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Testing Triac. Terms

Discussion in 'Electronic Basics' started by Chretien, Apr 6, 2005.

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  1. Chretien

    Chretien Guest

    Im a little confused by what is meant by infinity.
    I know what infinity mean in english eg goes forever.

    But in reading an article

    Point 8 says.
    You should set the meter to the highest setting
    And a Normal reading is infinity between T1 and T2 and T1 and Gate. When I
    set my DVM to 200M (the highest ) I get .09 and 1.3 Mega Ohms

    In my mind infinity is sort of the difference between Null and 0. 0 is a
    number null is nothing.
    Sooo infinity means that there is something out there but you will never see
    it. Different from Open which means there is no connection between the two

    Im still confused however by what an infinity number is suppose to be.
  2. Chris

    Chris Guest


    Hi, Chretien. If you look at 5) on the web page, they ask you to set
    your meter to the range that will read 40 ohms (usually 200 or 320 ohms
    full scale). If you do that, and measure something over the full scale
    range, you will get "O.L." or a blank. That's what they mean by
    "infinity", just overrange. Your meter (a pretty good one, I guess --
    most DVMs don't have a 200 meg range) should have the capability to set
    the range.

    By the way, those static tests mentioned on the web page for testing
    triacs could be augmented by this trick:

    * Meter + on MT1
    * Meter - on MT2
    (Meter reads O.L., indicating that the triac is not conducting)
    * While holding probes on MT1 and MT2, connect a 100 ohm resistor
    between MT1 and G
    (Meter will now read low ohms, indicating that the triac starting to

    This will work well for all but the biggest triacs with highest minimum
    gate current. With a little practice, it's not too difficult.

    Good luck

  3. Infinity means different things to different people and in different
    contexts. However, typically with regards to DVM readings infinity usually
    means any resistance that will measure as over range on the maximum
    resistance scale.

    Realize that you cannot touch the leads or anything while you are making
    very large resistance measurements. If you touch both leads with your skin
    you can get all kinds of resistance measurements anywhere from several kilo
    or tens of kilo ohms to many megohms. It varies significantly depending
    upon how wet your skin is and how much pressure you hold the leads with (as
    well as the metal surface area in contact with your skin).

    I suspect that is what you measured in the above test when you found 90k and
    1.3Mohm resistances. Try the test again but don't touch any metal in the
  4. Chretien

    Chretien Guest

    Chris thanks for your help and the added tips on triacs.

    So just for clarification, When you say over range what you mean is that if
    you set the meter to some mid range level say 1M Ohm your going to get a
    reading of 0.00 eg. Something but not enough to care about.

    The reason I had mine set to the highest setting was becasue the beginnig of
    point 8 says to set the meter to the highest resistance setting. I sort of
    wish that people would give a number like 1M or 2M. A bit of range. Rather
    than the highest. Lets face it if there were a "Cray super meter" it could
    mesure the resistance of anything.
  5. Chris

    Chris Guest

    Hi, Chretien. First, I guess I should say that, if you're sure about
    your pinouts on the triac you're testing, your test results indicate
    the triac's probably bad. You might want to double-check the pinout at
    the NTE cross-reference web site to make sure:$$Search?OpenForm

    I don't think you're quite getting what "over range" means. Let's say
    you have a 2000-count $10 cheapie DMM which has several resistance
    ranges, from 200 ohm to 2Meg. "Range" means what the full "2000"
    means. On the 200 ohm range, "1999" means 199.9 ohms. On that range,
    201 ohms would be "over range" (it can't show 201.0) and would probably
    show "O.L." (over load) on the DMM display. If you bumped up to the 2K
    range, though, it would show as 0201 just fine, and be in range.

    On the webpage, I guess the author was using the word "infinity" as a
    rather inelegant way to say the resistance reading is over range.

    If you want to measure high resistances, you need another instrument,
    because you generally need a much higher voltage to get enough of a
    reading. I'll use an old HP3456A DMM to measure up to 200 meg with
    pretty fair accuracy, but I'll measure higher resistances with a
    megohmmeter, which applies a higher voltage (typically hundreds of
    volts) to a resistor to measure current and infer resistance (typically
    1000s of megohms, or gigohms). These are also typically used to
    measure insulation resistance for product and motor safety. The Mouser
    catalog lists megohmmeters from $179 USD (an Extech cheapie) to $3500
    USD for a 15KV Megger.

