1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Analog meters output more current to measure resistance at low currents? Vacuum Tube

Discussion in 'General Electronics Chat' started by Billy Mayo, Dec 6, 2013.

  1. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    well a DVM meter is high impedance VS a analog meter is low impedance , maybe a low impedance meter is better at measuring OPENs?
     
  2. ghostman11

    ghostman11 Well-Known Member

    Joined:
    Nov 22, 2009
    Messages:
    1,192
    Likes:
    99
    Location:
    Scotland
    Ok so for example your measuring a blown fuse (open circuit), which of the meters do think will give the most accurate reading?
     
  3. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    Analog , DVM might say OL or it might measure in the MEGS
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2

    My Fluke 87 meter Continuity checker outputs 3 VDC test current , it's the same as my Diode checker mode , it outputs 3 VDC test voltage also

    When using the DVM meter in Continuity it will forward bias a diode

    The Diode checker and Continuity modes are the same , they output 3VDC , the only difference is diode checker measures volts and continuity measurings in ohms , both will forward bias a diode or transistor even in continuity mode , So you have to reverse the Red and black probe in continuity mode when testing traces around diodes or transistors

    What do you guys use the continuity mode for?
     
  6. rumpfy

    rumpfy Active Member

    Joined:
    May 26, 2012
    Messages:
    384
    Likes:
    61
    Location:
    Melbourne, Australia
    Mr ghostman,
    In post #109 you said your little one was looking into a dud AVO?
    I have a model 8 and have most of the drawings, if its of any use.
    Two problems I had with it is:
    1. The 15 volt battery is hard to get so I use a 12 volt flash unit battery in series with a couple of hearing aid batterys. Pain in the neck BUT!!
    2. There is a 7361 ohm resistor as part of the current shunt which went o/c on me a couple of times.
    Apart from that it works beautifully. I think its a 1958 purchase. I got it from Philips when it got to its 10 year write-off point in 1968.
     
    • Like Like x 1
  7. picbits

    picbits Well-Known Member

    Joined:
    Jan 29, 2007
    Messages:
    2,007
    Likes:
    95
    Location:
    Northants, United Kingdom
    How can a DVM measure in the MEGS on an open circuit ?

    Why would a DVM say "OL" while an Analog meter may not move ? Why would this make either meter any different for measuring a fuse ?
     
  8. ghostman11

    ghostman11 Well-Known Member

    Joined:
    Nov 22, 2009
    Messages:
    1,192
    Likes:
    99
    Location:
    Scotland
    thanks, his are mainly model 7's (i think), and one or two are way older. I am not sure of the faults, but all work in some way, so they are not dead. Fist job is to try and sort the batteries out. I think most of his take 2 batteries, one of which may be 1.5V, the large 15V rings a bell with me, when i first looked into it for him, someone gave me an address to get replacement batteries, they were not cheap but i will look it out for you
    jason.
     
  9. ghostman11

    ghostman11 Well-Known Member

    Joined:
    Nov 22, 2009
    Messages:
    1,192
    Likes:
    99
    Location:
    Scotland
    You have made the point i was getting at perfectly ;)
     
  10. MrAl

    MrAl Well-Known Member Most Helpful Member

    Joined:
    Sep 7, 2008
    Messages:
    11,049
    Likes:
    961
    Location:
    NJ
    Hello,

    He could have been talking about digital meters that dont check diodes or LEDs very well unless they can output a high enough voltage at a high enough current to check either of those. This is why digital meters started appearing on the market that have a "diode" check function. It's a basic check that works better than the meter set to the Ohms range.

    Many meters still can not check LEDs very well. Some LEDs require over 3v to test properly. For these and other tests it is usually better to apply your own current with a power supply and resistor and check the voltage using the voltage scale on the meter, or something similar to that. The only caution here is that LEDs often have only a 5v reverse voltage rating, so if you go to test one with a 9v battery and 2k resistor for example and you connect the LED backwards, it can very well blow out before you get to test it properly. So using a power supply of 5v or less when possible is the best idea here.
    In the past i have used a three AA cell battery holder and three fresh AA batteries which develops about 4.5 volts when they are in series and that's just enough voltage to test many LEDs using a series resistor to limit current to a safe level. If you dont have a three AA cell holder you can use a four AA cell holder and jumper one cell position with a clip lead jumper, or use a two AA cell holder wired in series with a one AA cell holder.
    If you really want to use a 9v battery (these are handy for many tests) then you can use a resistive voltage divider with equal value resistors to knock the voltage down to about half, and the upper resistor will limit the current to the LED. The three AA cell tester is still a safer method however because there is no way the voltage can get higher than about 4.8 volts with three fresh alkaline cells.
     
