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Author Topic: brain storm an optimal human safety system for back line  (Read 95452 times)

John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #250 on: June 14, 2015, 05:41:06 pm »

ATTENTION... my beer powered analysis was absolutely correct, proving once again that the only sure-fire way to fail is to give up.  :o

While not ready for prime time I now have my one touch outlet checker working on my bench.  8) I have a little bit of leakage between the LEDs but it seems pretty clear what is being indicated.

To repeat I have 4 LEDS;

A green "ground connection present" that lights for both real grounds and bootleg grounds (even RPBG) so it doesn't tell you about the quality of the ground only if ground path is conductive or not.

A second green LED lights if you touch the touch contact and the outlet Line terminal is hot. (I tested this on several outlets around my house and this works).

A yellow LED lights if the Neutral is hot to suggest caution. Most product will work fine with line and neutral reversed.  The outlet powering my bench is neutral hot and this tested as expected.

A red LED (Danger Will Robinson) lights up if ground is hot, like if the outlet is wired RPBG.. Actually both the yellow and red LEDs light up for RPBG. I had to intentionally rig up a RPBG outlet adapter to prove this.

I get a little leakage in the off LEDs and will look at fine tuning the individual LED currents. My very old (from the '70s) LEDs are not as efficient as modern LEDs so the old red is probably 10x the efficiency of the old yellow and 2x the old green. That and the fact that my neutral doesn't seem to be 0V may contribute to some ghosting. Testing outlets with my VOM on AC Volts scale with my body serving as the ground, I measure 45VAC on the line and about 10VAC on neutral. At least at my bench, some other outlets around the house have less VAC on neutral measured that way, but the difference is probably my body's reference voltage which will be different depending on where I am standing/sitting.

Long story short it works... The good news is this version uses less parts too.. only one MOSFET with all 3 LEDs connected to the one FET. When I am not touching the probe, the source of the MOSFET is high impedance and swings all over. When I touch the probe which connects the high impedance MOSFET gate to my body's reference voltage, the MOSFET source becomes a low impedance and sources current to the LEDs, the LED with AC voltage on the other end of it lights up.

I'll scratch out a schematic for those following along at home but the values will not be completely final yet.

This could be a very tiny SMD PCB that could probably fit inside a conventional outlet tester.... or almost anything.

 ;D ;D ;D ;D ;D

JR
 

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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #251 on: June 14, 2015, 10:40:41 pm »

Here is a schematic but like i said subject to change.

Looks simple after it's done.  8)

JR
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #252 on: June 15, 2015, 11:32:47 am »

To explain how this works for any still following along, the MOSFET is configured as a high impedance source follower to buffer the human reference voltage, so the touch probe sees very low current (maybe 100uA max).

The three outlet terminals that we want to test all connect to the source of T4 through 10k current limit resistors and rectifier steering diodes, so they only conduct during the negative half cycle of mains voltage. Generally only one of those three terminals will have AC voltage on it, except for RPBG where two will be hot. The drain of the MOSFET connects through diodes to both Line and Neutral so it will always be able to pull power from the one that isn't hot (and negative when the LEDs are lighting). When one of the three LEDs is on and drawing current the MOSFET drain will be pulling current from the actual neutral through one of those two diodes.

When not touching the touch contact, the gate will be swinging 120VAC and the MOSFET will be off. When touched it will be roughly 0V (give or take). That MOSFET will turn on enough to light the LEDs with a few 1/10s of a volt so source will be < 1V below the DC voltage of the human capacitor. Since the MOSFET source is a low impedance the hot outlet terminal LED will light up to indicate wiring status.

One problem I experienced before was that the MOSFET gate drive would suck current from the human capacitor so the LEDs would only flash momentarily then go out. The 100k pull up resistor in parallel with the MOSFET recharges the human capacitor. I may experiment with making that 100k larger, maybe 1M. With 100k some very efficient LEDs might start to glow when not touching the probe, while that might be OK to indicate power present and LEDs working.

The zener diode and 1M connected from gate to source of the MOSFET protects the MOSFET from too high gate voltage, and turns it off quickly when touch is removed. The MOSFET can tolerate +/- 20V gate to source drive so I could just use a smaller resistor at R8 to divide down the max gate voltage to <20V but then the source voltage would sag too much when turned on and LEDs wouldn't get as much drive voltage.

The ground present LED is a simple LED and current limit resistor from ground to either Line or Neutral via diodes. This is crude and will indicate ground present for bootleg and even RPBG grounds. A true ground quality test is far more complicated. 

This would be a very tiny SMD PCB... it could probably fit inside a standard plug, or maybe build it into a KISS stinger GFCI outlet strip. Perhaps might fit in an outlet cover plate.

JR

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Tim McCulloch

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Re: brain storm an optimal human safety system for back line
« Reply #253 on: June 15, 2015, 02:33:07 pm »

John, you've really done some fine work on this.  Now go build something to keep you in hops.
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #254 on: June 16, 2015, 09:59:52 am »

John, you've really done some fine work on this.  Now go build something to keep you in hops.

