Test Rig Simple Resistance Measurement

Discussion in 'The Projects Forum' started by martinwinlow, Mar 9, 2016.

  1. martinwinlow

    Thread Starter New Member

    Nov 20, 2011
    6
    1
    Hi,

    I'm putting a test rig togeather to check cables which have a special plug on each end. I envisage pluging both ends into sockets built into the test rig and the rig shows the resistance on 2 of the cables multiple conductors. There are less than 6 valid resistances on each of the 2 cable conductors that are of interest.

    So, I figured I could have a simple DMM display for each of the 2 conductors to dispaly whatever the resistance is on each at the same time. I hoped I could use those cheap panel mount meters but they only come in 2 ohms or 200 ohms 'FSD'... I need zero to 2K or so.

    Alternatively (and maybe much easier to implement, cheaper to make and easier to use) I was thinking of a comparitor-based setup with an LED for each valid resistance range, one set of LEDs for each of the 2 conductors. Outside the valid range = no LED = duff cable...

    Anyone know of an existing setup I could use for either option... or indeed a better way?

    Thanks. MW
     
    andrew331 likes this.
  2. TheButtonThief

    Active Member

    Feb 26, 2011
    219
    38
    Have you considered using a micro controller?

    They're cheap and after some quick programming, will do everything you need it to. You'll also be able to re-programme if your criteria changes, for instance the acceptable resistance of the conductors.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,654
    632
    If this is for production test I would definitely go with your comparator-based approach.

    If you put an ohmmeter on the panel the technician will have the job of being a virtual comparator to make the Go/No-go decision. She would probably prefer the LEDs as long a they give positive indications: Positive that the cable is ok or positive that the cable is a dud. (I know from experience.)
     
  4. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,146
    204
    The LTC3092 http://www.linear.com/product/LT3092 will go to 200 mA. Add a differential amp and a comparator.

    You force a current and measure a voltage drop.

    The advantage of the LED approach is that you can wiggle the cables and get a meaningful result.

    I have a simple tri-state LED based tool for network/telephone cables. It tells me if its straight/cross or intermittent. It won't detect some cable to cable shorts.

    Cable mapping tools won't tell me if a cable is intermittent.
     
  5. martinwinlow

    Thread Starter New Member

    Nov 20, 2011
    6
    1
    Hi,

    Thanks for the helpful replies.

    I continued to research on the WWW after I started this thread and found the following:-

    1 - http://shop.eeinabox.com/DMM-Sparkfun-Multimeter-Kit-KIT-10338.htm but now discontinued by Sparkfun, sadly.
    2 - http://www.ebay.co.uk/itm/ICL7107-Digital-Ammeter-DIY-Kit-Electronic-Learning-Kit-/111679703881?hash=item1a00a10b49:g:WXEAAOSwrklVY2~d - an ammeter kit but which has the expensive bits for...
    3 - http://www.eleccircuit.com/digital-multimeter-circuit-using-icl7107/

    I shall post back with my progress in due course. Thanks again. MW
     
  6. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,146
    204
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