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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

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    I was having trouble measuring resistance on a low current board because I was using a Fluke 87 DVM.

    My Manager said I need to use a Analog Meter or a Vacuum Tube multiple meter to measure resistance because it outputs a high current than a DVM

    When do you use an analog meter because it outputs a higher current than a DVM?

    Does a Vacuum Tube output a higher current when measuring resistance better? but for what kind of circuits?
     
  2. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello,

    When you get down in to resistance values like 1 ohm, 0.5 ohm, and 0.1 ohms and lower like 0.010 ohms, it becomes a problem to measure with an ordinary meter because the voltage drop is so little with a normal test current like 1ma. Most regular meters can go down to about 0.001v but that's it. Going down further means very reduced accuracy in many cases.

    Consider a 0.1 ohm resistor with a standard 1ma test current. 0.001 times 0.1 is already down to 100 microvolts, and most standard DVM's can not measure that low very accurately.

    So the next step is to find other ways to do this kind of measurement. The first being a more expensive DVM. A more expensive DVM may measure down into the microvolts. That would mean you can use the same 1ma test current and get some usable results. But more expensive meters are, well, more expensive, so that option is usually left for last.

    Another cheaper method is to use a higher test current. Since we have been using 1ma, stepping that up to 10ma means we get a voltage reading that is 10 times higher. With the same cheap DVM we now measure 1000 microvolts with the same 0.1 ohm resistor. This gives us a reading of 1.0mv on many cheaper (but not super cheap) meters. That's two digits, which is often good enough.
    But why stop there. Increasing the current to 0.1 amps means we now get a reading of 10.0mv, which is far more accurate than before. Now we have three digits when before we only had two.
    But increasing the current has it's drawbacks, one being that the resistor under test has to be able to handle the extra current without breakdown. Often this may work, but sometimes it doesnt, and sometimes the resistance is even smaller than 0.1 ohms so we have another idea we can use.

    The next idea is to use a stable DC voltage amplifier to amplify the voltage we measure so that the meter reads a higher voltage. We simply scale the reading and that allows us some more flexibility.
    For the same 0.1 ohm resistor with a normal test current of 1ma, the voltage drop is again only 100uv, but with a 10x amplifier that gets boosted to 1000uv, and that gives us two digits on the cheap meter which would read 1.0mv.
    Similarly, using a 100x amplifier boosts the output to 10.0mv, which is much more accurate to read on the cheap meter.
    Just how we build this amplifier though is a little tricky. We need a good input offset spec down in the microvolts or tens of microvolts to get anything decent out of this. Here's where the chopper stabilized op amp comes in really handy, because these devices can have very low DC offsets and so we get decent readings. Without this we'd have to use an input offset null amplifier which means we'd have to adjust it. That works too but is more problematic.

    Another idea is to use a resistance bridge. These circuits use the ratio of higher value resistors to determine the value of a lower valued resistor. Low tolerance resistors should be employed here. and connections have to be super solid.

    So the choices include:
    1. Get a better meter
    2. Increase current when possible
    3. Use a low input offset op amp to create a 10x or 100x DC amplfier
    4. Use a special technique for measuring resistances usually called a "resistance bridge"
     
  3. Billy Mayo

    Billy Mayo Member

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    a DC amplifier? does this use op amps to amplify the DC voltage? converts DC millivolts or microvolts into Volts?

    So an AC meter on RX1 range can measure milliohms? or the RX10 range?

    DVM meter output a 1mA Test current
    AC meter outputs a .1 amp test current?
     
  4. dave

    Dave New Member

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  5. Billy Mayo

    Billy Mayo Member

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    A tech at my work using a AC meter when testing Switches, he said that the digital DVM meters don't show you the inbetween state of the OFF and ON , I'm not sure what he is looking for on the AC meter when testing Switches

    Do you guys know what he is talking about?
     
  6. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Yes an amplifier using an op amp to make the very small DC voltage larger. This means you use the voltage setting of the meter to measure Ohms, using Ohm's Law. A known current and measured voltage and you can calculate resistance from R=V/I. You dont use the Ohms (resistance) setting on the meter when you do it this way.

    It could be he is talking about some higher voltage AC switches such as the regular wall switch used to switch 120vac for use with bulbs and other stuff like that, turning them on or off.
    These switches sometimes measure very high DC resistance even with the toggle lever flipped to the "on" position. That's because of a small amount of oxidation. The resistance appears high because the DVM only puts out a couple of volts DC which isnt always enough to test one of these kinds of switches. Using a much higher AC voltage the test reveals a much lower 'on' state resistance which means the switch works ok.
    He could be talking about something else too though this is just one example, so you'll have to ask him.
     
  7. pilko

    pilko Member

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    @ MrAl,

    I'm just lurking, but I would like to thank you for your time and your
    great response to this thread.
     
  8. Billy Mayo

    Billy Mayo Member

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    Why does analog meters output more current for their low ohms range?

    Why don't digital meters output more current like an analog meter?

    Digital meters only output 1mA?

    What advantage does output 100mA do for testing? testing what?
     
