Ground currents blowing up my CM choke

It sounds like you have a ground problem in your lab. There shouldn't be

*that* much current flowing in grounds that are that close to each other. Other than fixing that problem, the obvious solution is to isolate the grounds/signals. Either an optical isolator or one of the ADI isolators would be good. RS-422 might be called for, too.

Allan Herriman schrieb:

Hello,

a MC1488 driver is current limited to 10 mA, 20 mA ground current should not be a problem. I suspect there is a ground current of more than some

100 mA caused by different ground potentials on both sides. Maybe the choke maximum current rating of 80 mA was not specified for each of the four lines, but for all four together. I would try to measure the ground current, but in a noisy environment with strong currents over the power lines the resulting ground current through the RS232 cable may be much higher than the measured value.

Bye

I'd get one of those 3-prong outlet testers at the hardware store and check all your outlets and power strips in the lab. I got bit once in my office while connecting test equipment that was plugged into a different outlet. I brought in my tester from home and found one wall outlet had an open ground. Some older test gear had really BIG capacitors in the line filter and would deliver a lot of current if the ground pin was open. Some of our power strips get open grounds from time to time and that outlet needs to be marked with red tape.

Jon

Most likely both. The PE connection on your box and that of the box at the other end of the line seems to have a slightly different potential. It shouldn't, but sometimes it does. Even if this is only a volt or two, when you connect that with a short piece of serial cable there can be hundreds of milliamps. I have seen several amps. Then the cleaning crew plugs their big hoover into one of the outlets, turns it on ... ZZZONK ... *PHUT*

Seriously, a client has seen an expensive laser controller come to grief that way.

One problem will be that any such current will likely saturate the core of your CM choke, thus making the choke inefficient as an "RF scrubber".

Ooooh, that raises a red flag. For example, in Germany I have some painful experience with legacy installations of what they call "klassische Nullung", loosely translated as "classic earth protection". In those cases there are only two wires going from outlet to outlet, phase and neutral. No PE. The PE-connection in each outlet is then bridged to the neutral wire. So now when there is a major load towards the end of the string of outlets you'll see a significant voltage drop. If the load is a power supply with large electolytics and no inrush protection you'd see a spike of several volts when someone turns that on. Vacuum cleaners are also lots of fun there.

And, of course, if the neutral comes off at some outlet early in the string .... bzzzzt .. ouch ... *POOF*

The real solution for a worldwide product would be to provide truly isolated RS232 at your box.

If you mean the ground that normally goes to pin 7, running that outside the choke will wreak havoc with your data lines. It puts an inductor right into the signal path, meaning a lowpass. But worst of all if TXD and RXD go through the same choke you will couple TXD into RXD. Static signals will look ok but the faster the data rate the more data errors you'll see.

--
Regards, Joerg

http://www.analogconsultants.com/

Sounds like you have a power mains grounding/earthing issue. If the equipment on both ends has a proper power mains ground, you won't have this problem. You'll probably find one side is properly earthed, and the other side isn't. If you go barefoot and lightly rub equipment (bare metal) with the back of your fingers, you'll quickly find the evil doers.

I have seen this in a lab where they removed all the earthing on the power plugs thinking this was a good thing to do. Unfortunately, their anti-static, slightly conductive mats were sitting at half the mains voltage causing them to blow out all sorts of stuff. I rewired the offensive bench I was working on, but next time I came back, they mangaged to make it unsafe again. They also had a heap of RS232 boards which were blown out due to earthing issues.

You are blessed with an opportunity solve potential end user problems right now. Assume your customer has this problem. Don't change anything in your test area. Fix your product to accommodate.

Cheers, John S

I meant to add: I would go with Joerg's suggestion and opto-isolate the link.

Don't worry about frying anyone in your lab. Power faults just aren't worth fixing.

If cost is not super-critical or rolling their own is not desired for some reason, here's the canned solution for the board level:

If it's a hindsight "oops, got to fix it fast" situation, maybe something like this:

Haven't tried either though, I've always rolled my own.

