1. Field of the invention
The field of the invention is AC digital voltmeters which derive their operative power from the AC voltage being tested, and not from a separately supplied external electrical energy source.
2. Description of the related art
Conventional precision digital AC voltmeters require that an external or internal source of electrical power be connected with the voltmeter in order to power the internal operative electronic circuits. There are direct reading AC and DC voltmeters which do not require an external power source, but read direct from the line being measured, such as when the line voltage to be read deflects a galvanometer type needle. To the Applicant's knowledge, however, all available digital AC voltmeters utilize an internal or external power source as their source of operative electrical power.
Those AC and DC analog voltmeters which operate with a galvanometer type meter action do suffer from the shortcoming in that they are of rather low input impedance, and require a substantial amount of current to deflect the meter. Consequently, these meters tend to pull down the voltage which is being measured such that, except for the cases where the output impedance of the device supplying the AC or DC voltage to be measured is extremely low (relative to the meter input impedance, such as the 60 Hz home or factory electrical power source), the voltage reading is distorted and a true and accurate AC or DC voltage is not displayed.
Other problems inherent with analog and digital meters known to the Applicant, aside from their lack of accuracy, are such things of bulkiness, fragility, and paralax in meter reading. Additionally, many conventional available analog and digital meters require an additional power source to illuminate its display for night viewing. Further, if a digital meter is to operate at sub-zero temperatures, conventional digital voltmeters require an external source to maintain temperature of certain elements of the voltmeter.
Now it would be possible to construct a digital AC voltmeter which utilizes the line voltage being measured to power the electronic circuits of the voltmeter, however, conventional design dictates that the AC input being measured also be directed to a transformer for rectification to supply the necessary DC voltage to operate the meter. However, digital AC voltmeters constructed along these lines suffer from at least three major shortcomings, the first being that the transformer presents a relatively low input impedance to the line being measured, and therefore becomes a drag on the AC voltage input, such as to distort the true voltage reading. Secondly, transformers are inherently frequency sensitive. A transformer which operates at maximum efficiency at 60 Hertz has fallen off considerably at, for example, 1000 Hertz, and may be totally inoperative as a transformer at 10 kHz. Lastly, transformers are bulky and heavy, making the AC digital voltmeter heavy and detracting from the ease of operation.
It is overcome these shortcomings that Applicant's inventive transformerless line powered AC digital voltmeter is directed. Accordingly, it would be useful to provide the accuracy of digital techniques by a transformerless line powered AC digital meter which obtains its operative power from the AC line being measured, but yet is of a high input impedance and draws very little current from the line being tested, while at the same time is compact, avoids paralax problems inherent with analog type display meters, and is highly accurate and very light weight.