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Please explain this resistance reading

Discussion in 'General Electronics Discussion' started by stspringer, Jun 21, 2019.

  1. stspringer

    stspringer

    97
    5
    May 10, 2019
    I have this circuit on a breadboard
    9 volt battery
    1 led
    1 resistor
    sorry no schematic
    I am studying how to find a short a circuit

    With NO short created I get the following:


    With 9 volts power, to the breadboard, I put my DMM on the ohm setting and place the black probe on the negative power rail and the positive probe between the resistor and the led load and I get OL on the DMM pic 2 which I know I should get if there is no short in the circuit.

    Question
    When I place the red positive probe on the negative side of the led I get very high resistance reading 50 ohms pic 3 and climbing. Why is that?

    With Short created I get the following:
    Notice the black jumper wire going from positive side of led to ground to create a short. Meter now reads 35 ohms,which I know means anything other than OL means I have a short.see pic short_created_1.jpg

    Now with short still active, meter red lead, after led "more negative side", meter reads 0 see pic short_created_2.jpg

    Please advise

    Thanks




    Short_4.jpg Short_3.jpg Short_2.jpg Short_1.jpg
     

    Attached Files:

  2. duke37

    duke37

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    You have good pictures so draw a schematic and take a photo of it. Mark on it where you are taking voltage measurements.
    Do NOT use the meter for resistance measurement when power is applied.
     
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  3. Bluejets

    Bluejets

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    Oct 5, 2014
    You will be lucky if you have not blown your meter.
     
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  4. Audioguru

    Audioguru

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    Sep 24, 2016
    Since you wrongly connected your ohm-meter to a powered circuit then you should check that it still works by measuring a resistor without any power on each range.
     
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  5. stspringer

    stspringer

    97
    5
    May 10, 2019
    Ok here we go from scratch with schematic. Using a 330 ohm resistor. Using a DMM meter set to ohms. 12 volt Power off

    simple_battery_led_resistor_schematic_1.jpg shows schematic.

    12_volt_led_resistor_circuit_1.jpg shows OL as expected, black negative probe on ground, red positive probe on positive side of led.

    12_volt_led_resistor_circuit_2.jpg shows .333 with black negative probe on ground, red positive probe on more negative side of led.

    12_volt_led_resistor_circuit_3.jpg shows 00.2 with black negative probe on ground, red positive probe on more negative side of resistor.

    short_to_ground_1.jpg I added black jumper wire from positive side of led to ground. Shows 00.3 with black negative probe on ground, red positive probe on positive side of led.
    ==============================================================================
    I understand 12_volt_led_resistor_circuit_1.jpg reading OL which means no short.

    Not really clear on the other readings. Please advise.
    Also would a continuity test be better than using ohms setting when testing for a short?

    Thanks
     

    Attached Files:

    Last edited: Jun 22, 2019
  6. Audioguru

    Audioguru

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    Sep 24, 2016
    Your meter is not lighted well enough to be sure but it looks like in picture #2 the meter not show 0.333 ohms. Instead it correctly shows 0.333k ohms which is 333 ohms.
    Pictures #1 and #3 correctly show the 0.2 ohm and 0.3 ohm resistance of your meter's leads and wires and contacts on the breadboard.

    Measuring resistance is measuring continuity. It also shows you the resistance of the wiring.
    My meter beeps when measuring low resistance on the low resistance range that you call continuity.
     
  7. stspringer

    stspringer

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    May 10, 2019
    My meter is reading the resistor as 0.333K which is like you say 333 ohms.

    not clear on what you mean by Pictures #1 and #3 correctly show the 0.2 ohm and 0.3 ohm resistance of your meter's leads and wires and contacts on the breadboard.
    what about the reading on short_to_ground_1.jpg
     
    Last edited: Jun 22, 2019
  8. Audioguru

    Audioguru

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    Sep 24, 2016
    Simply hold the red and black meter's probes together to measure the 0.3 ohm resistance of the leads. If you squeeze the probes together very hard then you might measure 0.2 ohm.
     
  9. davenn

    davenn Moderator

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    Sep 5, 2009
    that is what AG is referring to

    you are just shorting out the meter leads via your 2 black wire links .... follow the path between the 2 meter probes.

    upload_2019-6-23_9-32-34.png


    see the blue path I have traced out in blue ?


    The same thing in the second pic but just through that single link
     
    Last edited: Jun 23, 2019
  10. Bluejets

    Bluejets

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    Oct 5, 2014
    OL means open loop.

