Non-loading digital voltmeter

A substantially non-loading circuit for measuring voltages in a range with relatively high upper limit, comprising:

• a voltmeter with an input circuit responsive to voltages applied to its input circuit to provide indications in digital form of that voltage, so long as those voltages are within a range with relatively low upper limit;
• potential divider means for applying to the input circuit of said voltmeter a fraction of the potential appearing between first and second nodes, which potential divider means includes
• a first resistance with a first end at said first node and with a second end, and which includes
• a second resistance with a first end connected at the second end of said first resistance and with a second end connected at said second node, said second resistance connecting across the input circuit of said voltmeter;
• means for applying the voltage to be measured between said first node and a third node;
• current monitoring means connected for generating an electric control signal responsive to the polarity and amplitude of the flow of current between said second node and said third node;
• and
• electrically adjustable voltage supply means applying its output voltage, the value of which is adjusted responsive to said electric control signal, between said first and second nodes in such poling that a feedback loop is completed which reduces the flow of current through the input circuit of said current monitoring means, said electrically adjustable voltage supply means including
• a source of alternating-current voltage of relatively low amplitude controlled by said electric control signal, including
• a step-up transformer having a primary winding to which said alternating-current voltage of relatively low amplitude is applied and having a secondary winding at which an alternating-current voltage of relatively high amplitude consequently appears, and including
• a high voltage module for converting said relatively high amplitude alternating current voltage to a substantially direct voltage, the output voltage of said electrically adjustable voltage supply means.