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Intra-conduit coupling.

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itsmoked

itsmoked

Electrical
Feb 18, 2005
19,114
Anyone have a feel for AC coupling into a long line?

If I run a dry contact at the end of 300 foot run. The contacts close to run 20mA thru an opto LED. Do I have to worry about the 120VAC they foolishly run in the same conduit coupling in enough AC to turn on my LED?

Keith Cress
Flamin Systems, Inc.-
 
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I'd worry about it.

When I was screwing around with them, 20 years ago, visible LEDs rated at 20mA were still easily visible at 2mA. Are IRLEDs similar?

Worst case: The opto you use today doesn't have enough gain to be affected by the noise. Its replacement does.





Mike Halloran
Pembroke Pines, FL, USA
 
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IRLEDs are probably worse...


Anyone got any favorite suggestions for protection from a coupled AC signal driving the LED?

2cxj9s9.jpg


Keith Cress
Flamin Systems, Inc.-
 
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A thought: Some optoisolator packages bring out the phototransistor's base lead, so a base- emitter resistor can suck charge off the base, speeding turn-off, and bit rate. The same resistor would also reduce the CTR, I think.

OR,

You could put a, say, 10Hz low pass filter downstream.



Mike Halloran
Pembroke Pines, FL, USA
 
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Twisted-pair (for the low voltage signal) would seem to be the obvious solution.
 
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The wire is already in the conduit, and I'll bet it's not twisted, even accidentally. In which case, it might be possible to use it to pull STP.

Which, I think, would still be in violation of the (1996) NEC article 300-3 (c) 1, requiring that all conductors within the conduit be insulated for the maximum voltage on any conductor, i.e. 240V.

I don't have a newer copy of the code, and I don't work with it, so don't take my word.



Mike Halloran
Pembroke Pines, FL, USA
 
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How many amps are we talking about in the power circuit?

 
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dpc; This is theoretical stuff. You make a product and have no say in its installation. We expect the runs to be 20-40 feet but you also expect someone to run 2-3 hundred feet and being cheap to mix wires in a conduit like they shouldn't.

Likewise I can expect the lack of twisted wires to boot.


Keith Cress
Flamin Systems, Inc.-
 
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Twisted pair wiring is the best solution, as I'm sure you already know. For a typical 20 A or lower 120 V circuit, I'd not expect any issues, but I'm not betting any money on it either. The ac circuit will include both the hot and neutral so there will be some cancellation of the magnetic field created by the ac circuit.


 
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I assume that the switch is slow ( manually operated)

If the switch is open you have only capacitive coupling
which is negligeable at 60 Hz.

If it is closed, the noise can be coupled magnetically.
It can increase or decrease the current.

I would suggest to move the resistor between the LED and the switch and add a cap from the common point of the R
and the LED to GND. Select as large cap as possible to
attenuate the 60Hz.

Plesae read FAQ240-1032
My WEB: <
 
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If there is a possibility of a variable load, your circuit may work fine at normal loads but fail under conditions of higher than normal current.
respectfully
 
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put as low a resistor from +12 to opto connector that your drive circuit and target load supply will tolerate.

drown out the noise with low impedance
 
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Since the wire is in a conduit, could an electrican pull a new pair of twisted wires? I've seen electricians pull wires much worse than small twisted pair through 300' of conduit.
 
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itsmoked, that's a very nice circuit diagram.

Perhaps some of the learned posters could explain to me how an open circuit wire (when the switch is open) can have a signal coupled into it by magnetic induction. Likewise how making a twisted pair out of the switch wires will have any effect whatsoever on the magnetic induction. This is using "standard solutions" to an incorrect situation.

Magnetic induction occurs into loops and you don't have one when the switch is open! You will get a significant capacitive coupling, but what you have not shown is where the ground is for the (isolated) mains load. Interestingly a twisted pair would reduce the capacitive coupling! 2dye4 has the right idea with a load on the opto end, reducing the capacitive coupling time-constant significantly.
 
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Thanks everyone.

Thanks logbook for the complement. It's what we Engineers live for.[lol]

2dye4; That's just what I've decided to do. Swamp things so that noise won't turn on an LED.

The cap is being considered to "clean contacts on the field contacts in case they are some grungy T90 30A relay or something. On closure and bounce the contacts oxidation will theoretically be 'cleaned off' by dumping that cap.

The cap will also sink coupled-in AC to ground.

The resettable fuse is for those 'morons', (and believe me I have been one of those!), who hook 12VDC to the wrong screw.
{That being the raw 12VDC from other parts of the system.}

30v2c1i.jpg


Keith Cress
Flamin Systems, Inc.-
 
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Keith:
Move the current limiting R from the 12V into the upper
lead of the switch ( between the SW and LED.c ).
Otherwise the swich and the PTC will be subject to unlimited discharge current. The R will lower the corner frequency,
too


Plesae read FAQ240-1032
My WEB: <
 
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