer

Would like to use just the one supply as you say (and I'm doing it now) -- not sure if I did a start ground to the supply ground itself (I'll check). I also thought of the separate supply because I'm only using 5v -- 9 or hig her would probably be better for long cable runs ? But using 18 gauge also.

Reply to
mkr5000

a KV or something of isolation?

up to 100ft or possibly longer

- not sure if I did a start ground to the supply ground itself (I'll check) . I also thought of the separate supply because I'm only using 5v -- 9 or h igher would probably be better for long cable runs ? But using 18 gauge als o.

that should be STAR

Reply to
mkr5000

I don't think the wire gauge will have much impact, but if you wanted you could run with a higher supply voltage, but the improvement would not be large.

I would split the series resistor putting half in the ground line and half in the switched line. This will all but eliminate any potential ground current issues.

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Rick C
Reply to
rickman

OK -- that's what Phil was trying to tell me. -thank you all

Reply to
mkr5000

other than popular electronics magazine and a distaste for math, I have the following embarrassing question for the design wizards.

esn't get hot we're good to go".

lems but my guess is there probably is one somewhere.

supply is ample with a simple LM317 regulator or whatever.

100' max) of good gauge wire that simply open/close the ground circuit to a n optoisolator(s) (which is located on the pcb). And not just 1 but several (think burglar alarm etc). ground closes -- opto triggers otherwise it's i dle.

never had a problem). I'm aware of any induced voltage through a lightning strike down the block etc but what are you going to do about that? -- shit happens.

cally being floated around everywhere? The same ground I'm using on the pcb ? Should there maybe be some isolation (diode or something?) or also, would problems depend on the complexity of the circuit onboard?

f them). -- anyway hope this makes sense

You haven't shown us your circuit, so people are inevitably making guesses/ assumptions. The main issues can be, depending on what you're doing: pickup on an open wire leading to rf/instability, false logic triggering, n oise, hum. defeatability if it's a burglar alarm etc vulnerability to static, which can easily come from people and of course lightning strike vulnerability.

NT

Reply to
tabbypurr

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