    If you happen to need "knock your socks off" accuracy in measuring high
    resistances, and/or if you can't apply that high a voltage to the DUT,
    Keithley makes a great line of instruments which will definitely do the
    job, but they're definitely out of the s.e.b. price range. On the
    other hand, maybe you'll win the Lotto, or come into an inheritance,
    and be able to bump up to the s.e.d. range. In that case, a Keithley
    current source/picoammeter would be in range. ;-)

    Good luck with your repair
  6. Chretien

    Chretien Guest

    To all responders. Thanks for the feedback.

    I think I understand. Actually I think my last reply was right it just was
    not perhaps properly spelled out or clear ( I wish sometimes when people
    spoke Electron'eze they would use some better descriptions)

    My meter probably cost about 50 or 60 bucks. Not the best but not 10
    Dollars. It goes from 200 to 200meg Ohm.

    When I measure say a resister that is out of range, as now I have learned
    the term, it displays 0.00. With an open circuit it measures "1 ."

    Out of range then if I dare make another attempt at defining this means then
    (or infinity in the description of testing the triac) that the resistance is
    so high that the average meter in the lower settings will not register it.
    I'm afraid the O.L comment fooled me a bit I thought when you said it, it
    meant Open Line or Open. not Over Load. I know now that I think when my
    meter says 0.00 it means "over load". eg its detecting something but not
    able to display at that setting.


  7. No - over range is open circuit - the meter will show the same as it
    shows if the probes are not connected to anything (my meter shows
    "OL", others may flash, or show some obviously invalid inidcation.)
  8. John Smith

    John Smith Guest

    I would suggest that, without reading the article, it was written with an
    analogue multimeter in mind. By 'infinity' they probably mean open circuit.
    However, 'open circuit' needs to be taken with a pinch of salt: it is still
    a piece of silicon (or other semiconductor) and will not be a perfect open
    circuit, just a rather high resistance. Several megaohms is rather high, and
    for most purposes could be taken as open circuit (unless you start playing
    with high voltage).
  9. John Smith

    John Smith Guest

    Having just glanced at said article, where it clearly states 'Digital
    multimeter' :)....

    As other poster's have said, set your DMM range to a low resistance range.
    If this is not possible, then treat your readings of several megaohms as
  10. No. A reading of 0.00 indicates zero resistance - a short circuit -
    the lowest resistance the meter can measure.

    "Over Range" or "out of range" means a higher resistance than the
    meter can measure.
    One of my meters displays "OL" for open circuit/over range - it really
    isn't a "proper" indication for this condition when measuring
    resistance, but is easily displayed on a 7-segment digital display,
    and does make sense if you try to measure a current or voltage greater
    than the meter can display.

    My other meter displays "-1 " for an open circuit - it is clearly an
    invalid value, so you know the meter can't display what you want it
  11. Chretien

    Chretien Guest

    Its unfortunate that the meter people cant get together.
    I checked with one of my other meters and when I put a 15k R on it and set
    it for 200. It just does not change at all stays at "1 ." which is what it
    stays at when there is nothing on it. I wonder what it would take the
    manufacturers to start with a - dash lets say when the meter is on but not
    measuring resistance. And then when out of range say something else like
    +++. Frankly I find my original meter the bigger of the two with fancier
    options confusing when it also displays "1 ." for no resistance and 0.00
    for out of range. Normally when I think there is a dead short i use the
    diode/Beeper setting. Frankly it is a little disconcerting that out of
    range gives an output of 0.00 but I wonder if it is actually producing an
    output but that there are not enough digits on the display to show it ??.
    Anyway I'm sort of smart enough to know what I expect to get and so if I
    think its a short rather than an over limit I test it with the diode/Beeper

    Thanks again. It all just goes to show each step of the way that every area
    of electronics has a lot of learning.

  12. Unfortunately, meters can't tell the difference between a resistance a
    little more than full-scale, and open circuit, so your display of
    "1 ." just means "the resistance between the test leads is more than
    I can measure on this range".
    I am certain that the reading of "0.00" indicates a low resistance -
    too low to show any digits on the display. For example, if you set a
    three-digit meter for 2K full scale, it will be unable to display a
    resistance under 10 ohms, so would show "0.00" for any resistance
    below 10 ohms.
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