{COURSE}204.1{COURSE}
{LESSON}3{LESSON}
{LESSON TITLE}Resistance Measurement{LESSON TITLE}
{TOPIC}4{TOPIC}
{TOPIC FILENAME}20020403-04-HowDoesaMultirangeOhmm.dita{TOPIC FILENAME}


{SCENE 1}

{SCENE TITLE}Working of a Multirange Ohmmeter{SCENE TITLE}

{VIDEO SYNC}

{TIMING 1} 3.19 {TIMING 1}

{TEXT 1} If ohmmeter has manually adjusted range setting, set instrument to desired range of resistance{TEXT 1}

{TIMING 2} 11.63 {TIMING 2}

{LEVEL OF BULLET 2}2{LEVEL OF BULLET 2}

{TEXT 2} Usually expressed as R×1, R×10, R×100, etc.{TEXT 2}

{TIMING 3} 26.12 {TIMING 3}

{TEXT 3} To measure resistance accurately, pointer should be about mid-range in its scale{TEXT 3}

{TIMING 4} 39.92 {TIMING 4}

{TEXT 4} Note that total resistance in meter circuit is sum of meter movement resistance, plus R{SUB}1{SUB} (fixed resistor), R{SUB}2{SUB} (variable resistor), and external R{SUB}x{SUB} (unknown or test resistance){TEXT 4}

{TIMING 5} 0 {TIMING 5}

{INSERT VIDEO 5}Multirange_ohmmeter_circuit{INSERT VIDEO 5}

{VIDEO HEIGHT 5}440{VIDEO HEIGHT 5}

{VIDEO WIDTH 5}600{VIDEO WIDTH 5}

{NARRATION}
How Does a Multirange Ohmmeter Work?
If your ohmmeter has a manually adjusted range setting, the next thing you do is set the instrument to the desired range of resistance.  
This is usually expressed as Rx1, Rx10, Rx100, and so on.
Let’s see how a multirange ohmmeter works and why several ranges might be needed.  
In order to measure a resistance reasonably accurately, the pointer should be about mid-range in its scale.
How large an unknown resistance (Rx) would give a mid-range reading?
As you can see in this illustration, the total resistance in the meter circuit would be the sum of the meter movement resistance, plus R{SUB}1{SUB} (the fixed resistor), R{SUB}2{SUB} (the variable resistor), and the external R{SUB}x{SUB} (the unknown or test resistance).
{NARRATION}

{SCENE 1}


{SCENE 2}

{SCENE TITLE}Working of a Multirange Ohmmeter (Cont.){SCENE TITLE}

{VIDEO SYNC}

{TIMING 1} 0.49 {TIMING 1}

{TEXT 1} An ohmmeter is essentially an ammeter with reversed indicating scale{TEXT 1}

{TIMING 2} 5.53 {TIMING 2}

{LEVEL OF BULLET 2}2{LEVEL OF BULLET 2}

{TEXT 2} Makes it is easy to find mid-range value{TEXT 2}

{TIMING 3} 8.65 {TIMING 3}

{LEVEL OF BULLET 3}2{LEVEL OF BULLET 3}

{TEXT 3} For meter movement to indicate half-way on scale, it must detect current half the value of full-range deflection (30 {ITALIC}µ{ITALIC}A){TEXT 3}

{TIMING 4} 23 {TIMING 4}

{LEVEL OF BULLET 4}2{LEVEL OF BULLET 4}

{TEXT 4} Resistance (R{SUB}x{SUB}) that would produce such current would be combined internal/external resistance that is double original resistance (internal resistance alone){TEXT 4}

{TIMING 5} 32.03 {TIMING 5}

{LEVEL OF BULLET 5}2{LEVEL OF BULLET 5}

{TEXT 5} With 50 kΩ resistance already inside meter, another 50 kΩ between test leads would be required to cut current in half{TEXT 5}

{TIMING 6} 41.67 {TIMING 6}

{LEVEL OF BULLET 6}2{LEVEL OF BULLET 6}

{TEXT 6} Therefore, R{SUB}x{SUB} would be 50 kΩ{TEXT 6}

{TIMING 7} 45.81 {TIMING 7}

{TEXT 7} Similarly, a one-third pointer deflection from infinity mark ({SUPER}1{SUPER}⁄{DENOM}3{DENOM} of the way from left) indicates current that is one-third of full current (20 {ITALIC}µ{ITALIC}A){TEXT 7}