    You are testing for continuity whether using ohm range or diode test range.
    Just what amount of "continuity" would depend on the resistance of what you are measuring.
    A short will normally read around 0.3 as the meter leads have some resistance.

    Best advice would be if measuring resistance or suspect short, stay with the ohm scale.
    Semi conductors require the diode scale to provide enough bias to turn on the semiconductor to get any useful reading.

    Probably a good idea to get a bunch of bits and test them out of circuit with your meter as an exercise.
    For capacitors you will need another meter.
     
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  11. hevans1944

    hevans1944 Hop - AC8NS

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    Sure. First, dig out whatever documentation came with your digital multimeter and read and understand it. If there are some things you don't understand about your multimeter after reading the docs you have available, visit this link and some of the web pages you will find there. If none of those links help sufficiently, come back here with specific questions.

    Second, NEVER try to measure resistance or conductivity (ohms function) with electrical power applied to your circuit. Resistance always requires power to in order to measure it, but this power is supplied by the ohms function of your multimeter. Always remember to disconnect or remove external power sources (batteries, line-operated power supplies, etc.) before trying to measure resistance or conductivity.

    Failure to follow this advice when using old-school analog multimeters will usually blow an internal fuse in the meter, damage the D'Arsonval meter movement, burn out precision resistors in the multimeter, or all of the above. The situation is often aggravated by the multimeter using the same test lead connections, whether measuring resistance or measuring voltage. I have personally destroyed at least one analog multimeter while trying to measure voltage with the meter set to measure resistance, or while trying to measure resistance in a circuit that is already under power. Digital multimeters may be a little more forgiving, and not immediately self-destruct when used improperly, but it is never good practice to rely on that to save you from your bad practices. Learn how to use electronic measuring instruments safely and properly!

    Thirdly, why are you trying to measure short circuits? These are usually immediately visible from the effects they have on circuits in which they occur... sagging power supply voltages, blown fuses or open circuit breakers, smoke and sometimes flames, and certainly improper circuit operation... all usually preventable by a careful visual inspection before power is applied for the inevitable "smoke test."

    Fourthly, understand that the multimeter you show in your (very nice!) pictures has three ways to measure continuity: resistance mode, diode mode, and audible continuity mode. All three modes will apply a voltage to the test leads. All three modes MUST be used without power applied to the circuit you are measuring.

    The resistance mode is quite simple: the meter provides a constant current to the pair of probes and measures the voltage between them that results. A simple internal calculation translates the measured voltage into a measured resistance.

    A similar arrangement is used in diode mode, except the measured voltage across a forward-biased diode is displayed. This allows you to easily distinguish germanium diodes (about 0.4 volts) from the more common silicon diodes (about 0.7 volts). Diodes that are open, or with test leads applied to reverse-bias their junction, will display O.L on your meter.

    The audible continuity mode is similar to the resistance mode, with the addition of a comparator and an audible alarm that will sound if the measured resistance is less than some threshold value set by the manufacturer of the multimeter... usually somewhere between ten and a hundred ohms. This is handy for tracing out circuit paths without having to maintain visual contact with the multimeter. Very useful when wiring, and checking the wiring, of a large panel you are building containing hundreds of wires... all of the same color!

    As an end-note, realize that all wiring has resistance, including your breadboard jumpers and breadboard connections and your meter test leads. If need be, you must subtract this "wiring resistance" from your measured resistance to obtain an accurate value for the measured resistance. Some multimeters (not yours) will do this automatically if you tell it to do so. Mine has a REL button (for RELative) that "zeroes" the meter reading whenever I press it momentarily, thus allowing me to remove the short-circuit test lead resistance or the open-circuit test lead capacitance from the measurement that follows.
     
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  12. stspringer

    stspringer

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    May 10, 2019

    Thanks hevans1944

    You explain things well.

    BEGIN UPDATE on this POST:

    I went to your link and see that is says set your meter to Diode mode. I did that and I got .000 volts whith the short "jumper wire" active, and OL with the short "jumper wire" inactive.

    I like these diode readings better they seem "cleaner" to understand. :)


    Check this link

    IMPORTANT!
    After watching the above video she says to put red probe on negative side and black probe on positive side in diode mode. Why do that? Please explain.




    Check this link louis rossmann

    Do you know how can anyone get schematics for a PC motherboard not apple?

    END UPDATE on this POST:

    ============================================================



    Thirdly, why are you trying to measure short circuits?

    I just want to wrap my head around short circuits.






