Increased Range for DVMs

By Elektor Magazine


Panel mount DVMs with a standard range (around 0.2 volts) often times are insufficient for what you may need.

But, according to Elektor magazine all is not lost. It's not difficult to increase the measuring range to higher voltages. In addition, it is also possible to measure current with these modules.

So, how do we suggest you go about increasing the range of your DVM? You simply need to reduce the voltage with a divider, adding an R1 and an R2.

The R1 is connected to the positive input of the DVM; the R2 is then connected in parallel with the inputs. In the chart below, you'll see the correct ratios of R1 and R2.
DVM

 
Range
R1
R2
VIn
2V
910k
100k
20V
1M
10k
200V
1M
1k
2000V
1M
100Ω
IIn
200mA
1k
2mA
100Ω
20mA
10Ω
200mA

Because modules typically have an input impedance of 10MΩ, the addition of the attenuator reduces that impedance to 1MΩ, which is still high enough for most measurements.

Then there's the art of converting a DVM so that it can measure current.

The key is converting the current in to a correlating voltage, because the input impedance for a current meter needs to be as low as possible and is dependent on range of the circuit being measured.

Just keep in mind that there is a chance that lethal voltage may be present in the circuit, especially in the 200V to 2000V range. So, when measuring current this strong, make sure that the resistors you are using are sufficient for the job.
schematic