  9. chemelec

    chemelec Well-Known Member

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  10. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    When we choose a DMM for a given application like measuring resistance we make sure we choose a meter tailored for the job. Take a look at this digital meter. Note the specifications listed. The lowest range is
    0 - 5.999 m ohms and at that low range (milli ohms) the current is 10 amps. Meters designed for measuring low resistances use high currents, they also use a 4 wire kelvin configuration.

    Ron
     
  11. Billy Mayo

    Billy Mayo Member

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    So a Digital DVM can't do this? why not?


    Measuring the resistance of a germanium diodes and transistors?

    So digital DVM can't do this? why can't it?

    Are you measuring the resistance of the silicon diode junctions?
     
  12. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    You mentioned a Fluke 87 way back in the first post. The Fluke 87 handheld general purpose DMM is perfectly capable of doing basic testing of small transistors and diodes (silicon and germanium). Have you looked at the owners manual which explains how to do it as well as the readings one can expect to see. Taken from the Fluke 87 User Manual:

    Ron
     
  13. Billy Mayo

    Billy Mayo Member

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    I know but I want to Focus on how an analog meter outputs a High Test Current

    Since it outputs a higher Test current, what are some things I can test with an analog meter that I can't test with a Fluke DVM meter 87?
     
  14. chemelec

    chemelec Well-Known Member

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    NOT MUCH of Anything.
    The Current is not very High.
    On the Analogue Meter I Have, Measuring on the Lowest Resistance Range, (0 to 10 Ohms) the Maximum Current is just 3.1 mA.
     
  15. Billy Mayo

    Billy Mayo Member

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    At my work they use an Analog Simpson 260 meter, the tech said that the output test current is VERY high compared to a Fluke DVM 87 meter

    Maybe different analog meters have different output test current?

    How do you find the output test current on all the ranges for an analog meter or digital meter? I have look it up in the manuals but I can't find to know the name of it? is it called the output test current?

    What is the Analog Simpson 260 output test current?
     
  16. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Simply by design. Take a look at this old Simpson 260 Analog VOM manual. Nice thing about these meters is the manuals include a good set of schematics. Take a look at page 17 where they show examples of the resistance mode. The meter movement is a 50 uA movement. RX1 is a 0 to 2000 Ohm range. The meter movement is linear so just do the math. The test current isn't really what I would call "High" but does exceed what most digital meters use.

    Ron
     
  17. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Different meters use different currents and different ranges use different currents on the same meter. Take a look at the link I posted for the Simpson 260 and keep in mind the Simpson 260 has several different versions as it evolved over the years.

    Ron
     
  18. Billy Mayo

    Billy Mayo Member

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    My manger said I should use the Simpson 260 when measuring resistance on circuits boards that have very low current because the DVM can't pick it up and measure it, only the simpson 260 can pick it up and measure it

    What does he mean by this? and why do i have use the simpson 260

    T
    This is good for measuring AC current and why? to look at the fluctuating?
     
  19. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    I am not sure why your manager said that. It would depend on the board and the actual circuit. Components on boards like resistors have other components in parallel with them. Earlier the thread mentioned semi-conductors and how they can be biased when taking measurments with an ohm meter. So without seeing the actual circuit(s) it is hard to say why your manager said what he said.

    Now as to this:
    With slow repetitive signals like pulses an analog meter has some merit. A digital meter will generally just show scrambled digits but an analog meter will show the bump, bump, bump of pulses. Not as good as a scope but it will give the user an idea of what is going on. That is about all I can think of.

    Ron
     
  20. Billy Mayo

    Billy Mayo Member

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    So an analog meter is better to measure resistance in circuit test because of parallel components?

    I was getting weird readings with my DVM fluke meter, My manager said use the Simpson 260 because the output test current is higher he said , I think because there is components in circuit that are parallel component , so when measuring resistance in circuit a DVM meter is to low of test current?
     
  21. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    The Simpson 260 can put out over 100ma with the leads shorted on the Rx1 scale. That's considerably higher than a DVM which puts out 1ma. Case closed.

    However, mid scale on the Rx1 range is 12 ohms. Not sure how well that pans out when trying to measure very low resistances, perhaps a close up of the meter face would tell us the answer but it would have to be a good picture not a fuzzy non focused picture, and it would have to show the Ohms scale clearly.
    We could then see if the meter can read below 1 ohm.
    My Simpson 160 can not read below 1 ohm because that's the smallest grad on the face, but the Simpson 260 should have it stretched out a bit more so we'll have to see a close up of the face.

    But we know from the schematic that it can definitely put out more current than the typical DVM.

    ABOUT 5 minutes later:
    Found a nice pic on the web. The 260 definitely has a much better resolution for Ohms. It has 10 grads between 0 and 2 ohms, which means each small grad is 0.2 Ohms. So this thing can tell the difference between 0.2 Ohms and 0.4 Ohms, and maybe 0.3 Ohms by interpolation, but it could get a little questionable as to the true value. If your looking for really good resolution this isnt it though, as a DVM fairs better. If you need more current, then measure the current using one DVM and the voltage using the other DVM and calculate the resistance.

    At those low values depending on the application, there could be a slight heating effect that causes the resistance to go up slightly, and that would be more in line with what the device might do during actual use. But i would favor a test current adjusted to suit if i was going to be that particular.
     
    Last edited: Dec 8, 2013

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