    Last edited: Dec 18, 2013
  11. ghostman11

    ghostman11 Well-Known Member

    Joined:
    Nov 22, 2009
    Messages:
    1,192
    Likes:
    99
    Location:
    Scotland
    Except we were talking open circuit, and the example i gave was a blown fuse. Your a really nice guy and obviously see the best in everyone. The world could do with more like you, if i was younger i would probably like to be more like that, but i am old and tired. and more and more i become cynical!
     
    • Like Like x 1
  12. MrAl

    MrAl Well-Known Member Most Helpful Member

    Joined:
    Sep 7, 2008
    Messages:
    11,049
    Likes:
    961
    Location:
    NJ
    Hello there Ghostman11,

    Thanks very much, and thanks for pointing that open circuit issue out. I was just really throwing some more information out there just in case the original poster misinterpreted the person who told him that or there just happened to be something else somehow missing in the translation. I've read about humans telling humans other things verbally and by the time the 10th person gets the information it could be just the opposite of what it started out to be. If he's really talking about an open circuit then yes i agree it will have to be a very strange thing to say that it takes a lot of current to test an open circuit (hee hee, cant help but chuckle a little here). But in case it was just something that LOOKED like an open circuit without the proper current or voltage (an LED or diode will look like an open circuit without the proper test voltage and that was a big issue some years ago with the original DVM's), then maybe my reply will do some good. If not, then i guess it falls on dead ears, or else maybe someone else will be able to use that information in the future for something else.
     
  13. ghostman11

    ghostman11 Well-Known Member

    Joined:
    Nov 22, 2009
    Messages:
    1,192
    Likes:
    99
    Location:
    Scotland
    A great deal of what now gets posted, in this and some other threads, is mainly directed at someone,sometime searching and finding a lot of information. For some of the OP's that are helped lately, much of the high quality information given to them is of little use. The great thing about forums and google, is that many more benefit from alot of what is posted, and no one ever know's about it. Using LG as an example, he researches and reads up on thing's for his project, he looks at many online sources for this, and reads many post's in many places, and yet none of the original author's, are aware they have just helped a young lad learn something, so i think sometimes it is worth helping the odd :troll:, because someone will benefit.
     
  14. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    Yes at my work they will say it's an open circuit, path or component if the DVM meter is in the MEGS or OL

    DVM meter are high impedance, it reads MEGS or OL when it's an open circuit , path , component
    Analog meters are low impedance , the needle doesn't move at all, stays at zero , it never measures MEGS resistance when there is an open

    it's a big difference


    That's a good point true
     
  15. chemelec

    chemelec Well-Known Member

    Joined:
    Apr 4, 2004
    Messages:
    2,123
    Likes:
    77
    Location:
    Hedley, B.C. Canada
    "OL" is "OverLoad:.
    This Means it is a Higher Resistance than the DMM is capable of testing.

    On a VOM, the Needle Doesn't Move because this Overload (INFINITY RESISTANCE) is the LEFT SIDE of the Scale.
     
    • Like Like x 1
  16. ghostman11

    ghostman11 Well-Known Member

    Joined:
    Nov 22, 2009
    Messages:
    1,192
    Likes:
    99
    Location:
    Scotland
    We need to be careful, and try not to get over technical!Explaining the meaning of OL is probably getting us into far more technical aspects, than the OP could be expected to grasp, seeing as he only has a few years experience, also try and remember he isnt the one they let PLAY WITH use the Oscilloscope. So for the OP's sake and this discussion, lets just say that OL means you need another meter to take the measurement ;)
     
  17. 4pyros

    4pyros Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2010
    Messages:
    4,368
    Likes:
    282
    Location:
    Pennsylvania, USA
    I learned as a teenager how to test, troubleshoot, and repair PCBs in a design and manufacturing plant.
    I could find solid shorts on boards no one else could find with a pocket meter from RS.
    I also wired all the outlets not just on my bench but all the benches.
    I can not see how someone can have such a hard time testing for shorts and opens.
    It's mind boggling.

    Billy;
    Sorry to say if you really want to learn something from us than you need to stop listening to the guy sitting next to you.
    Your co workers seem to just want to confuse you.
    The 2nd thing you need to do is answer our questions, that way we can help you faster instead of getting frustrated with you because you keep going in circles.
     

Share This Page