[Elecraft] Resistance Readings with Digital Volt Meter

[DolfinDon@aol.com]

In a message dated 1/2/2002 3:40:20 PM Central Standard Time, 
rlewis@staffnet.com writes:


Okay, I've got some questions. How does a "true DVM" differ from an "analog" 
DVM with a digital readout? How does a "true DVM" measure resistance without 
putting a voltage across the device being measured?

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

Hi

There seams to be some confusion about meters here. First, all meters whether 
analog or digital read out are analog meters. Voltage current and resistance 
are all analog measurements. The digital meter converts the analog value to a 
digital display with an analog to digital converter and some form of numeric 
display. The so called analog meter is the type with a mechanical movement 
(Simpson 260) is really measuring the current flowing in the circuit made up 
of the external resistance an internal range resistor the meter movement and 
a battery. The resistance scale is usually non liner and the accuracy is not 
as good as the digital meter. Also on some ranges the battery may be 15 to 67 
volts and that's why it may damage semiconductors. When measuring voltage the 
battery is out of the circuit, the range resistor is a different value and a 
liner scale is used on the meter.

The digital meter measures voltage with an analog to digital converter. The 
range switch is designed so a constant resistance is always across the 
measure terminals (usually 10 megohms). To measure resistance the meter has a 
constant current circuit. This current is applied across the measurement 
terminals and the A to D measures the voltage drop. By selecting the right 
current the voltage reading is a liner relationship to the resistance. The 
constant current circuit will only apply 2 or 3 volts to the circuit and the 
current is limited to a small value. It requires .5 to .7 volts to turn on 
most semiconductors, so many meters have some ranges where the voltage 
applied to the circuit is below .5 volts. This way incircuit measurements can 
be made without diodes or transistors upsetting the readings. The answer to 
your question -- all meters apply some voltage to the circuit when measuring 
resistance it's just the amount of voltage and current that is different. The 
DVM is a safer choice.


Don Brown
KD5NDB

K2 #1808,#2174,#2153
K1 #1000,#542