2
\$\begingroup\$

I'm trying to sleuth out a wrongly wired connection on a small injection molding machine. I'm starting by verifying the connections match the diagrams from the manufacturer. However, some of the connections go from the front of the machine to the back of it, and it's physically impossible to do a continuity check with my hands holding the probes.

What's a good way I can do this? I was thinking I might have two options:

  1. Just grab a long piece of wire, stick it in the spot I'm testing, route it over to the other side of the machine, and then use the multimeter as normal.

  2. With the machine off, I could hook up a DC power supply to the spot I'm interested in. I was thinking a low voltage (eg, 1-3 V) and limiting the current to a nominal value (eg: 15 mA). Then, I could check to see if voltage is present or not.

Are there any major concerns with these approaches? Is there a better alternative?

\$\endgroup\$
3
  • \$\begingroup\$ Also change the polarity for possible diode elements and wait for possible charged capacitors. \$\endgroup\$
    – Circuit fantasist
    Commented Sep 24, 2023 at 19:58
  • \$\begingroup\$ Max, I might consider just building/buying something like this as a signal generator (without the mic circuit.) The antenna wire/node is all that's needed here. This gets hooked up on one end. The tester you use at the other end is just a series pair of 1N4148 diodes with an LED across them. The test point to use is the center node of the series 1N4148 pair. Leave one end of the LED as a 1 or 2 inch antenna, cut short the other end of the LED. The LED will light if there is continuity. Cheap. \$\endgroup\$
    – periblepsis
    Commented Sep 25, 2023 at 2:04
  • \$\begingroup\$ Max, if you look at this site here you will find still simpler examples such as this one for the FM signal injector. The ARRL Handbook for Radio Communications also provides still better examples and more of them. The chapter on transmitters starts immediately with some crystal controlled examples. In short, if providing a galvanic connection is a hassle (and it sounds as though it may be) then use a bit of low-power RF. \$\endgroup\$
    – periblepsis
    Commented Sep 25, 2023 at 2:15

1 Answer 1

Reset to default
1
\$\begingroup\$

I wouldn't recommend using a voltage, besides the possibility of damaging circuitry it's not necessarily going to be a reliable indication of continuity. A high resistance in a circuit will still give you a voltage reading if there's no load.

schematic

simulate this circuit – Schematic created using CircuitLab

If you have more than one connection you're trying to check or a common ground you can use as a return path you can put a known resistance across at one end and measure the resistance at the other end.

schematic

simulate this circuit

If you read the test resistance you have continuity. A higher or open reading indicates a fault, a lower reading indicates some resistance in parallel or a short. I've used this to do a quick check on antenna cables in vehicles, put a 50 ohm dummy load on instead of the antenna and read the resistance at the other end of the cable. 50 ohms is good, anything much above or below that is bad.

As for connecting a long wire to the point you want to test, that should work as long as it's not so long that the wire resistance becomes significant, and it doesn't sound like that should be a problem.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.