[Elecraft] dmm vs vom

[John W2XS]

Be careful with some of the DMM's too. I did a study on the voltage produced
at the leads for several brands of industry-standard meters at work.  Here
is one of them:

Resistance Range        Voltage on the leads
0.2 ohms                       12 volts
2 ohms                          12 volts
20 ohms                        12 volts
200 ohms                       6 volts
2M ohms                        6 volts
20M ohms                      1 volt

We found out why certain semiconductor devices were being destroyed after
"meter checking". 

73,

John W2XS

Generally it is not a problem, if you are checking resistors. The area you  
can get into trouble is when you have semiconductors connected to the
circuit - 
 Elecraft Rigs are loaded with them. The problem can come from most VOMs not  
being able to limit the current applied to the circuit under test. Even at 
low  voltage you could apply a voltage with enough current to bias a
transistor 
to a  destructive level. Most DMMs use a high impedance amplifier on input
to 
reduce  current flow to/from the meter. The resistance circuitry can also be 
made to  sample E/I data at low enough levels to provide resistance
measurement 
at much  lower current and voltage. IMO, with many DVM/DMMs on the market at 
low cost,  it's probably not worth the risk.

Al WA6VNN
 



**************New MapQuest Local shows what's happening at your destination.  
Dining, Movies, Events, News & more. Try it out!      
(http://local.mapquest.com/?ncid=emlcntnew00000001)
_______________________________________________
Elecraft mailing list
Post to: Elecraft at mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft    

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com



-- 
View this message in context: http://n2.nabble.com/dmm-vs-vom-tp1307371p1314193.html
Sent from the Elecraft mailing list archive at Nabble.com.