[Elecraft] Resistance Readings with Digital Volt Meter

Mon Dec 31 16:39:33 2001

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In a message dated 12/31/2001 10:36:07 AM Central Standard Time, 
[email protected] writes:

For the test points that should read >100k Ohms, I get different readings 
depending on what scale I am reading on the ohm meter.  I am using a Micronta 
meter I purchased at Radio Shack several years ago that has the following Ohm 
meter ranges 200, 2k, 200k 2m 20m.. For the test points in question, I get 
different readings on the 200k, 2m and 20m range.. All readings are OK on the 
highest range, but may be close in the 200k range..

The meter reads purely resisitive loads (resistors) fine on all ranges.. 

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

Digital voltmeters measure resistance by forcing a constant current across 
the test leads and measuring the voltage across those leads. For example by 
Ohm's Law if the meter forces 1.000ma with a 1000 ohm resistor across the 
test leads, the voltage will read 1.000 volt. The current forced changes when 
you change ranges. Also most DVM's limit the voltage on some ranges below 0.5 
volt so semiconductor junctions will not be biased on. While other ranges 
allow about 2 or 3 volts maximum. This is why the resistance is different on 
some ranges. When a diode, IC or transistor is biased on with the higher 
voltage the current flowing in the semiconductor will upset the reading. Some 
meters (Fluke is one) have a little diode symbol on the ohm range switch to 
let you know it will bias on a semiconductor. When the manual asks you 
measure >100k any value larger than 100k is OK. I would measure this on the 
200k range because the lower ranges will just read overrange. If the meter 
reads overrange on the 200k range I know the reading is over 200k. Sometimes 
you will have to wait a second or two for any capacitors to charge before the 
reading is stable.

BTW the ranges that allow the biasing on semiconductors can be used for 
matching diodes. Remember the meter is measuring the voltage across the test 
leads so the number is the forward voltage drop of the diode or Vf. Just 
select diodes that have the same reading.

Don Brown
KD5NDB

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<HTML>In a message dated 12/31/2001 10:36:07 AM Central Standard Time, [email protected] writes: 
 
 
For the test points that should read &gt;100k Ohms, I get different readings depending on what scale I am reading on the ohm meter.&nbsp; I am using a Micronta meter I purchased at Radio Shack several years ago that has the following Ohm meter ranges 200, 2k, 200k 2m 20m.. For the test points in question, I get different readings on the 200k, 2m and 20m range.. All readings are OK on the highest range, but may be close in the 200k range.. 
 
The meter reads purely resisitive loads (resistors) fine on all ranges.. 
 
===================================================== 
 
Digital voltmeters measure resistance by forcing a constant current across the test leads and measuring the voltage across those leads. For example by Ohm's Law if the meter forces 1.000ma with a 1000 ohm resistor across the test leads, the voltage will read 1.000 volt. The current forced changes when you change ranges. Also most DVM's limit the voltage on some ranges below 0.5 volt so semiconductor junctions will not be biased on. While other ranges allow about 2 or 3 volts maximum. This is why the resistance is different on some ranges. When a diode, IC or transistor is biased on with the higher voltage the current flowing in the semiconductor will upset the reading. Some meters (Fluke is one) have a little diode symbol on the ohm range switch to let you know it will bias on a semiconductor. When the manual asks you measure &gt;100k any value larger than 100k is OK. I would measure this on the 200k range because the lower ranges will just read overrange. If the meter reads overrange on the 200k range I know the reading is over 200k. Sometimes you will have to wait a second or two for any capacitors to charge before the reading is stable. 
 
BTW the ranges that allow the biasing on semiconductors can be used for matching diodes. Remember the meter is measuring the voltage across the test leads so the number is the forward voltage drop of the diode or Vf. Just select diodes that have the same reading. 
 
Don Brown 
KD5NDB</HTML>

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