Thanks.. I may build a small batch of these but can't see tooling up a fancy package. The parts cost even in small quantity don't amount to much, while the expectation of a fair price for an outlet checker is likewise low.

Even if this is the only one that actually detects the dangerous wiring scenarios.  8)

JR 
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Mike Sokol

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Re: brain storm an optimal human safety system for back line
« Reply #255 on: June 17, 2015, 08:30:02 am »

Thanks.. I may build a small batch of these but can't see tooling up a fancy package. The parts cost even in small quantity don't amount to much, while the expectation of a fair price for an outlet checker is likewise low.

Even if this is the only one that actually detects the dangerous wiring scenarios.  8)

JR

JR - The input buffer is brilliant.... When can I get one to play with?

Mike
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #256 on: June 17, 2015, 12:43:54 pm »

JR - The input buffer is brilliant.... When can I get one to play with?

Mike

I can send you my one prototype, I'm kind of finished with it for now. It is a mess and I will need to check the solder connections, since this same prototype has been torn down and rebuilt at least three times. It is kind of dangerous with lots of exposed energized components, but I wrapped some clear tape around the circuitry. I literally don't have enough 400V diodes laying around to build another, I used up all I bought the last time between this and my smart GFCI outlet.

I am thinking of laying out a PCB for this using all SMD parts, but before I do that I need to figure out how I want to package this (I can probably get 20-30 boards from a single prototype panel).

I bought a cheap 3 lamp outlet tester, and sure enough it says that my RPBG test outlet measures as good.  >:( >:( Why does UL let them sell those? (They are UL listed)  That's F'n dangerous. My way is slightly more expensive, but mine actually works.  I've seen the 3 lamp testers as cheap a $2 something so nothing much inside, my guts are probably <$2 in decent qty, which means a $10+ tester.  While the only one that works correctly should command a price premium.

I plan to take apart the cheap 3 lamp tester to see if i can easily fit a PCB inside, but first I need to get it apart without destroying it.

Let me know your snail mail address (you have my email). I may send you my smart outlet prototype also, which works,  while I really don't plan to ever make more of those.

JR
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Mike Sokol

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Re: brain storm an optimal human safety system for back line
« Reply #257 on: June 17, 2015, 03:33:41 pm »

For those of you (still) following along, here's the schematic of a simple 3-light outlet tester you can buy anywhere for less than $5 or so. As you can see, it's just a handful of resistors and neon lamps. Probably millions of these have been produced over the last 50 years or more. But as JR alluded to, and I've written about extensively, they can't identify several very dangerous miswiring conditions, especially an RPBG (Reverse Polarity Bootleg Ground).

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Bob Leonard

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Re: brain storm an optimal human safety system for back line
« Reply #258 on: June 18, 2015, 05:12:06 pm »

Hey guys,
I've been following along and props for all the hard work that's been done. Makes me wish I had more leisure time. How about a couple of pictures showing construction and component placement. Specifically JR, are the type and values relevant for the diodes in your schematic. Thanks.
 
I've been using one of these lately. Simply brilliant for the price.
 
 http://www.sears.com/craftsman-circuit-breaker-finder/p-03482021000P?prdNo=9&blockNo=9&blockType=G9
 
 
« Last Edit: June 18, 2015, 05:16:22 pm by Bob Leonard »
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John Roberts {JR}

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Re: brain storm an optimal human safety system for back line
« Reply #259 on: June 18, 2015, 06:06:04 pm »

Hey guys,
I've been following along and props for all the hard work that's been done. Makes me wish I had more leisure time. How about a couple of pictures showing construction and component placement. Specifically JR, are the type and values relevant for the diodes in your schematic. Thanks.
 

Thanx Bob-

The rectifiers in my prototype are typical 1n4004 400V 1A. They don't need to be 1A but 400V reverse breakdown is useful.

The proto is kind of ugly, built up on a generic .100" centers standard pads board. The spacing was a little tight for the MOSFET which is actually .050" centers for pin spacing but they put the drain on the tab too and made it the center pin so i could bend up the center pin and solder the gate and source on 0.100" pads and grab the drain from the tab on the other side of the part. These modern SMD parts are crazy small and hard to breadboard with. 

I already sent my only working proto to Mike to run through it's paces on his torture bench. I hope it still works after the USPS gets finished with it. I built up and tore down 3 different versions, before hitting on that final design. And that one took two tries to get it working adequately, so lots of questionable tack solder connections, by an old man with bad eyesight.  ;D ;D

I am thinking about laying out a SMD board for this that could be really tiny... Absolutely none of the parts need to be very large, or dissipate much power. I suspect the form factor will be dominated by what I decide to build this into.

This could be built into any product with a 3 wire line cord. I could build it into an empty plug housing.

Any suggestions?

I am not enthusiastic about tooling up a slick package, but could see selling assembled working PCBs in small quantity that others can built into whatever.  A mounting screw head could double as the touch contact. Color code is pretty simple green=good, red=bad.

JR 
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