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Author Topic: Common mode current on AC wiring?  (Read 3503 times)
KC1BMD
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Posts: 608




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« Reply #15 on: September 04, 2017, 12:53:15 PM »

WB6BYU and W9IQ: There is no room in the AC wall outlet to really fit much of anything. I tried plugging the CO detector into an extension cord to the outlet and used ferrites (mix-31) on the cord to see what effect it had. I used snap on ferrites of varying sizes on the extension cord putting as many turns in as would fit. It did not silence the detector, although the ppm indicator on one of the CO detectors with such an LED display showed a lower reading, indicating that the RF current was reduced. These CO detectors are so sensitive, I think it would need to be reduced to near zero to have any effect and I was unable to accomplish that.
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NK7Z
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« Reply #16 on: September 04, 2017, 03:34:20 PM »

Why not just make some filters?
Some .1 to .01 uf disc caps (proper voltage) and a couple of coils like the old "Brute Force" filter in many ARRL handbooks should do the trick.
You don't even need toroids to build one.

-Mike.
When doing this on AC lines, I use X and Y caps, not disk caps...  See:
https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
G8JNJ
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« Reply #17 on: September 05, 2017, 03:39:56 AM »

Hi Norm,

You may need to optimise the type of ferrite and number of turns for the frequencies that are problematic.

If you are just using small single clip-on ferrites, you would probably need at least 10 of them attached to have any sort of noticable effect at 7MHz.

That's why multiple turns on large cores are more cost effective.

Steve, G3TXQ's balun chart is very useful in helping to understand what is likely to work the best.

http://www.karinya.net/g3txq/chokes/

Adding X and Y capacitors or an off the shelf mains filter may also be required if the problem is really severe.

If this still doesn't work, it may be necessary to modify the detector and add additional RF decoupling capacitors to the circuit. However this may invalidate your insurance, so personally I wouldn't consider it, unless the manufacturer can provide modified units.

Regards,

Martin - G8JNJ




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KH6AQ
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« Reply #18 on: September 05, 2017, 05:01:04 AM »

G3TXQ has a nice chart that will show you how many turns of power cord thru which core will give you maximum CM impedance at the frequency of interest. What makes a CO detector a challenge to CM filter is that it is small and has little capacitance (that means a high impedance) for the CM inductor to work against.

http://www.karinya.net/g3txq/chokes/
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KC1BMD
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Posts: 608




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« Reply #19 on: September 05, 2017, 05:42:44 AM »

G3TXQ has a nice chart that will show you how many turns of power cord thru which core will give you maximum CM impedance at the frequency of interest. What makes a CO detector a challenge to CM filter is that it is small and has little capacitance (that means a high impedance) for the CM inductor to work against.

http://www.karinya.net/g3txq/chokes/

Thanks, I've also seen that nice chart. I think I convinced myself that no reasonable amount of ferrites will solve it.
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W6EM
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« Reply #20 on: September 05, 2017, 06:06:43 AM »

Have you contacted the manufacturer of the detectors and explained your situation to them?  Perhaps they have a modified detector for use in RF environs.  I would think that by now, CO detectors installed in RF environments would have had a solution from the manufacturer side, just like AFCIs of recent manufacture have.

It is, by definition, a classic appliance Part 15 problem.  And, you've done almost everything imaginable to try to suppress RF externally.  Manufacturing devices highly susceptible to RF falsing would rule out their application in many buildings subject to mild RF.  And, that's probably a lot of places.

The fact that you don't have the problem when operating from batteries should steer the manufacturer in the right direction.  Perhaps some internal filtering would solve the problem, but doing it yourself would risk invalidation of warranties, liability (in the case of your neighbor's  etc., and insurance company denials if ever a bad situation).

73.

Lee

 
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KC1BMD
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« Reply #21 on: September 05, 2017, 06:07:53 AM »

AA4HA: You commented: "It would not surprise me to find that the radiated pattern for your antenna is more like a "cloud burner" and is going more upwards than to the horizon."

I've been fairly lucky with DX running at most 85W or so (if my ARRL DXCC and WAS over the last year or two says anything about that). However, I have not been able to reach Japan and other parts of Asia. Close contacts have been difficult but not impossible (e.g. immediately surrounding states). It seems intermediate distances are easier than the close distances leading me to believe that at least on some bands the take-off angle is reasonably low as opposed to more straight up.
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KC1BMD
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Posts: 608




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« Reply #22 on: September 05, 2017, 06:23:12 AM »

Have you contacted the manufacturer of the detectors and explained your situation to them?  Perhaps they have a modified detector for use in RF environs.  I would think that by now, CO detectors installed in RF environments would have had a solution from the manufacturer side, just like AFCIs of recent manufacture have.

It is, by definition, a classic appliance Part 15 problem.  And, you've done almost everything imaginable to try to suppress RF externally.  Manufacturing devices highly susceptible to RF falsing would rule out their application in many buildings subject to mild RF.  And, that's probably a lot of places.

The fact that you don't have the problem when operating from batteries should steer the manufacturer in the right direction.  Perhaps some internal filtering would solve the problem, but doing it yourself would risk invalidation of warranties, liability (in the case of your neighbor's  etc., and insurance company denials if ever a bad situation).

73.

