Help please for tech college

Thread Starter

Aprout124

Joined Feb 16, 2020
7
The question is says
Calculate the value of R1 which allows the meter to read voltages up to 100v

Now calculate the value of R1
(Rtotal =10000000ohm)
R1=____ohm_____ohm
R1=_____ohm
 

Thread Starter

Aprout124

Joined Feb 16, 2020
7
I’m still stuck I can’t move on till I get it and 99999970ohm I’ve tried and I’ve tried with 10000000 -3000 and 30000 all wrong running out of options
 
Resistance of the meter is 30 Ohms
and the FSD (Full Scale Deflection) requires 10ua

We still don't know what the readings on the meter are .... if you don't know the readings then how can I move to a specific number?

I suppose if 100V is full deflection then you need to choose a resistor value that is in series with the 30 Ohms that will deliver 10ua throught the resistor combination.
 
10ua x 30Ohms = 300uV

100V - 300uV = 99.9997V

99.9997V / 10ua = 9.99997 Meg


I believe you have an extra "9" in your second answer ... you had 99.99997 Megs

" ... I’ve tried 9970000 and 99999970 and there both wrong so I’m stuck at moment..."
 
Last edited:

ci139

Joined Jul 11, 2016
1,696
if your meter is (basically a µAm) 10µA you should expect that value to pass it at 100V external Voltage
normally the value is U=I·R=1E-5A·3E+1Ω=3E-4V=300µV -- so -- R.tot=1E+2V/1E-5A=1E+7Ω=10MΩ
- thus - R.ser=R1=R.tot-R.m=10M-30=9.999970MΩ . . . the difference of the resistances is huge
you may consider bypassing some current with the shunt
uAm - Cfg - 1.gif
 

MrAl

Joined Jun 17, 2014
7,803
Hi,

To estimate you can skip the 30 ohms unless you have to get super accurate results. But in practice the larger resistor will swamp the smaller by many orders of magnitude.
One gives 100v exactly and the other gives 100.0003 volts (ha ha) or in terms of current 9.99997ua instead of 10.00000ua.
The meter resistance is more important when the series resistor value is more comparable to it.
For example if the series resistor was 3000 the error would only be around 1 percent but if 300 then the error would be something like 10 percent which is usually not acceptable.
I dont know if the practical aspects of the design are part of the assignment though.
 
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