Hi folks,

I have something of an elementary question.

I'd like to get started on some 6502 projects but I don't have grounded wiring in my house. That is, all the electrical outlets have 2 prongs. I can tell that all of the appliances are "live" because when I put my hand on the computer, fridge, space heater, etc. there is that low level electric feel that goes away when the appliance is unplugged.

My question is, will this be an impediment to using an oscilloscope, mains powered circuit board, and so on? I don't want to blow anything up by connecting devices at different potentials.

Thanks for the help.

patchberri

I see you're in Brazil. I expect that means your floors are concrete tile on concrete floor (which do not insulate like the linoleum, wood, and carpet in most American houses' floors), and that you probably have 230V at the wall outlets and non-polarized plugs, and your conduits (if there are any) may not even be metal, so you can't use them for a ground connection. I grew up in Argentina, and I remember some of the shocking problems that resulted under these situations. Even though there was no direct short between the power lines and the chasis in our electronic equipment (which mostly had tubes back then), the power transformers had plenty of capacitance between windings that would pass enough current to really wake you up if you touched the chasis while standing on a dry floor barefooted. No fun!

In 1989 I installed an FM stereo broadcast transmitter in southern Spain where the building had similar conditions. Even though I knew I'd be safe touching "live" things while wearing rubber shoes, I again felt a little of the same effect when I was wiring something up near the floor and I was sitting on the floor cross-legged, which increased the capacitance between my body and the concrete floor with concrete tiles on it. I don't remember what all the test equipment was that I might have had plugged in at the time, but there didn't seem to be any problem there. However, partly for safety's sake, I did have the owner get an electrician come in to install a real ground connection. We couldn't just put a 3-prong plug adapter at the wall outlets to use the metal box and conduit to find a ground, because the conduit was plastic!

I initially wrote a lot more here about other experiences but decided to omit it for brevity. So here's the "short" version. I've worked with low-voltage solid-state electronics whose reference (circuit ground) was as much as 12,500V above real ground. That in itself is ok, as long as there's no current passed where it shouldn't be. This applies even to handling ESD-sensitive devices. If everything is at the same potential, you won't have actual ESD, even if you and your work environment are charged to astronomical voltages.

The more subtle problem that you refer to with things like oscilloscopes occurs even here in the U.S. where the ground prong connection at the power outlet on the wall may actually have several volts AC on it. Sometimes I've had to use an adapter to isolate the ground connection because of all the noise it was putting on the readings. Other times, in moments of exasperation, I've used the pliers to actually break the third prong off of plugs.

In your case, I think I would just run a wire to all the grounds of the various pieces of equipment so they're all at the same potential regardless of things like probes and power supply sources getting connected and disconnected during your work. Then for your own safety if for no other reason, connect that common ground wire to a water pipe somewhere, even if you have to run it to another room! This should take care of all the problems you mention. Then if there is an actual short from one side of the AC mains to the circuit ground in one piece of equipment, and from the other side of the mains to the circuit ground of another piece of equipment, the house's fuses or circuit breakers should keep the condition from damaging anything.

Beyond that, just observe standard static-handling precautions as you would anywhere else you work with NMOS and CMOS parts. I have anti-static mats on the workbench which are grounded through the oscilloscope's ground connection on the front panel. And yes, in this office the oscilloscope's third prong is intact and gets its ground connection through the conduit.

Garth

It shouldn't be a problem; just connect the ground from your 6502 circuit to the ground of the oscilloscope (through the probe or through the front panel). In grounded houses, the neutral lines are connected to the same bus as the ground lines anyway, but it's done at the circuit box for protection; don't ground any of your circuits using the neutral from the outlet! For a power supply, you might be best with a commercial power adapter--perhaps an old one from an unused portable CD player or something. My 6502 project used a simple wall transformer with a 7405 voltage regulator for a power supply; toss in a diode and an electrolytic cap if you want, but for small projects, this should be sufficient. If you're using NMOS 6502, you might need at least 300mA, but with 65C02, you can probably get away with anything, even a 9V battery.

Incidentally, I almost got electrocuted from an old tube oscilloscope, ungrounded, with a short to its metal case some time ago.

Garth, you beat me to it; you must have submitted your reply just after I started mine, but before I previewed it! You make a good point about attaching to a water pipe for a safe ground.

Scott

Even if it did have a ground connection, you do not want to use it for a ground for RF transmission purposes. I'm assuming you're working in the VHF region for FM use (and even the top-end of HF, e.g., the 11 or 10m bands). At these frequencies, the ground lead, and any metallic conduits that exist, will form a nice antenna of their own, and will consequently feed back the emitted RF right back into the transmitter.

I have a similar problem where I live, and I *am* in the USA. I have a Kenwood TS-2000 DC-Daylight transceiver, and no matter if I'm working on HF or VHF, any power above 20W will make the chassis of the rig "hot". I've zinged myself a couple times before I realized what was happening. Any power above 35W will cause enough ground voltage variations to cause erratic front-panel behavior of the rig, and sometimes will crash my computer. Not fun at all. I'm pretty positive that if I'd exceeded 50W of RF power, I could have damaged the rig. Unfortunately, being on the second story of an apartment complex built over sandy (e.g., poorly conducting) soil, there's no way I can rectify this problem without moving.

Thus, when working with RF-producing equipment, it's always best to provide a local, single-point connection to ground.

The transmitter put out 500W actual power at 107.1MHz. The ground I mentioned was for protecting personel and equpipment from the 50Hz power which was mostly capacitively coupled through power transformers. Several years earlier I was one of three who installed a 50kW AM broadcast transmitter on 870kHz. It's rather off-topic for this forum, but I can give details of the RF aspects of these projects if anyone wants to E-mail me privately. We did not have any trouble with RFI on either of these though. In any case, patchberri, I'm sure you won't have any ground-related powerline-originated trouble with your 6502 projects using the methods mentioned earlier.

Garth
engr@drecomm.com

Reminds me of an old building I lived in back in the '70s. Some idiot had used some hefty fuses in both hot lines and common after they entered the building, when the building had been rewired from DC to AC back at the dawn of time. Something (lightning strike? a rat?) blew out the common line fuse.

Some fun! One casualty was a mains-powered LED digital clock I had built from 7400 series ICs. Melted right down, I'm guessing from transformer overheating (the rest of the P/S could handle the overvoltage). Nothing else I had was on when it happened, it was the middle of the night.

I have heard that one should avoid putting a fuse in the common/neutral line like the plague, for that same reason.
That's sad about the clock. Were the ICs hurt, or only the p/s? Good thing it was at night!

_________________
-Slimey

It was in a plastic case that got slagged pretty good. I didn't try salvaging it.