{TIMING 8} 58.77 {TIMING 8}

{LEVEL OF BULLET 8}2{LEVEL OF BULLET 8}

{TEXT 8} Could only result from unknown resistance (R{SUB}x{SUB}) of 100 kΩ{TEXT 8}

{TIMING 9} 0 {TIMING 9}

{INSERT VIDEO 9}Multirange_ohmmeter_circuit_2{INSERT VIDEO 9}

{VIDEO HEIGHT 9}440{VIDEO HEIGHT 9}

{VIDEO WIDTH 9}600{VIDEO WIDTH 9}

{NARRATION}
Since an ohmmeter is essentially an ammeter with a reversed indicating scale, it is easy to find the mid-range value. 
For the meter movement to indicate half-way on the scale, it needs to detect a current half the value of its full-range deflection, in this case, 30 µA.  
What resistance (R{SUB}x{SUB}) would produce such a current?  
It would be a combined internal plus external resistance that is double the original resistance (internal resistance alone).  
With 50 kΩ resistance already inside the meter, it would take another 50 kΩ between the test leads to cut the current in half.  
Therefore, Rx would be 50 kΩ. 
Similarly, a one-third pointer deflection from the infinity mark (i.e., {SUPER}1{SUPER}/{DENOM}3{DENOM} of the way from the left) indicates a current that is one-third of the full current, or 20 µA, which could only result from an unknown resistance (R{SUB}x{SUB}) of 100 kΩ.
Total up the internal and external resistances and prove it for yourself!
{NARRATION}

{SCENE 2}


{SCENE 3}

{SCENE TITLE}Working of a Multirange Ohmmeter (Cont.){SCENE TITLE}

{VIDEO SYNC}

{TIMING 1} 1.17 {TIMING 1}

{TEXT 1} Pointer deflection {SUPER}2{SUPER}⁄{DENOM}3{DENOM} of the way from left to right{TEXT 1}

{TIMING 2} 5.19 {TIMING 2}

{LEVEL OF BULLET 2}2{LEVEL OF BULLET 2}

{TEXT 2} Caused by current that is {SUPER}2{SUPER}⁄{DENOM}3{DENOM} of full-deflection value (40 {ITALIC}µ{ITALIC}A){TEXT 2}

{TIMING 3} 13.99 {TIMING 3}

{LEVEL OF BULLET 3}2{LEVEL OF BULLET 3}

{TEXT 3} R{SUB}x{SUB} value that would cause this current is 25 kΩ{TEXT 3}

{TIMING 4} 0 {TIMING 4}

{INSERT VIDEO 4}Multirange_ohmmeter_circuit_3{INSERT VIDEO 4}

{VIDEO HEIGHT 4}440{VIDEO HEIGHT 4}

{VIDEO WIDTH 4}600{VIDEO WIDTH 4}

{NARRATION}
Can you now see how a pointer deflection {SUPER}2{SUPER}/{DENOM}3{DENOM} of the way from left to right would be caused by a current that is {SUPER}2{SUPER}/{DENOM}3{DENOM} of the full-deflection value?  
And what would that be?
40 µA, of course.  
What Rx value would cause this current?
If your math is correct, you’ll find the answer to be 25 kΩ.
{NARRATION}

{SCENE 3}


{SCENE 4}

{SCENE TITLE}Working of a Multirange Ohmmeter (Cont.){SCENE TITLE}

{VIDEO SYNC}

{TIMING 1} 0.13 {TIMING 1}

{TEXT 1} Second difference between analog ohmmeter scale and voltmeter or ammeter scale{TEXT 1}

{TIMING 2} 7.79 {TIMING 2}

{LEVEL OF BULLET 2}2{LEVEL OF BULLET 2}

{TEXT 2} Ohmmeter scale is reversed and non-linear{TEXT 2}

{TIMING 3} 15.46 {TIMING 3}

{LEVEL OF BULLET 3}2{LEVEL OF BULLET 3}

{TEXT 3} Divisions get smaller from right to left{TEXT 3}

{TIMING 4} 19.03 {TIMING 4}

{LEVEL OF BULLET 4}2{LEVEL OF BULLET 4}

{TEXT 4} In digital ohmmeter{TEXT 4}

{TIMING 5} 19.03 {TIMING 5}

{LEVEL OF BULLET 5}3{LEVEL OF BULLET 5}

{TEXT 5} These effects are not visible{TEXT 5}

{TIMING 6} 23.9 {TIMING 6}

{LEVEL OF BULLET 6}3{LEVEL OF BULLET 6}

{TEXT 6} Infinite or over-range resistance is often indicated by flashing display{TEXT 6}

