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.

  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.