E.H.T. supply

EHT supplies are designed to provide a high voltage to a high resistance load (e.g. cathode ray tube). If the terminals or wires connected to the EHT were accidentally touched this could provide an electric shock. One way to deal with this is to connect a large resistance in series with the output (positive) terminal. If the terminals are shorted (e.g. by contact through a person) the current drawn is limited to I = E / r.

Typically, an EHT supply (up to 5000 V) is protected by a 5 MΩ resistor so the maximum current if shorted is just 1 mA. EHT supplies often have a further ‘safety resistor’ (e.g. 10 MΩ) to reduce the maximum current still further. This resistor can be by-passed when necessary.