modified GFCI

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: modified GFCI

Post by JR. »

gfci_1.jpg
gfci_1.jpg (43.57 KiB) Viewed 17762 times
I've been working on this for a while and as often I had to mentally stretch my gray matter to get this to work. :D I need a little more field testing to find all mistakes but think I've caught most of them.

The basic premise is to start with a standard GFCI outlet that protects against leakage from it's hot that doesn't find it's way back to it's return. BUT this does not protect musicians from the classic hot mic problem. If the mic is hot because the console ground is hot a standard GFCI will not protect the muso.

The 3 pole relay not only removes power but also releases the ground. The relay coil is powered from the GFCI output so anytime the GFCI trips, ground is lifted too. An additional circuit senses for current flowing in the ground, and when more than a few mA is detected an opto-triac creates and artificial leakage fault in the GFCI to trip it.

i had some fun figuring out why the shunt opto-triac was tripping the GFCI at start-up. Apparently there is a 1000V/uSec voltage rise time limit, which start-up apparently exceeded, causing the opto-triac to conduct, and trip the GFCI. by adding a C from the shunt R to output side neutral I am able to slow down the edge rate of that voltage, while the RC current in completely in the output side so looks like valid current to the GFCI.

In addition to protecting against improper power to the console (mic). Another problem is RPBG outlets. When converting 2-circuit wiring to 3-circuit outlets some people "bootleg" the ground by shorting it to neutral. This is wrong for multiple reasons, but even worse if the line and neutral are reversed, so the bootleg ground instead of grabbing neutral, is now hot. As I have posted before people have been killed by these RPBG (reverse polarity bootleg ground) outlets. My solution to protect against RPBG, is to use a touch switch start up circuit that only works if line and neutral are wired correct polarity.

I accomplish this touch switch start-up using a high impedance MOSFET device with it's source connected to the (hot) line circuit. A small (.1uF) reservoir cap and diode from neutral charges up this floating DC power. Since the MOSFET is swinging at 120VAC, connecting the touch contact to anything near 0V will turn on the MOSFET drawing current through the two opto-triac LEDs, which in turn turns on two 16A triacs to apply power to the GFCI. (two TRIACs are needed to prevent issues from RPBG), Once the GFCI is powered up, the relay coil winding connected to the GFCI output turns on the relay and latches power on.

One downside to requiring a touch switch input to turn on could be momentary power outages causing power to stay off until reset again. This is not cool since power can blink in the middle of a performance and you don't want to have to hunt around to reset power. My design fix for this is to provide an alternate touch input that when the power relay is open, senses the output side neutral. As long as some equipment is plugged into the GFCI outlet, this will probably be enough to automatically reset and latch the relay back on. With my prototype an empty outlet stays off, but plugging as small hand drill into the outlet is enough of an environmental 0V reference to reset power when restored.

This is not very cheap... In small quantity the GFCI outlet was $15, the 3-pole relay another $15... the 2x 16A triacs and 3x opto-triacs were maybe $1 each... the small MOSFET was relatively inexpensive.

I will provide an accurate BOM for any who ask.

JR

PS: This touch switch technology is unlike anything that I have seen in my research. Most touch switches use either finger DCR to complete an electrical circuit between two external contacts, Another touch switch uses capacitance to shift the frequency or rise time of some AC signal. A third uses two sandwiched electrical overlays where finger pressure completes an electrical circuit. My approach basically assumes the free standing human body is sitting at a low modest AC voltage, especially relative to the mains 120V. The high impedance MOSFET will turn on from as little as 1V AC so a direct comparison between the human body and either line or neutral would turn on. I had to reduce the sensitivity so that it would ignore a direct comparison to neutral, like for a reversed outlet wiring situation. I pad down the 1M input R into a 240k R so the voltage difference is reduced to 1/5th actual. In a really noisy environment a noisy human might turn on a reverse wired outlet, but the ground sense currect circuit will still protect the talent.

PPS: For another data point when I probe my line voltage using a relatively high impedance VOM, using my body as the low reference, I measure around 50VAC on the 120VAC hot line, but I also measure 10VAC on the cold neutral line. I am not sure how exactly to interpret this. Probing other outlets around my house found neutral readings as low as <1V but I suspect I am really reading my body voltage.
Cancel the "cancel culture", do not support mob hatred.
emrr
Posts: 570
Joined: Sun Mar 02, 2008 10:27 pm
Location: NC, USA
Contact:

Re: modified GFCI

Post by emrr »

Nice. I'd like to see the BOM. Having at least one of these around seems wise, given the junk amplifiers clients sometimes show up with.
Best,

Doug Williams
Electromagnetic Radiation Recorders
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: modified GFCI

Post by JR. »

emrr wrote:Nice. I'd like to see the BOM. Having at least one of these around seems wise, given the junk amplifiers clients sometimes show up with.
I'll pull a BOM together but I do not advise building one of these for studio use.

