50hz Calibration aid for Multimeters

Posted on Feb 6, 2014

The purpose of this prototype unit does away with the need for an oscillator. It offers a mains-derived and fully variable 50 hz AC volts down to millivolts for comparison and to check accuracy, of two or more parallel connected multimeters. Sometimes older instruments of this type are favoured to respond better at low frequencies than higher audi

50hz Calibration aid for Multimeters
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o frequencies, which are better suited for measurement by wideband audio millivoltmeters. It is to be called to attention that unless otherwise stated by the manufacturer, both mechanical and digital multimeters display all pure and irregular wave shape inputs including square, triangle, sawtooth and integrator, as readings in Mean or Average Form-factor. Obviously the higher the mains transformer secondary voltage output delivered, the higher the AC switched ranges or auto-ranging which can be measured. The choice of transformer and resistor attenuator values can thus be adapted to suit individual voltage range requirements. Caveat: To comply with Health and Safety, ideally a small adhesive warning sign should be affixed to the box lid to state "Danger Mains Inside". The prototype does not have one displayed as not being available. However in this case a screwdriver is required to open the lid, therefore that situation is in my control and at my risk. AC feeds a multiturn pot VR1 which sets the reduced base voltage and then in series to two tandem-wired 1k lin pots labelled coarse and fine. For convenience of adjustment nearer to the hole in the box lid, VR1 is recommended to be supported on the same tag strip. So to improve the waveform to a more acceptable sinusoidal shape, this can be further low-pass filtered by a single 2. 2 mfd or two 1mfd polyester capacitors wired in parallel across the two pots combination as...

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