Question
How can I measure the HV on the myQA CCU-X accurately? Why do I get deviations of more than 2% when measuring with a multimeter?
Answer
When measuring the high voltage (HV) on the myQA CCU-X with a standard multimeter, deviations of more than 2% may occur. The same measurement method shows smaller deviations on the older CCU because its circuit design is different.
This difference results from the new design of the myQA CCU-X. In the older CCU, the circuit design allowed a higher current during HV operation. To meet electrical safety requirements, EMC regulations introduced a current limit for HV. As a result, the electrometers were fitted with a different resistor to reduce current, which increases the measurement deviation when using a standard multimeter.
If you measure 500 V on an electrometer with a multimeter that has an input resistance of 5 MΩ, a load error occurs. Because the multimeter loads the source, the displayed voltage will be lower than the electrometer’s actual open-circuit voltage.
Cause of the discrepancy
Electrometers are designed to measure voltage with almost no power consumption and therefore often have a very high internal source resistance. When a multimeter with an input resistance of only 5 MΩ is connected, it forms a voltage divider with the electrometer’s internal resistance. This causes the measured voltage to drop below the true value.
Calculating the discrepancy
The voltage displayed on the multimeter can be calculated using the following formula:
with:
Open-circuit voltage of the electrometer (500 V)
Input resistance of the multimeter (5 MΩ)
Internal resistance of the electrometer
Recommendation
For more accurate HV measurements on the myQA CCU-X, we recommend using a multimeter with an input resistance greater than 10 GΩ.