You should be able to confirm that your studio outlets are properly wired.

Installing $15 GFCI outlets into the studio where you expect amps to be plugged in, should protect against killer amps.

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
mediatechnology
Posts: 5444
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: modified GFCI

Post by mediatechnology »

No studio or venue should be without these.
There should be an insurance discount for those that have them.
emrr
Posts: 570
Joined: Sun Mar 02, 2008 10:27 pm
Location: NC, USA
Contact:

Re: modified GFCI

Post by emrr »

JR. wrote:You should be able to confirm that your studio outlets are properly wired.

You should see my studio! They generally aren't, and it's not my building. Consider it your average nightclub.....it actually was once, along with being a house, an antiques place, a carpet store, and a bar and grill. Each change and addition done in the lowest rent sort of way.
Best,

Doug Williams
Electromagnetic Radiation Recorders
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: modified GFCI

Post by JR. »

emrr wrote:
JR. wrote:You should be able to confirm that your studio outlets are properly wired.

You should see my studio! They generally aren't, and it's not my building. Consider it your average nightclub.....it actually was once, along with being a house, an antiques place, a carpet store, and a bar and grill. Each change and addition done in the lowest rent sort of way.
You can find a portable GFCI power strip in a hardware store (maybe $30). Be sure to put your name all over it so it doesn't grow legs and leave with some musician.

If you add the 0,15uF mic in place of the ground wire, it may reduce ground loop noise from pedal PS in guitar rigs.

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: modified GFCI

Post by JR. »

mediatechnology wrote:No studio or venue should be without these.
There should be an insurance discount for those that have them.
Insurance may offer discounts on new builds but the new technology seems to be "arc flash" protection, as that wiring fault often leads to burning down the house, not just stinging the occupant.

JR
Cancel the "cancel culture", do not support mob hatred.
emrr
Posts: 570
Joined: Sun Mar 02, 2008 10:27 pm
Location: NC, USA
Contact:

Re: modified GFCI

Post by emrr »

JR. wrote: You can find a portable GFCI power strip in a hardware store (maybe $30).

I hadn't noticed those anywhere, I'll have a look.
Best,

Doug Williams
Electromagnetic Radiation Recorders
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: modified GFCI

Post by JR. »

emrr wrote:
JR. wrote: You can find a portable GFCI power strip in a hardware store (maybe $30).

I hadn't noticed those anywhere, I'll have a look.
http://www.homedepot.com/p/Defiant-2-ft ... /203741466
http://www.tripplite.com/product/Safety ... fgodySUALQ
http://www.walmart.com/ip/Tripp-Lite-TL ... d/11077963

I actually modified one of that first design to add a cap internally. It used a tamper proof security screw so I had to buy the correct screw bit to get it apart.

Just a standard GFCI will protect them from nasty guitar amps.

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: modified GFCI

Post by JR. »

While I have lost enthusiasm for the glorified "smart" outlet strip with relays etc, I still like the stinger cap ground replacement GFCI.

On a related theme, the classic ($5 cheap) outlet tester from the hardware store does not detect RPBG (Reverse polarity bootleg ground). It indicates that they are all OK. This is scary dangerous. Some building inspectors use and trust these. :oops:

As a mental exercise if anyone has too much time on their hands, try to think of a simple cheap way to detect mis-wired outlets. This is exactly the kind of problem I enjoy attacking. :roll: :roll: :roll:
out_test.jpg
out_test.jpg (43.28 KiB) Viewed 17563 times
My outside the box solution was to grab a local environmental zero or ground reference by buffering a human touch probe. Using a high impedance MOSFET my touch probe current is limited to microamps (via 1M R). This MOSFET source follower presents a low impedance environmental reference ground. Then I use simple color LEDs driving this pseudo ground, to detect which outlet contacts are hot and which are not. I also threw in a Ground lead present or not LED to detect an open circuit ground (like all the outlets in my house). :oops:

http://forums.prosoundweb.com/index.php ... msg1423617 Here is a post on an AC safety forum where they test my "Johnny outlet tester" against known outlet wiring faults or dangerous practices.

I indicate OK (Green LED) for bootleg ground and don't claim to detect for that, but i correctly indicate Neutral hot (yellow LED) and the even more dangerous Ground hot (red LED). There is some leakage in the red LED which is way more efficient than my few decades old yellow and green LEDs. These LEDs are parts I've had laying around since the '70s/80s. You can see the red LED light brightly when it is really indicating the RPBG

This is far cheaper and more practical than my fancy outlet for sound professionals to test their power in venues.

JR
Cancel the "cancel culture", do not support mob hatred.
Post Reply