Multimeter testing

Discussion in 'FAQs' started by Herc Driver, Nov 12, 2008.

  1. Herc Driver

    Herc Driver Active Member

    OK guys...I'm gonna swallow my pride and fess up to this fact. I was never taught how to use a multimeter. I'm trying to figure out if my soldered joints are carrying enough (or better) current than the non-solder joints, but even after reading the directions, I don't know where or how to actually test the wires. I've got a Craftsman Multimeter and actually read the directions - but it didn't really tell me where to test much less how. Do you simply turn on the power pack and touch each rail with both the black and red testors? Or do you only touch one rail with the red testor and ground the black? I'm clueless...any help would be greatly appreciated.

    On the plus side, thanks to the Gauge emag, I read up on and practiced soldering (another skill I didn't have) and after a few attempts, actually pulled up some of the layout track and soldered them together. Not a bad job I guess...nothing melted and I didn't get any on the inside of the rails. I used an electrical solder the LHS said would be perfect - since RC modelers use it for their connections on cars and planes.
  2. pgandw

    pgandw Active Member

    Unfortunately, measuring the voltage across the rails without a steady load won't tell you much - it will either tell you there is electrical continuity or there isn't. With a load applied, a high resistance joint will cause a voltage drop, and you will read a lower voltage beyond the poor joint. But you have to have about a 25 ohm load in place across the rails (rated at about 10 watts if using a resistor) to have enough current to cause a readable voltage drop.

    An alternative method for checking for voltage drops is to use a 10 watt automobile bulb with 2 wires. Simply check the bulb across the track in different places and compare brightness levels. No brightness change means no voltage drops.

    Instead of measuring voltage drop directly, you can measure the resistance of your joints directly by using the ohms scale on your multimeter. Use the most sensitive scale (1X ohms). You should see 0 ohms across every non-insulated track joint. On most multimeters, you have to calibrate the ohms scale by crossing the probes and adjusting the meter reading to zero. A few have automatic calibration. Just make sure there is no power on the track when you are measuring resistance. The resistance across the rails should be infinite (assumes no locomotives or lighted cars on the track).

    hope this helps
  3. nachoman

    nachoman Guest

    Part of your issue may be that the joint you wish to measure can't be isolated electrically from the rest of the layout. In other words, the current must have only one path through the joint, and there must be no other way for the circuit to be completed (through power feeders or a complete loop of track). You want to be sure you are measuring the resistance of the joint in question, and not the resistance elsewhere in the circuit. If you have the joint isolated, you can use the resistance setting on your multimeter to test.

    Other than that, Fred offers good advice.
  4. 60103

    60103 Pooh Bah

    Herc: one approach might be to measure the voltage coming out of the power pack and again at the rails. (same setting of power pack). If there's a drop of a volt or more (in a reasonably short distance) you may have a problem. You might need a small consistent load like a lamp.
    Another trick might be to measure resistance between the feeders -- just after the block selector switches (turned off). Set the meter to ohms and put a short across the rails (wire with 2 alligator clips). You should get a reading just above 0. If you have a substantial reading, you might try wiggling the rail attachments. (Try measuring a length of wire to get an appropriate reading).
    Then try different blocks.
  5. baldwinjl

    baldwinjl Member

    Driver-What you are trying to test really doesn't need to be tested, and you'll pretty much drive yourself crazy trying! If the solder joints look good, meaning shiny, and not bubbling up, they are fine. Unless you notice a particular problem, any further checking is honestly a waste of time, and might even be misleading.
  6. Herc Driver

    Herc Driver Active Member

    I appreciate the info...thank you.

    I've been practicing my new found solder technique and so far, I've been making the joints smooth with no solder getting to the inside of the rails. Since the connections seem firm, I'm assuming they're conducting electricity without problems. Running engines over them seemed to prove this...but I thought there'd be a way to positively know the voltage on the rails and see if I was getting better current flow on the soldered sections than the non-solder track sections. Once again, this hobby is teaching me new things...and I guess that's good.
  7. baldwinjl

    baldwinjl Member

    As Fred mentioned, measuring the resistance with no current draw doesn't really prove anything, so it would be hard to compare a soldered and unsoldered joint that way. And a nice fresh rail joiner probably conducts fine, at least at first. Over time, the rail joiner loosens, gets some oxidations, gets pulled apart a bit, you get the idea! That's when the soldered joint is clearly superior, in the problems you'll never see!
  8. Herc Driver

    Herc Driver Active Member

    Good point. I have rail joiners over the entire layout and think I'll solder those joints anyway. That way the rail joiners will serve to hold the rails in place as I accomplish the soldering process. Hopefully, that's a good idea.
  9. pgandw

    pgandw Active Member

    Soldering every joint is probably not a great idea unless your layout room has pretty steady humidity and temperature. I prefer to leave the joints on straight track unsoldered and a very tiny gap at the joint. This gives the benchwork a chance to move with changes in humidity without throwing the track out of kilter. Just run feeders every few track sections on the straights, or solder a jumper wire to both rails either side of the rail joiner.

    my thoughts, your choices
  10. Herc Driver

    Herc Driver Active Member

    Funny you mention that...I had planned on doing just that...adding a few more feeders every few feet or so. I planned on soldering the longer straight-aways of flex track to make sure the rails stayed in close contact. (I only used snap track on the curves to make sure the rails would hold a true radius.) So I thought I'd place a feeder at each end of the curved loops, and solder the straight-aways along the mainlines, sidings, and yard tracks. I'm covering almost 15 feet of straight flex track and although the layout stays in a bonus room that is climate controlled like the rest of the house, any mistake I made in cutting the exact length of joined track is magnified over that length. The gaps might be only 1/32 or 1/16 of an inch, but I've noticed that the rails do move around a bit on the flex track and just enough to sometimes momentarily break electrical contact thereby causing the engine to hesitate ever so slightly. But, I made a mistake of spray painting large sections of joined flex track, and unwittingly allowed the paint to get in and around the rail joiners which, when combined with the tinyest amount of rail gap, adds just enough resistance to decrease the conductivity. I need to replace all the painted rail joiners before soldering the rails. (Just another mistake I've learned a lesson from.) I figured...fresh rail joiners, a few more feeders, and soldered rails should give me the best possible conductivity for both DC and DCC operations.
  11. sgtcarl

    sgtcarl Member

    Sounds like you're on the right track. :thumb:
    (my apologies to Atlas):wave:

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