Ac/Dc Common? 1

[controldude]

Stupid question probably

I am using a 120 ac single phase motor, a NC switch opens when a cam hits. I want to use the NO part of the switch to ground an input to a microcontroller (24 Vdc). What implications are there if the common part of the switch is tied to the Vdc- and AC neutral of the power supply? Noise?
Thank you.

dude

 

[IRstuff]

Plus, any wiring error or excessive neutral current will find its way into your controller circuitry.

TTFN

 

[buzzp]

I assume the AC and DC grounds are already tied together in your design since your microcontroller would not see a 'low' signal if they are not tied together (micro uses DC ground and your using AC ground to tell it a low or high signal) unless your using some isolation device.
Is the input into your micro through an optoisolator or is there any other isolation going on? I don't see any real issues with doing this other than the input signal may have to be cleaned up a bit from a noisy ground and the possibility of ground loops (probably not a big concern since this is an input into micro).

 

[controldude]

Thank you both for your quick replies. In response to buzz, there is no isolation to the input. I agree that the input may need need filtered someway, but once the switch activates it will be open for a long period of time ( > 1 sec). I was just worried that this may be damaging in some way. Thanks again!

 

[buzzp]

No isolation then if your looking at an AC signal then the DC and the AC 'grounds' HAVE to be tied together anyway or your micro wont see it.
One gray area to me on this is the NEC. They may have something to say about this in their LV wiring.

 

[IRstuff]

My previous posting was more a caveat about the fact that neutral is often not neutral and connecting that to your digital ground means that you potentially have a hot digital ground.

I nearly had a eye taken out by an IC that exploded when I connected my scope ground to the circuit ground which was connected to AC neutral.

TTFN

 

[bigbillnky]

NEC strictly forbids this type of wiring, owing to the fact that the neutral will also carry unbalanced current from other sources back to ground thru the controller. Check NEC Article 200 and Article 250 for further reference. If necessary, you could utilize a relay with the coil controlled by the cam switch. That would eliminate the need to create a dangerous situation like what IRstuff experienced.

 

[controldude]

Thank you Bill. I was hoping to avoid the extra use of relays but that seems to be the best solution for my design. I have never done controls wiring, so this advise has been a much appreciated.

 

[buzzp]

One solution might be to use an optocoupler seperate from your controller. This would keep the DC system seperate from the AC system. This should eliminate any possible NEC violations concerning wiring DC and AC neutrals together. I don't know for sure they have one with a 120V input but they do make AC/DC optocouplers. I am sure they have some with 120VAC inputs but here is one with a lower voltage input

http://cp.literature.agilent.com/litweb/pdf/5953-0406.pdf

It may require some current limiting resistors or voltage divider. Not sure but definately worth checking out.

 

[busbar]

 
A longshot solution may be something like…

http://www.crydom.com/pdf/crydom_io_oac.pdf

 

[controldude]

Hmmm, unique and interesting solutions, but the cost savings are important too. Thanks for the advice though. I wonder if anyone makes a dpdt single break lever type snap switch, or something like it. I've looked but to no avail. This switch would have 2C, 2NC, and 2NO (6 terms) and I could use one side as my ac and the other as dc. Oh well...i guess i will have to use a relay. Thanks everyone!