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Author Topic: Advice on ac line filter  (Read 2534 times)
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WA2SQQ
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« on: December 19, 2018, 09:19:28 PM »

RF occasionally gets into the thermostat of my bathroom radiant heat system and trips the ground fault protection. I’m going to try making an RF filter using some torroids. Looking for suggestions on whether I should use a separate torroid for each leg, or one torroid with a dual winding? I used the latter on my phone lines and it completely eliminated all the RF problems.
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AB2EZ
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« Reply #1 on: December 20, 2018, 11:15:28 AM »

Hi!

The following works for me (no more GFCI trips since I did this):

You will need two modern 0.01uF "orange drop" capacitors... with a voltage rating of 1000V or higher. [I use 3000V-rated capacitors because I have some in my parts cabinet, and they are not very expensive.] The voltage rating needs to be high enough to keep the 120VAC that will be across each capacitor from causing heating of the capacitors.

Directly at the location of the GFCI outlet, install one capacitor from "load side" hot (black wire/narrow slot on the outlet) to safety ground (green/round pin). Install the other capacitor from "load side" neutral (white wire/wide slot) to safety ground.

For convenience, I did this by installing the capacitors across the terminals of a 3-prong plug... and then inserting the plug into one of the 3-prong outlets of the GFCI.

The modern capacitors should have very little leakage at 60Hz, and a nominal impedance at 60Hz of -j265,000 ohms each. So, at 60Hz, they should not bypass enough (unbalanced) 60Hz current around the GFCI's internal ground fault sensor to disturb the proper operation of the GFCI.

However, RF... including unbalanced-to-ground RF... flowing through the load-side hot and neutral wires... will bypass the ground fault sensor inside the GFCI, and flow directly to the safety ground.

Be careful to properly insulate all of the wires. Also, test the GFCI after installing the capacitors, to ensure that it is still working properly.  
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WA2ONK
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« Reply #2 on: December 20, 2018, 12:06:50 PM »

I did almost the same as Stu. I have GFCI breakers in the power panel, so I did the by-passing inside the panel right at the breaker. I also placed some ferrite snap on beads on the romex where it exited the panel. It cleared up the false tripping...
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WA2SQQ
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« Reply #3 on: December 20, 2018, 12:47:01 PM »

The problem is, the GFCI is an integral part of the thermostat. I have two caps installed which worked last year. This year when I turned the heat on, my 75m transmissions trip it - only 75m. I have a few RF filters that were removed from some TV's, where the AC line cord connects. Going to wire that in series with the power connection. If it works, I'll have to disassemble and hardwire the PCB inside a larger junction box. Thanks for the suggestions.
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #4 on: December 20, 2018, 03:02:21 PM »

For your situation:

Place one of the capacitors from line side to load side (input of the thermostat to the output of the thermostat)... on the “hot side” (black wire) of the power line.
Place the other capacitor from line side to load side... on the neutral side (white wire) of the power line.

This should enable any RF entering the thermostat to bypass the GFCI / unbalanced current sensor inside the thermostat.

Stu
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w3jn
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« Reply #5 on: December 20, 2018, 05:08:49 PM »

Not a good idea to use orange drops, or other random types of capacitors on line filters.

Y-class capacitors were developed for this very reason.  https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/
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« Reply #6 on: December 20, 2018, 06:36:39 PM »

One thing to remember when using split-core toroids is that they don't work at all unless the two halves are in direct contact with each other. Trying to squeeze too many conductors into one may cause a separation of the halves rendering it useless.

Be wary of snap-ons that hold themselves in place by gripping the conductor as this may push the ferrite halves apart. When I have no choice but to use a snap-on core, I wrap it tightly with electrical tape.

The only sure-fire way is to use a solid ferrite core. Properly deployed, they can work wonders.

Don
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WA2SQQ
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« Reply #7 on: December 21, 2018, 09:53:19 AM »

I've never had any luch with split cores. I use the regular torroids and wind the chokes. It's on the list for the next few days - thanks for all the tips.
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« Reply #8 on: December 21, 2018, 11:05:10 AM »

Not a good idea to use orange drops, or other random types of capacitors on line filters.

Y-class capacitors were developed for this very reason.  https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

Thank you!

Admins: How about this being condensed and made a safety sticky?
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