MEASURING HIGH VOLTAGES & CURRENTS SAFELY
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Measuring High Voltages
If one wants to measure the voltage of a 0 to 100V line, one option is to use a 100 to 1 voltage divider to convert the 0-to-100V signal to a 0-to-1V signal, and and then place the 0-to-1V signal across the instruNet Vin +/- screw terminals. This voltage divider is in parallel with the voltage source and in parallel with the load. This means that the 1V shunt resistor must be next to the GND node an not the 100V node, in order to keep the voltage at instruNet at 1V instead of 99V. If the 100V accidently became 1000V in a overvoltage scenario, then 10V would go into the instruNet, and the instruNet is protected at this level indefinitely, and protected for higher voltages for shorter durations. The resistors must be able to handle the power:

resistor power dissipation (Watts) = volts * volts / resistance

For example, a 100 ohm and 10K ohm resistor for 100V would mean the 100 ohm resistor power dissipation would be: 10V * 10V / 100 = 1Watt. And the 90V drop across the 10K would be: ~100V * ~100V / 10K = 1Watt. These are fairly high, since most resistor are 0.25 Watts or so; therefore, 330K and 1K would be better, and would yield power dissipation of 0.03 Watts for each resistor and a 0 to 0.33 V measurement range.

To reduce the chance of overvoltaging instruNet internal electronics, one might set up their voltage divider circuit to apply less than half the maximum input voltage. For example, if the instruNet maximum input voltage is ±5V, then one might try to apply voltages less than ±2.5V.

The sum of the accuracy of your resistors will limit the accuracy of your measurement. For example, if you use 2 resistors with initial accuracy of 0.05%, then your final measurement will incur a maximum error of 0.1% due to resistor error. For typical precision shunt resistors, click here.

For safety, one might want to place the two resistors at the high voltage line, and then run the low voltage signal (e.g. 0 to ±.6V) to the instruNet box, further away. Or place the larger resistor close to the source and the smaller at instruNet hardware.

To reduce noise with instruNet hardware that does not have multiplexors (i100 and i423 are ok, i420/i430/i60x are not ok 12), one can add a capactor across the instruNet Vin+/- screw terminals. This creates a 1pole low pass filter with FrequencyCutoff (Hz) = 1/(6.28 * R * C); where R is the high value resistor in ohms and C is the capacitor in Farads.

If working with multiplexors (i420/i430/i60x have these, i100/i423 do not), one should set the delta field to the source impedance of the voltage divider:

   Rsource (Ω) = (R1 * R2) / (R1 + R2)

where R1 and R2 are the two resistors in the voltage divider.