原创 Is continuity really easy to check?

One of the most basic tests engineers can make when verifying or debugging a circuit is to check for DC continuity. After all, it's important, it's easy to do, and having poor or no continuity will ruin performance of many systems.

Fortunately, in most cases continuity is also easy to check. If it's a PC board, an installed short cable, or a long cable on the bench, you can usually check it out with a meter in a few seconds. To make it easier, many DMMs have an audible continuity mode which lets you know if the resistance is below a fixed amount, such as an Ohm, so you can "buzz out" the pathway without having to look at the meter reading. Instead, you can look at your probes and where you are placing them.

But there are times when something as simple as continuity is actually hard to assess, due to the physical layout. I'm not talking about a tight PCB, or a multi-layer one with internal ground and power planes, populated with vias that could have hairline cracks. I mean where the two ends of the single wire under test (I'll call it the WUT) are very far apart.

I thought about this a few weeks ago, when I caught a part of an episode of This Old House. The show mentioned a meter that lets you check continuity of a wire from one end alone, or so it seemed to me. Great, I thought, tell me more: Maybe there's some sort of transponder you put at the far end and it somehow it relays back to the source (your end) the continuity of the single wire.

The one-wire continuity problem was one I had faced in the past, when I was helping a friend fixing up a very old two-story house. We found a single red-insulated wire coming out of a defunct wall-switch box on one floor, and similarly situated wire on the other floor. Were they the ends of same wire? Did they have continuity now? Did they ever have it?

Since there were no other wires of known good continuity nearby, we had a challenge. "Continuity" means a path for electrons to flow, but they need a return path to the source. We tried using the grounding screw on the junction box at one end and a similar one at the other end for the return path connections, but the reading indicated "open circuit." We had little faith in this result, however, because ground connections in old buildings are often corroded or disconnected. So the determination problem was still there.

In the end, we did the simple, obvious thing: We took a long piece of basic #20 wire, ran it from one floor to the other through the windows and outside, and used it as the return path. That solved the problem, which let us run a valid quick test (and the WUT was OK).

I thought this Extech continuity-test unit would solve a long-standing problem, but I misunderstood its function.

Out of curiosity, I looked into the unit they cited on This Old House, the Extech CT20 Remote & Local Continuity Tester. It's certainly handy, but it doesn't let you check one-wire continuity. I must have misunderstood what they said it did. To use it, you connect your two wires at the far end (one of which must be known to be continuous), then buzz them out from the near end. It even has some diodes in the far-end jumper cable so you can even buzz out a bunch of wires in the cable from the source end, if you attach their multiple-clip jumper.

Next time, I'll just have to get out the TDR (time domain reflectometer) to check that one wire—perhaps that will do the trick. (Well, not really an available option at my job site.) As far as I can tell, there is no easy or low-cost way to verify the continuity of a single wire over a distance without a known good return-current path.

Are there any basic, low-cost test instruments you'd like to see, that would solve such "basic" and obvious problems?