Quote:
Originally Posted by Bazz4CQJ
Could you elucidate?
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I'll try...
This is how I originally did it...
If RS is source resistance of meter where RS = Range x 100k
and RL is load across terminals
then
RP = RS//RL = RS*RL/(RS+RL)
and voltage across terminals is given by
VT = 10uA*RP
However, having thought about it, it is easier to apply a Thévenin equivalent which gives a voltage source equal to the Range (VRange) and a source resistance of RS = Range x 100k
The load across the terminals (RL) now forms a voltage divider of RL/(RS+RL) and hence VT = VRange * RL/(RS+RL)
A quick way of estimating the test voltage voltage (VT) for a given reading is to note the meter deflection in terms of percentage FSD (say D) (eg 50% for half way across the scale) and use
VT = Range * (100 - D) /100
So on the 1000v range 100M gives 50% FSD and the voltage is
VT = 1000*(100-50)/100 = 500v
on the same range 10M gives about 90% FSD so
VT = 1000*(100-90)/100 = 90v
etc
I made an error in my last post, the table should read
25v______20.0v
50v______33.3v
100v_____50.0v
250v_____71.4v (My excuse is the actual value is 71.43 an I dropped a digit)
500v_____83.3v
1000v____90.9v
Finally WME_bill's post made me think about the meter movement and its vulnerability especially as I sometimes use mine with an external power supply instead of batteries.
I shall place additional protection on my to do list.
Jim