    Here is a google search answer:
    Connect one lead of the meter to one end of the circuit to be tested. Connect the other lead of the meter to a good ground. If the DMM does NOT display infinite resistance (OL), there is a short to ground in the circuit. Remove the power feed (i.e. fuse, control module) from the suspect circuit.

    This explanation doesn't say what to set your meter to, either ohms or continuity.

    In my circuit I placed a jumper wire from positive side of my led to ground. I then set my dmm to ohms and placed red positive lead of dmm to positive power rail and black probe of dmm to ground power rail and I got a reading of 00.2 or 00.3

    With the meter set to continuity setting I get an audible beep, and readings that vary from 00.8 to 00.1

    QUESTION:
    What setting is best to use ohms or continuity. I know they are pretty much the same, I think, why choose 1 over the other?

    Also if I was testing a laptop motherboard "no schematics available for the laptop motherboard" for a short will this method work to see if the motherboard has a short in general? Or do you have to isolate a particular area of the motherboard?

    I really appreciate you informative and lengthy explanations. :)
     
    Last edited: Jun 23, 2019
  13. Audioguru

    Audioguru

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    Sep 24, 2016
    Your laptop motherboard might have a defective IC or capacitor. It might have a broken pcb trace. Then why measure for a short circuit when it probably does not have a short circuit?

    Why did you measure the resistance of a piece of wire here:
     

    Attached Files:

  14. Bluejets

    Bluejets

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    Oct 5, 2014
    It is like most things , there is a time and place for certain things.

    Diode test is not the be-all end-all as you seem to think.
    There are times when the resistance ranges suit better.
    This is also the reason some multimeters have ranges, yours is auto-ranging by the look of it.

    Primary reason for diode test is to bias on things like transistors and diodes so a meaningful reading can be taken.
    If you require more explanation on bias, look it up .

    This is usually described as "measure across the resistor".
    .333 with black negative probe on ground, red positive probe on more negative side of led.

    Your terminology is all a-up and the last thing you want to do is to start some new fangled way of looking at or describing circuits. Same as Audioguru query in #13, a short is a short, Only reason to measure things like that in a circuit is when you know where both ends are but difficult to know if a track is broken or some soldered joint is suspect.
    A 1" piece of wire in a breadboard certainly does not apply. You may later learn voltage readings in that situation are more helpful.
     
    Last edited: Jun 23, 2019
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  15. stspringer

    stspringer

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    May 10, 2019

    Dear sir

    I don't have a broken laptop motherboard. I am just asking general questions. I want to learn how to detect a short in a simple breadboard circuit using a dmm and find out which setting is best to use, ohm diode or continuity. I don't think you are getting the questions I am asking in my posts. Read post 11 He understands what I want to learn

    Thanks anyway :)
     
    Last edited by a moderator: Jun 24, 2019
  16. davenn

    davenn Moderator

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    Sep 5, 2009
    continuity is the best

    diode test is for checking diodes or transistors


    you didn't even respond to my previous post :(
     
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  17. stspringer

    stspringer

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    May 10, 2019
    Sorry thanks for the previous post I must have forgot to respond to it. Sorry
     
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  18. stspringer

    stspringer

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    May 10, 2019
    Dear Bluejets,
    I don't consider my post to Audioguru to be "nasty"

    "Don't think you are qualified to start judging peoples level of understanding or qualifications."

    I just pointed out that perhaps Audioguru was not reading my entire post.

    Sorry if I came on as being "nasty"

    I am an old timer too. I am a combat Vietnam Veteran, I know what "nasty" is. I think your statement was more judgemental than my statement to Audioguru.

    Thanks
    Sorry if I offended anyone.
     
    Last edited by a moderator: Jun 24, 2019
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  19. hevans1944

    hevans1944 Hop - AC8NS

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    Please, let's not let this thread wander off-topic into back and forth accusations that may or may not have any substantial substance, relevant to measuring short circuits with a digital multimeter.

    I think it more important that we try to discuss why anyone would even want to do this. No one I know tries to measure a short-circuit. They may try to identify a short circuit, or remove a short-circuit, or prevent a short-circuit from occurring in the first place, but once identified who cares how much resistance a short-circuit measures? And will measuring resistance help you find a short-circuit? Someone please explain (better than I have been able to do so far) just how that works. I know it works because I have used my dmm to locate short-circuits.
     
    davenn likes this.
  20. Audioguru

    Audioguru

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    Sep 24, 2016
    A few times in my career I had a short circuit in a circuit and needed to cut pcb traces to isolate the area on the pcb the short was located. Then after replacing a shorted capacitor or semiconductor I needed to splice my cuts back to normal.
     
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