# Differences in voltage measurements using DMM

#### Bhope691

Joined Oct 24, 2016
38
Hi,

I am trying to understand high voltages and am in the process of trying to measure the voltage of a high voltage source a friend made for me.

The problem is I am getting different readings depending on what probes I use on the DMM. I am using a Fluke 80k-40 HV probe and standard DMM crocodile clip probes. The DMM has Vrms upto 1000V.

When I attach the Fluke probe to the DMM and measure the voltage I get a Vrms of around 450V (0.450V on the DMM multiplied by 1000 for the probe). As the DMM goes upto 1000V Vrms I thought I could just use the original DMM crocodile clips and get the same 450V reading. However, I get 0.005 Vrms.

Taking measurements on different DC voltages and the mains AC the measurements match up but with the HV source they do not. How come?

Thanks.

#### MrChips

Joined Oct 2, 2009
29,208
Resistance (or impedance) of the meter compared with that of the voltage source.

Your DMM has an internal resistance of typically 10MΩ. The DMM will give a reasonably reliable reading if the resistance of the voltage source is lower that 1/100 of 10MΩ, that is, any thing lower than 100kΩ will result in an error less than 1%.

The high voltage probe will boost the resistance to a value orders of magnitude above 10MΩ.Your DMM with resistance of 10MΩ is too low for your voltage source.

Joined Jan 15, 2015
7,218
The Fluke 80K-40 High Voltage Probe is designed for use with a meter having a 10 Meg Ohm input impedance:
Specifications The 80K-40 will achieve rated accuracy when used with a voltmeter (ac or dc) having an input impedance of 10 MΩ ±1.0%.* Specifications for the probe are as follows: †Voltage Range: 1 kV to 40 kV dc or peak ac, 28 kV rms ac Input Resistance: 1000 MΩ Division Ratio: 1000: 1 (1000X attenuator) Accuracy DC: 20 kV to 35 kV: ±1% at 20°C to 30°C; add 1% at 10°C <20°C and >30°C to 45°C. (For total measurement accuracy add accuracy specification of voltmeter being used.) 0 kV to <20 kV and >35 kV to 40 kV: ±2%. Accuracy AC: 60 Hz, ±5%.

Keeping in mind the probe itself is a high voltage divider and due to its design needs to work into a 10 Meg Ohm meter input.

Theory Of Operation The 80K-40 High Voltage probe, is designed to extend the voltage measurement range of an ac/dc voltmeter up to 40,000 volts. Electrically, the probe is a passive attenuator as shown in Figure 1. Its high input impedance (1000 MΩ), as well as its accuracy and stability characteristics are achieved through the use of special thick film resistors. When the probe is connected to a voltmeter with a 10 MΩ input resistance the probe becomes an accurate 1000:1 divider. Notice that the divider depends upon a ground lead to complete the low side of the circuit path. Therefore, this connection must always be secure before attempting a voltage measurement. Otherwise, instrument damage or a shock hazard will result.

Figure 1 of the linked Fluke Document includes a simplified schematic of the probe which does reflect the input impedance of the meter the probe is used with. There is also this supplemental data sheet where they cover in more detail voltmeter comparability.

I used those probes for years and never had a problem but they need to be clean inside and outside with no dirt on the high value resistors used in the divider network. If the probe fails calibration a new resistor matching set is available from Fluke.

When I attach the Fluke probe to the DMM and measure the voltage I get a Vrms of around 450V (0.450V on the DMM multiplied by 1000 for the probe). As the DMM goes upto 1000V Vrms I thought I could just use the original DMM crocodile clips and get the same 450V reading. However, I get 0.005 Vrms.
Keep in mind the probe was designed for use above 1 KV.

Ron