--
Regards, Joerg

http://www.analogconsultants.com/

But ALL the currents return through the ground pins, so those lines see higher currents which could push things over the edge.

Jon Elson mentions checking the Earth line. I would also check that L and N are not swapped (as they were in a light fitting in my house... and the switch only broke "L", ie Neutral); and I found a neighbour's house had E and N swapped in their garage. So much for qualified electricians.

Check the voltage (AC) between the chassises (what's the plural?) of the two items of equipment you are connecting. Even a few tenths of a volt will cause hundreds of mA to flow. A common solution is adding a small resistor (10 ohms) in the ground line, this cuts the current of a minor earth difference considerably.

I had a cheap (Chinese) lab power supply which was incredibly noisy. Eventually I realised that there was an interesting tingling sensation when I touched the terminals. It seemed to have one output, which I'd assumed was isolated, tied to Neutral and was floating at around 20V. I've since learnt that mains-to-DC switched power supplies come in 2 flavours, both seem to have a bit of deliberate leakage from earth to their output to stop it floating away (I think: I'm saying this in the hope one of the Great Old Ones on the newsgroup will correct my gross simplification and I will learn Stuff). Imagine if that goes to Neutral rather than Earth.... 8)

Nemo

I had some respect for you a few weeks ago, but that has gone to hell now. You just have to pop up with some curt comment to anyone who doesn't agree with your thought process, don't you? - Ahhh - Never mind. I won't be reading responses from you in the future.

ESAD

until maybe 20 years ago all installations around here was made with outlets without a ground only exception being in a kitchen maybe and the standard plug still doesn't have a ground prong since it wouldn't fit many outlets

many soundcard have been killed by computers floating at 110V from the Y capacitors getting grounded when plugging in a line signal from the tv etc. grounded via the antenna installation

-Lasse

I've seen those two-prong outlets in Denmark and wondered why they kept them for so long. Is it because you guys are decendents of the Vikings and thus everyone feels invincible? :-)

--
Regards, Joerg

http://www.analogconsultants.com/

maybe we are invincible :) at the moment I can't recall hearing of anyone getting seriously hurt

"hpfi" (high sensitivity ground fault interrupter) is mandatory even in old installations

most thing come with a schuko plug it fits perfect but doesn't connect ground no one wants a plug with ground because half the time it won't fit the outlet

so you can imagine lots of plugs with sawn off ground pin and extension cords with a hole for the ground pin, but only a two pin plug with no ground at the other end

-Lasse

I have used this one. It works well. I might reverse engineer it and use it in a design. And I have also rolled my own.

Cheers

In many cases, we do not directly connect the shield inside, we cap to it.

This kind of problem is very common when you are connected to unknowns of other equipment plugged into outlets down stream or completely different circuits.

It sounds to me that you have a bad electrical system and line loading down stream is causing your grounds to get induced currents.

Normally, we connect only one end of a shield to a true ground. This prevents induced currents with in the shield and currents being past through the shield between devices.

This also gives you an opportunity to put a ground fault detector in your device and report it to the user some how.

A lot of safety equipment have ground fault detection circuits in it so that it can get reported via automation of visual inspection other wise, it can hamper the operation of the device at some point.

Jamie

GFCIs are used to retrofit circuits with 2-pin outlets on this side of the pond, too. This brings an existing circuit "up to code".

Same deal here. It's not so common now because three-pin outlets have been standard for fifty years. Older houses still have these issues.

Sure. My inlaws had a ton of extension cords with the ground pins cut off. I threw them all away when I cleaned out the house.

Oh, I'm so sorry I didn't include a smiley but the simple fact is that you're advocating intentionally continuing to use an unsafe electrical installation. OSHA must love inspecting your lab.

You really are like Slowman.

That's like peeking into a map when driving somewhere. Real men don't do that :-)

There you go!

--
Regards, Joerg

http://www.analogconsultants.com/