{TIMING 7} 0 {TIMING 7}

{INSERT VIDEO 7}Multirange_ohmmeter_circuit_4{INSERT VIDEO 7}

{VIDEO HEIGHT 7}440{VIDEO HEIGHT 7}

{VIDEO WIDTH 7}600{VIDEO WIDTH 7}

{NARRATION}
You have now discovered the second difference between an analog ohmmeter scale and a voltmeter or ammeter scale.  
Not only is the ohmmeter scale reversed, but it is non-linear. 
It looks like the one in this diagram.  
The divisions get smaller as you go from right to left.  
Of course, these effects are not visible in a digital ohmmeter.  
On a digital meter, infinite or over-range resistance is often indicated by a flashing display, with the left-hand digit changing to 1 and all the others registering 9.
{NARRATION}

{SCENE 4}


{SCENE 5}

{AUDIO FILE}204-03-04-05{AUDIO FILE}

{SCENE TITLE}Working of a Multirange Ohmmeter (Cont.){SCENE TITLE}

{TIMING 1} 0.23 {TIMING 1}

{TEXT 1} Scale works for resistances of 25 to 100 kΩ{TEXT 1}

{TIMING 2} 8.1 {TIMING 2}

{TEXT 2} If system has resistances in 500 to 1000 kΩ range{TEXT 2}

{TIMING 3} 15.65 {TIMING 3}

{LEVEL OF BULLET 3}2{LEVEL OF BULLET 3}

{TEXT 3} "Switch gears"{TEXT 3}

{TIMING 4} 23.18 {TIMING 4}

{TEXT 4} When switched to position 2, internal resistance of meter is now ten times as great as it was{TEXT 4}

{TIMING 5} 32.78 {TIMING 5}

{LEVEL OF BULLET 5}2{LEVEL OF BULLET 5}

{TEXT 5} Midpoint of scale becomes 500 kΩ{TEXT 5}

{TIMING 6} 37.3 {TIMING 6}

{LEVEL OF BULLET 6}2{LEVEL OF BULLET 6}

{TEXT 6} Position allows you to read accurately any resistance between 250 and 1000 kΩ{TEXT 6}

{TIMING 7} 44.79 {TIMING 7}

{LEVEL OF BULLET 7}2{LEVEL OF BULLET 7}

{TEXT 7} Meter circuit can be selected from R×10 position on selector switch{TEXT 7}

{TIMING 8} 0 {TIMING 8}

{IMAGE 8}204.3-5.png{IMAGE 8}

{NARRATION}
So the scale we have just examined works fine for resistances of 25 to 100 kΩ.  
What if we have a system that is supposed to have resistances in the 500 to 1000 kΩ range?
Here is where we “switch gears.”
This illustration shows the circuit diagram for an ohmmeter with a selector switch. 
If we switch to position 2, the internal resistance of the meter is now ten times as great as it was, or 500 kΩ, and the midpoint of the scale now becomes 500 kΩ as well.  
This position allows us to read accurately any resistance between 250 and 1000 kΩ.  
This meter circuit can be selected from the “Rx10” position on the selector switch.
{NARRATION}

{SCENE 5}


{SCENE 6}

{SCENE TITLE}Working of a Multirange Ohmmeter (Cont.){SCENE TITLE}

{VIDEO SYNC}

{TIMING 1} 1.61 {TIMING 1}

{TEXT 1} Wide range of internal resistances gives ohmmeter great flexibility{TEXT 1}

