Tuesday, September 18, 2012

Project Clean Up Part 1 Chapter 2

Chapter 2 "Why the White Wire is Hot"

This is a continuation, the First part is Here,  a complete list is available under "Ham Radio Master links" on the sidebar.

I didn't want to button my power supply up before figuring out why the hot wire was white. Basically, I wanted to make sure this radio could work anywhere, and having a hot white wire was troubling. Like David suggested in his comment, I suspected the outlet, and then the wiring in the Mains box. Time to flip breakers and do some inspecting!
First, I flipped the breaker to the shack, and trundled back into it, hoping to find a black wire where a white wire should be. The Shack in the Back was built before we moved here, and is 'unfinished'. The guy who owned the place two owners ago built it with scraps left over from some of his contracting jobs I think. When I unscrewed the socket from the recepticle, everything looked, well, fine. I was impressed with the craftsmanship of the socket to tell you the truth, connections were tight, and in the right place. Time to check the mains box, but I knew it was going to look fine there too. It did.
Back to the thinking bucket.
Yon Thinking Bucket

I juiced the shack back up, grabbed my VOM, and began doing things my mother told me not to do, namely sticking metal things in electrical outlets. By metal things, I mean VOM probes. I always feel weird sticking things into electrical outlets, makes me make sure I do so with fear, and caution. That's not a bad thing, I suppose.
First, I checked the socket for power, Hot wire to neutral, and I got 120v AC. Good!
Then I checked for what should be the neutral wire to ground, and got no AC voltage, Good!
for the kicker, I checked what should be the Hot wire to ground, and got 120v AC, Good!
So the socket is good! Should have done that first instead of assuming it wasn't... hmmm.

Next test, the power supply strip I plug into the socket. Maybe there's something miswired there,
Just checked Hotside to ground, 120v AC. Good!
So the power supply is not the issue.
must be the power cord.

The Cord- Not Polarized

notice that in the picture of the plug end, the plug is not polarized. This plug and wire was salvaged from some piece of computer equipment, and at one point in time had a female chassis mount plug on it, the kind that looks like this:

Female Chassis Mount

On a polarized plug, the narrow slot/prong is hot, the wide slot/prong is neutral and the round prong is ground, my non polarized plug fits in the socket the way a polarized one would, but, was it wired like a polarized socket? Just where did the white wire go?
I made sure the plug was unplugged, clipped the cord out of the power supply, and I decided to idiot check the green wire first, green for ground right? Switching my VOM to measure resistance, I continuity checked the green wire to the round prong, and it was Good! Then I checked between the other two wires, and no other wire lead to the round prong, Green is good!

I then oriented the plug like I was about to plug it in.

Right is hot, left is Neutral...

As this picture shows, the prong on the right, goes to the hot side of the socket. That's the one I want to check.
First I check the black wire, no continuity.
Then I check the green wire, no continuity.
Finally, I confirm that the right prong, the prong that normally connects to the hot side of the plug is connected to the white wire.
The white wire is hot because it goes to the hot side of the socket.
I have no idea why this cord is made this way.
I just know that I can safely button up the power supply knowing It's the Cord, not my shack!

Next Up: Chapter 3: "Totally Tubluar Transmission and Technical Tidbits"


David, ex-W8EZE said...

So paint the white wire black, and the black wire white ;-) .

G. Brandon Hoyt said...

Heh, don't think that didn't cross my mind! ;-)

Steve Smith WB6TNL said...

I never realized that the Twin-Tube 80 used a transformer-less, AKA "line operated" power supply for the B+. Not a problem provided you take precautions. -Never- assume that the A.C. line polarity is correct. -Always- check between the transmitter chassis and earth ground to ensure the chassis is not 'hot'. Best of luck with the project and 73.......Steve Smith WB6TNL "Snort Rosin"

G. Brandon Hoyt said...

To be fair, Mike originally stated that if you weren't going to use a three pronged plug, you should use an isolation transformer. Another way to solve this particular problem is to use a full wave bridge rectifier. Then polarity becomes moot as both cycles transmit power. Thanks for the safety reminder. I will be getting another post of this soon, showing the transmitter in action. Then it will be on to an accompanying receiver, I'm thinking something tubed and regen although the Desert Ratt is tempting. Am taking suggestions.

David, ex-W8EZE said...

You wrote: "Another way to solve this particular problem is to use a full wave bridge rectifier. Then polarity becomes moot as both cycles transmit power."

Draw the circuit diagram, and consider the waveform on the hot side of the AC line (relative to ground). On negative half cycles it will be connected (through a conducting diode) to the chassis. So you'll only electrocute yourself on the negative half cycles. On the positive half cycles the chassis will be a diode's voltage drop above ground potential so you'll live through those half cycles.

Don't do it!!!

G. Brandon Hoyt said...

Wait, what? This is the circuit I'm talking about: http://en.wikipedia.org/wiki/Diode_bridge
I see this circuit as keeping the positive side positive, and the negative side negative no matter which side of the AC side is 'hot'.
although this brings up something I've always wondered about, if the return line and the ground are connected at some point in time (Like at the supply box), shouldn't there always be a potential difference between ground and hot line?

G. Brandon Hoyt said...

Dave wanted me to post this, it's good to take into consideration when building power supplies:
"Oh, it will work all right. But read my comment again and look at the Wikipedia article you referenced, specifically the second of the diagrams, where the upper AC wire is negative and the lower positive (happens for 1/120 of a second, 60 times a second, in North America). If your line voltage is 120 V rms, during that half cycle the blue wire will swing between 0 V and about -169.7 V relative to ground. And during that half cycle the blue diode will be conducting and will make the negative output terminal (which is connected to your rig's chassis) the same voltage as the blue wire (except for a fraction of a volt drop inside the conducting diode). That will kill you, good and dead, if you're grounded but touching the chassis during that half cycle. The fact that it's negative, not postive, doesn't help at all. (Well, it might, if you had a diode inside your body between your hand and your brain or your heart, connected with the proper polarity so as to conduct only when your hand was positive!) Remember those half cycles occur 60 times per second and last for 1/120 of a second, each.

To be safe your chassis MUST be grounded to a good earth ground. And it can't be both grounded and connected to the negative output terminal of the circuit in the Wikipedia article, or your house fuses or circuit breakers are going to blow. But suppose you do ground the chassis, use the full wave bridge rectifier circuit, and then plug the thing in. If the first half cycle after you plug it in happens to have the upper AC wire go positive, then the fuse or circuit breaker won't blow until the next half cycle comes along, 1/120 of a cycle later.

All this assumes that your AC wire is connected in such a way that the upper wire in the diagram is the hot side and the lower one is the cold side. But if it's the other way around then read my argument above again, but this time use the first diagram. Your chassis will still go negative varying between 0 V and -169.7 V relative to ground (less the drop in the diode of a fraction of a volt), but it will do so during those half cycles where the lower AC wire goes negative, not the upper one.

So this circuit is guaranteed dangerous, whichever way you plug it in. The original way is NOT dangerous, provided your chassis is really grounded. But the original circuit simply won't work 50% of the time as you found out (when the plug is in the wrong way around). The full wave circuit will work 100% of the time whichever way the plug is plugged in, and will also be dangerous 100% of the time (ignoring the opposite half cycles from the ones I'm worried about, so I guess in that sense it's only dangerous for 50% of the time, 1/120 of a second at a time and as I said it won't kill you for that first 1/120 of a second).
From Dave, ex-W8EZE