microcontroller picking up noise?

[Kevin Sevcik]

Phil,

Most consumer "UPS"s are, in fact, "SPS"s. Standby Power Supplies. So while there's still adequate power available from the mains, the SPS only filters it slightly before passing it along. A true UPS is always running its inverter from the DC supply that's charging the battery. FYI, anything that's going to really filter your power is going to run you around $100. That said, you're looking for a power conditioner or line conditioner. Be suspicious of anything that looks like a standard strip-plug.

[RyanN]

With my experience with micro controllers, a steady power supply is definitely required. Even with my Homework Board that I used to run my "Weather Alert" system with, simply flipping a switch on and off would cause the micro controller to freeze and reset. After I added a large capacitor to the input power insulated with diodes and a voltage regulator, the problem seemed to be fixed. I think a 5V voltage regulator, some diodes, and a fairly large capacitor may help you. I'm not familiar with PIC controllers though, so this may not work, but hopefully it does. Also, have you considered that something may be wrong with your oscilloscope? Maybe like it is "shocking" your setup causing your problems. I would listen to Al though, he seems to be a veteran at everything electrical. Good luck.

[Al Skierkiewicz]

Phil,
Since you had power line related noise when on battery, using a power conditioner is not going to improve anything. The distortion on the power line is an indication of a transformer failure. It can be caused by a variety of things including too much of a load for the transformer size. It can also be caused by the transformer arcing internally. The arcing coincidently can cause generated noise. Remember spark gap transmitters? Everything points to something locally generated. Where is the power line transformer in relation to your house? Have you tried taking your experimenter board to a friend's house a couple of blocks away? The display on the scope indicates a rather strong field.

Just to be sure, many appliances still maintain a power supply even when turned off. I came across a rather nasty TV power supply when I was trying to setup a ham station in a hotel room a few years ago. The only way to cure some things is to unplug them from the wall.

[sciguy125]

Originally Posted by Al Skierkiewicz

Phil,
Since you had power line related noise when on battery, using a power conditioner is not going to improve anything.

If it's running only on the battery, it's fine. However, when I connect the scope (even only the ground), it starts acting up. I just tried a different board (smaller and without a metal back plate) and found that the problem is much worse. I can't even get the thing to turn on unless I start poking at things with my finger. And, yes, I'm sure it's the noise, not a bad connection.

Anyway, I'm two houses down from the transformer (probably ~100'). If I make one more run and eliminate everything inside the house, what should I do? Should I talk to the power company?

[Al Skierkiewicz]

Phil,
This is a strange one, that is for sure. It is possible you have a bad ground connection in the house. At the entry point, (usually the power meter or the electrical breaker panel) the neutral must be tied to a good earth ground. Usually this is a green wire tied to water pipe. (It depends on local electrical codes.) Sometimes this wire goes inside the panel and sometimes it just connects to one of the conduits near the panel. Check that all connections outside the box are tight and not corroded. DO not enter the box. This is the realm of a professional. If you still can't find a cure, it might be time to call the power company. You said in an earlier post that line operated AM radios had hum (off station) but battery operated radios inside the house did not. If this is the case, it just adds one more piece that doesn't fit.
I still would like to see what the effects are at a friends house or at school. It sure seems like there is a lot of noise riding on power wiring or ground connections at your house.

[Qbranch]

Uh... can you try making up your own power supply on board? Maybe as simple as just a low-value resistor and a capacitor to filter out high/low frequency noise... maybe wrap your incomming power wire around a ferrite to cut ultra-high-frequency harmonics.

Strange problem... but i sympathize. I had a motion control board I built this summer that worked ONLY when the scope was connected. So... I feel you.

-q

[sciguy125]

I emailed the power company, but I haven't gotten a response yet (not that I'm really expecting one). I looked at their power quality info and, from what I can tell, this kind of noise isn't their responsibility. If I were feeling really adventurous, and I could prove that the power lines are radiating, I could probably get the FCC involved...

Anyway, I wanted to run an experiment, but I wanted to make sure that I won't hurt myself. My scope, of course, has a ground screw on the back. If I find a suitable ground for it (maybe a water pipe or the one that the cable guy seems to have installed on the breaker panel), I want to try grounding it directly. Is this a worthy/safe experiment?

[Al Skierkiewicz]

Phil,
If you cannot find a connection to ground for your breaker panel then you might try a different experiment. If you scope has a three wire power cord, plug it in near where the water enter the house. Then touch the scope probe center pin to the pipe and see what kind of noise you get. If you get no noise, then the electrical system is likely grounded to the water pipe. If the noise is a higher level then at your bench, there may be something wrong with your house. If that is the case and you cannot find a tie in to the water pipe, the cable attachment may have made things worse. I would suspect you have a ground connection but it may be loose or corroded. If you do find the connection, you may want to use an alligator clip to shunt it directly to the pipe and see if things improve.
The power company first response is to say they are not responsible for fixing noise. If it originates in their equipment, it is most definetely their responsibility. I have never had to call the FCC on these matters when I have told them I am looking at their line with a scope. The only way for you to actually look at the line with a scope is through an isolation transformer. Some scopes allow you to display the trigger signal which would give you the same thing. If your scope has a "trigger view" or "view trigger" or "display trigger" function, then select that and then select "line" trigger to see what is coming in the power line.

[sciguy125]

I figured it out! (mostly...)

It was indeed a ground problem. None of the outlets in the house are grounded. I know that because I personally replaced all of the old 2 prong outlets with GFCI about 2 months ago.

I discovered, using my meter, that the outlets in the garage are properly grounded. (Acutally, I guess it only proves that ground and neutral are connected at some point, not that the system is properly grounded...) My mistake was thinking that the distortion I was seeing in the line was the cause of the problem. So, when I was trying to find what was causing it, I only looked at my scope, not the problemed circuit. After using an outlet in the garage to test my circuit (with a real power supply rather than the transformer...I can't find it right now) it works fine. Not surprisingly, the distortion in the line is still there. So, that distortion isn't the problem, the ground is.

So, I've figured out that to work, all of my equipment needs earth ground. Using a 2 to 3 prong adapter (isolating ground) makes it mess up again. Why is this? I would have thought the opposite would be true.

[Al Skierkiewicz]

Phil,
You have to look at what is causing the noise to be displayed on the scope. If the scope does not have a ground reference either through the power cord or the ground clip on the probe (or both) then everything that the scope is connected to is now 'raised' above ground. This means that everthing between the scope and a real ground is acting like an antenna and inducing noise in that resistance. Old Ohm's Law comes into play, an induced current through a finite resistance develops a voltage across the resistance. You still have a noise source in your neighborhood and I still suspect it is power related due to a transformer or insulator defect. You have just found out why everything is picking up the noise and one method to minimize it. I think you will continue to be plagued by this and will eventually find it. I am reminded of a broadcast problem where a radio station had lots of noise and it was being picked up by everything in the studio. A search with a scope showed excessive noise on the ground buss of the audio console. A bright engineer noticed that the noise was audio modulation. When they detected the noise they found it was the competitors transmit signal. A quick search showed that the competitor was licensed at an address outside of town so no one thought it would be a problem. However, the studios were located outside of town, but the transmitter and antenna were on the building across the street. Normally radio studios keep the drapes closed but when someone opened them for some light, there was this antenna directly across from the studio window. Keep searching and you will find it.
Good news that annother problem has been solved.