# How to Measure a High Voltage With an Oscilloscope

By John Papiewski

An oscilloscope is a piece of electronic test equipment that graphically displays waveforms and other changing voltages over time. Scientists and technicians use them to measure many kinds of electronic signals. These sensitive devices generally measure in a range of between a few thousandths of a volt to a few hundred volts. To safely measure higher voltages, connect a high-voltage probe to an oscilloscope. The probe has a long insulated handle and contains a voltage divider, which scales high voltages down by a factor of 1,000, permitting measurements up to 40,000 volts.

Plug the high-voltage probe's dual male banana connectors into the female banana to BNC adapter.

Twist the BNC adapter onto the oscilloscope's male BNC connector.

Connect the high-voltage probe's ground clip to a solid grounding point on the high-voltage equipment.

Turn the high-voltage equipment on, then turn the oscilloscope on. Set the oscilloscope's vertical sensitivity to one volt per division. Adjust the vertical position so it matches the zero point on the scope's display. Set the horizontal sweep rate to a time scale suitable for the high-voltage equipment.

Grasp the probe by the plastic handle at the end opposite the metal tip. Touch the probe's metal tip to the high-voltage point you want to measure.

Observe the waveform on the oscilloscope and count the vertical divisions above the center line where the waveform peaks. Multiply this by one volt per division. Multiply again by 1,000 to obtain the true reading scaled by the probe. For example, if the waveform peaks at 2.5 divisions above the center line on the oscilloscope, 2.5 x 1 volt per division is 2.5 volts. Multiplied by 1,000 gives a true reading of 2,500 volts at the probe tip.

#### Tip

If the top and bottom of the waveform do not fit on the screen, turn the vertical gain knob to a less sensitive setting, such as 5 volts per division.

#### Warning

Always pay close attention when working with high voltages, as they can be lethal. If possible, work with a partner or under the guidance of a supervisor who is familiar with high-voltage safety.