Lee

 

Thanks Lee. Yes, I have contacted the manufacturer (Kidde in this case). I'm not saying all their detectors have issues but the 2 or 3 that I tested did (e.g. model KN-COB-DP-LS, 900-0235). Their tech-support said they have not had any reports of the issue and there are no suggestions for fixing it. I provided information from my testing but I doubt it went anywhere. Switching to a different brand (First Alert model CO615) helped somewhat. Note that some people have had issues with some models from First Alert also. You only have to search this and other sites (e.g. "Kidde") to see some of the issues reported -- albeit, many of those posts could have come from me Smiley). I suppose there are two sides to many issues and someone could possibly make an argument that my installation is not typical, nor recommended, in the amateur radio community. So I have to wonder at what point the blame starts to shift to my side (causing the interference) compared to blame on their side for not being able to adequately filter and suppress it. I am not sure if they are required by the FCC to eliminate any and all interference regardless of antenna/equipment installation and proximity to their device. This is a  gray area and point of confusion for me.
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KH6AQ
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« Reply #23 on: September 05, 2017, 09:00:05 AM »

You are not required to fix the interference problem and the CO detector manufacturer is not required to fix the interference problem. I believe the the FCC's position is that the user of the CO detector can have it modified to reject RF or they can stop using it. If you are renting, or even if you own the condo I can see the management making things hot for you.

The simple fix is to disconnect the AC and operate it off the battery. You might supply lithium batteries for maximum battery life. Another way to deal with the issue is to reduce RF power.

Infinite CM filtering will not solve the problem if the dominant RF signal mode is DM (Differential-mode). There can be CM-to-DM conversion on the AC wiring due to asymmetry of the line, neutral, and GND wiring. The fix could be to install an EMI filter having both DM and CM attenuation. But, even using an off-the-shelf, UL listed EMI filter requires getting into the AC wiring and that's a potential safety issue.  

  
« Last Edit: September 05, 2017, 09:04:27 AM by KH6AQ » Logged
N7EKU
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Posts: 702




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« Reply #24 on: September 05, 2017, 10:51:19 AM »

Hi,

Both battery and AC CO detectors perform the same.  I don't see why anyone would object to replacing AC ones with battery operated ones.  Plus an advantage to battery ones is that you have more freedom to optimize their placement (height above floor level) and they can leave more outlets free.  Perhaps you can "sell" you neighbor using these points.  The only disadvantage is that you eventually have to replace the batteries, but they beep at you just like smoke detectors when they need replacing so there is no worry there (just the cost).

Another advantage is you can replace CO-only detectors with combined smoke-CO units and add even more safety!

Otherwise, probably you could build up a small plug-in box that contains filter capacitors with inline choke, and an AC socket, to plug between the detector and wall outlet.  As using a choke alone on different bands doesn't seem sufficient, I think you need the capacitors to bypass the RF.

73,


Mark.
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Mark -- N7EKU/VE3
KC1BMD
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Posts: 608




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« Reply #25 on: September 05, 2017, 11:14:52 AM »

KH6AQ: Thanks for the reply. Even if I could somehow apply a filter, I have no clue whether the intruding RF is CM or DM. I could always just unplug my offending detector when I want to operate but would never expect my neighbor to do that and would never ask. That's only a short term workaround anyway, since when unplugged and on battery, I believe it beeps every couple of minutes to remind you.

N7EKU: If it becomes more of a problem affecting my neighbor's detectors, I'll probably ask if they would consider switching to battery-only operated detectors. Not really knowing what I was dealing with when it started, I tried plugging th detector into this TrippLite surge suppressor/noise filter. It didn't have any effect. Maybe I could use such an empty case to add caps and choke. Anyone have a design they tried already?
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KH6AQ
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« Reply #26 on: September 05, 2017, 11:26:42 AM »

Having an AC powered CO detector might be required by the local (national?) electrical code. I'm getting my new house inspected soon and was told that the smoke detectors had to be AC powered.
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KC1BMD
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Posts: 608




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« Reply #27 on: September 05, 2017, 11:45:13 AM »

Last time I checked (in MA, about 3 years ago) battery operated CO detectors met the requirement. I just checked on-line and they have the requirement from 2015 which states in part:

There are several types of alarms that are allowed; they include:
Battery powered with battery monitoring;
• Plug-in (AC powered) units with battery backup;
• AC primary power (hard-wired – usually involves hiring an electrician) with battery backup;
• Low-voltage or wireless alarms with secondary power; and
• Qualified combination smoke detectors and CO alarms.

Of course, that could differ by location.
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N7EKU
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Posts: 702




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« Reply #28 on: September 05, 2017, 05:46:10 PM »

KH6AQ: Thanks for the reply. Even if I could somehow apply a filter, I have no clue whether the intruding RF is CM or DM. I could always just unplug my offending detector when I want to operate but would never expect my neighbor to do that and would never ask. That's only a short term workaround anyway, since when unplugged and on battery, I believe it beeps every couple of minutes to remind you.

N7EKU: If it becomes more of a problem affecting my neighbor's detectors, I'll probably ask if they would consider switching to battery-only operated detectors. Not really knowing what I was dealing with when it started, I tried plugging th detector into this TrippLite surge suppressor/noise filter. It didn't have any effect. Maybe I could use such an empty case to add caps and choke. Anyone have a design they tried already?

Hmm,

I looked at that TrippLite unit and it seems pretty good as includes ferrites and capacitors designed for HF/VHF, so I would have expected that to take care of it.  Probably there is a lot of HF getting into the AC line just because it is of an unfortunate length.

If battery-only is OK in your area, that seems to be the way to go.  Other than that, you could try rotating the dipoles 90 degrees and see if that makes a difference.

Or QRP to a level that stops tripping things and don't go above that.

Good luck and 73,


Mark.
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Mark -- N7EKU/VE3
KC1BMD
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Posts: 608




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« Reply #29 on: September 06, 2017, 03:12:40 AM »

When I purchased it, I don't recall seeing the inside shot with the included components. It only had a general description about surge suppression and 'noise filtering'. You would think with all those components it would have cured it! I don't think I could do any better job building one.
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