Photo - This Voltage Divider is not doing what I expected - why?

Discussion in 'General Electronics Chat' started by Lumenosity, Dec 19, 2017.

  1. Lumenosity

    Thread Starter Member

    Mar 1, 2017
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    Hello and thanks for stopping to look at this thread.

    In the photo, I have created a voltage divider that mathematically should give me 3.3v at the red wire between the two resistors.
    But instead, I'm getting 1.79v. I tested the resistors with a VOM and they are accurate.

    Can you see what I have done wrong? I'm missing something obviously.

    Thanks

    VoltageDividerProblem2.jpg

    PS
    Are these resistors printed correctly?
    How is one supposed to know which end to begin reading the bands?
    I have to test each one just to verify it's value.
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Only two reasons for it,

    1) wrong value resistors.
    2) your voltmeter is adding to the parallel resistance.
     
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  3. Lumenosity

    Thread Starter Member

    Mar 1, 2017
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    Thanks Dave.
    I've carefully checked the resistors as mentioned and they are spot on.

    The IR of the meter appears to be 00.4 ohms (zero,zero,point four)
    What I mean is, when I set the meter at it's lowest value and touch the leads together.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    Where's the other probe on your voltmeter going when you measure the 1.79 V. Seems telling that the expected voltage across the 180 Ω resistor would be 1.76 V.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you had your meter probes inadvertently connected across the 180Ω resistor then ~1.79V is the reading you'd expect.
    As for reading resistor values, try to match the band colours with those of the standard E12 or E24 series. If left to right makes more sense than right to left, then L to R is the most likely sequence. But if in doubt, use an Ohmmeter.
     
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  6. WBahn

    Moderator

    Mar 31, 2012
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    How is 00.4 Ω different from 0.4 Ω?

    A voltmeter should have extremely HIGH resistance, like 10 MΩ.

    Are you sure you are on a DC V range and not on a current range?
     
  7. Lumenosity

    Thread Starter Member

    Mar 1, 2017
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    Hello,
    See the red wire coming from between to two resistors? I check the voltage at the end of the red wire with the other VOM lead connected to the 5v+

    Hello,
    Hmmm......I set the meter to the 200ohm range then touch the leads together. Result= 00.4 ohms
    (I used 00.4 just to clarify. Overkill perhaps. 0.4 would have been sufficient)
    I tried another VOM and the results were precisely the same.

    Yes, I'm positive I'm using the DC volts range.

    Am I connecting the +5v and ground leads to the correct places?
     
    Last edited by a moderator: Dec 19, 2017
  8. panic mode

    Senior Member

    Oct 10, 2011
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    are you sure you have correct polarity for supply? what is red output wire connected to? how about disconnecting it and trying measuring voltage just across voltage divider?
     
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  9. Lumenosity

    Thread Starter Member

    Mar 1, 2017
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  10. Lumenosity

    Thread Starter Member

    Mar 1, 2017
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    Hello Panic Mode,

    I have +5v from a DC power supply going to the lead marked +5v
    The Red output wire between the resistors is not connected to anything.
    I just use it to test the output voltage.

    Where should I place the leads to do that?
     
  11. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    In that case, 1.79 V is exactly correct. 1.79 + 3.3 = 5.09 V. You are measuring across the wrong resistor.

    With your meter set to the 20 volt range, connect one lead to the junction of the two resistors, and the other lead to the ground end of the 330 ohm resistor.

    Also, a meter cannot measure its ow input resistance. What you measured in post #3 is the offset error of that range of the ohms function.

    ak
     
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  12. Lumenosity

    Thread Starter Member

    Mar 1, 2017
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    When I measure across both resistors, I get 5v.
    But when I measure across just the 180ohm resistor I get 1.79v

    But if I measure across the 330ohm resistor, there is no voltage drop at all.
    However, measured by itself, the 330ohm resistor measures 330ohms resistance?

    It's as if they are doing nothing. I'm really cornfused.
     
  13. Lumenosity

    Thread Starter Member

    Mar 1, 2017
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    OK.
    When I do this I get 3.3v

    So I have 5v coming out of the power supply and 3.3v going back TO the power supply ground (negative) side?

    Now I have to try to figure out what I've learned
     
  14. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    Ideas:
    1. User error
    2. Intermittent contact in breadboard
    3. Meter error
    There's no way you have 5V across the two resistors together, 1.79 across one of them, and nothing across the other.
     
  15. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    Sorry, cross post there - I was replying to one post earlier.
     
  16. Lumenosity

    Thread Starter Member

    Mar 1, 2017
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    User error is a given :D (see avatar)
    Post 11 seems to have pointed me in the right direction....but I still have to figure out where I'm going
     
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  17. Lumenosity

    Thread Starter Member

    Mar 1, 2017
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    Thanks to everyone who replied.

    It is REALLY appreciated :)
     
  18. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    Nothing wrong with a little user error; that's usually part of the learning process (certainly has been for me!)
     
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  19. WBahn

    Moderator

    Mar 31, 2012
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    And therein lies you problem. If you want to measure the voltage across the 330 Ω resistor, then you need your voltmeter leads to be across the 330 Ω resistor. You are measuring the voltage across the 180 Ω resistor. In fact, with the black lead connected to 5 V and the red lead at the junction, you should be reading about -1.76 V. That's assuming that you have the leads plugged into your meter correctly (black to COM and red to VDC or similar labeling).

    That's the resistance of the probe leads, not the resistance of the voltmeter.

    No. Connect the leads across the component you want to measure the voltage across.
     
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  20. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Nope. Voltage doesn't "go to" something, current does.

    In conventional terms, current leaves the power supply + terminal, goes through both resistors, and returns to the power supply - terminal. A critical truth of DC circuits sigh as this is that all of the current goes through all of the components all of the time. That current causes a voltage drop across each component in the loop, and the size of the voltage drop is directly proportional to the size of the resistance. Ohm's Law: E = I x R

    With the sum of the two resistors you can use OL to calculate the current through the loop. With the current you can use OL to calculate the voltage drop across each resistor.
     
    Last edited: Dec 19, 2017
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