{TIMING 2} 12.36 {TIMING 2}

{LEVEL OF BULLET 2}2{LEVEL OF BULLET 2}

{TEXT 2} Allows for readings as small as 5 Ω, as large as 5000 kΩ{TEXT 2}

{TIMING 3} 18.67 {TIMING 3}

{TEXT 3} Older-model ohmmeter would have selector positions shown here{TEXT 3}

{TIMING 4} 23.23 {TIMING 4}

{LEVEL OF BULLET 4}2{LEVEL OF BULLET 4}

{TEXT 4} R×1{TEXT 4}

{TIMING 5} 25.28 {TIMING 5}

{LEVEL OF BULLET 5}2{LEVEL OF BULLET 5}

{TEXT 5} R×10{TEXT 5}

{TIMING 6} 27.31 {TIMING 6}

{LEVEL OF BULLET 6}2{LEVEL OF BULLET 6}

{TEXT 6} R×100{TEXT 6}

{TIMING 7} 29.74 {TIMING 7}

{LEVEL OF BULLET 7}2{LEVEL OF BULLET 7}

{TEXT 7} R×1K{TEXT 7}

{TIMING 8} 0 {TIMING 8}

{TEXT 8} These ranges are also available on digital ohmmeters{TEXT 8}

{TIMING 9} 0 {TIMING 9}

{LEVEL OF BULLET 9}2{LEVEL OF BULLET 9}

{TEXT 9} Internal resistance values established electronically{TEXT 9}

{TIMING 10} 0 {TIMING 10}

{INSERT VIDEO 10}Multirange_ohmmeter_circuit_6{INSERT VIDEO 10}

{VIDEO HEIGHT 10}440{VIDEO HEIGHT 10}

{VIDEO WIDTH 10}600{VIDEO WIDTH 10}

{NARRATION}
By extension, you can see that a wide range of internal resistances, each providing its own complete circuit via a selector switch, gives an ohmmeter a great deal of flexibility.  
It allows for readings as small as 5 Ω or as large as 5000 kΩ.  
Typically, an older-model ohmmeter would have the selector positions shown here.
{NARRATION}

{SCENE 6}


{SCENE 7}

{SCENE TITLE}Working of a Multirange Ohmmeter — Quiz{SCENE TITLE}

{AUDIO FILE}204-03-04-07Q{AUDIO FILE}

{QUIZ} MULTIPLE CHOICE {QUIZ}

{TIMING 1} 0.15 {TIMING 1}

{TEXT 1}Does the midpoint on a series ohmmeter scale always indicate a measured resistance that is equal to the internal resistance of the meter?{TEXT 1}

{TIMING 2} 8.88 {TIMING 2}

{OPTION 2} Yes{OPTION 2}

{TIMING 3} 9.88 {TIMING 3}

{OPTION 3} No{OPTION 3}

{TIMING 4} 10.85 {TIMING 4}

{ANSWER 4}1{ANSWER 4}

{SCENE 7}


{SCENE 8}

{SCENE TITLE}Working of a Multirange Ohmmeter (Cont.){SCENE TITLE}

{AUDIO FILE}204-03-04-08{AUDIO FILE}

{TIMING 1} 0.33 {TIMING 1}

{TEXT 1} Some ohmmeters have second, higher-voltage battery that may be switched into circuit{TEXT 1}

{TIMING 2} 0.33 {TIMING 2}

{LEVEL OF BULLET 2}2{LEVEL OF BULLET 2}

{TEXT 2} Expands range farther{TEXT 2}

{TIMING 3} 9.07 {TIMING 3}

{TEXT 3} Higher voltage battery requires higher resistance inside meter to keep meter current in microamp range{TEXT 3}

{TIMING 4} 18.41 {TIMING 4}

{TEXT 4} When selecting meter range, keep in mind expected approximate value{TEXT 4}

{TIMING 5} 24.74 {TIMING 5}

{TEXT 5} If expecting 1000 kΩ{TEXT 5}

{TIMING 6} 27.71 {TIMING 6}

{LEVEL OF BULLET 6}2{LEVEL OF BULLET 6}

{TEXT 6} Do not put selector switch in R×1 position{TEXT 6}

{TIMING 7} 32.19 {TIMING 7}

{LEVEL OF BULLET 7}2{LEVEL OF BULLET 7}

{TEXT 7} Must be higher setting{TEXT 7}

{TIMING 8} 35.55 {TIMING 8}

{TEXT 8} After changing range selector switch, recalibrate (reset pointer to zero){TEXT 8}

{TIMING 9} 0 {TIMING 9}

{IMAGE TITLE 9}Dual-power ohmmeter{IMAGE TITLE 9}

{IMAGE 9}204.3-6.png{IMAGE 9}

{NARRATION}
To expand the range still further, some ohmmeters have a second, higher-voltage battery that may be switched into the circuit.  
But the higher voltage battery requires a still higher resistance inside the meter in order to keep the meter current in the microamp range, as shown here.
When selecting the proper meter range, keep in mind the approximate value you are expecting to read.  
If you expect to see 1000 kΩ, you would not put the selector switch in the Rx1 position!  
It needs to be in a higher setting.
Note: After you have changed the range selector switch, you may need to recalibrate (reset the pointer to zero).
{NARRATION}

{SCENE 8}