Rectifier Voltmeters

Rectifier Voltmeters Points : Rectifier Voltmeters, Definition The arrows indicate the direction of current. It is obvious that direction of current in the instrument is same for both conditions i.e. when terminal “X” is positive and terminal “Y” is negative and vice-versa. The permanent magnet moving coil (PMMC) instrument reads the mean values but the instrument is calibrated to read rms values assuming the sinusoidal shape for which form factor is 1.1 1. For example if the instrument circuit resistance is 0.5 M Q and the applied voltage is 500V (rms), the rms current will be 1.0 mA, the mean current will be 0.9 mA, and the 0.9mA point on the scale by marked as 500V. It will be apparent that the scale rating of the rectifier instrument will be in error for any type of a.c. wave other than a sine wave unless the form factor happens to be 1. 11.

The current-voltage characteristics of rectifiers are non-linear. At low values of forward current, the rectifier operates in an extremely non-linear part of its characterizes curve and the resistance is large as compared to that at high current values. In addition, the resistance in the reverse direction is not infinite, and there will be a small current in the reverse direction. Thus the effective resistance of the instrument circuit varies with strength of current following, and the scale will not be uniform. From it follows that, for a low range voltmeter whose series resistance is not large in comparison to that of the rectifier bridge, the lower part of the a.c. scale is often crowded. Most of the manufactures provide a separate low-voltage scale, calibrated especially for this purpose. The variation in rectifier resistance may lead to difficulties in designing multi-range rectifier instruments incorporating low range. The high resistance in the early part of the rectifier characteristics also limits the sensitivity which can be attaifled in micro-ammeters and voltmeters.

The resistance of the rectifying element falls with the increase of temperature. However, the accuracy of the instrument is ordinarily satisfactory under normal working conditions at room temperature and usually is of the order of about’±5% of full scale deflection for sinusoidal wave-forms. The effect of temperature variation is very serious either at very high or at lower temperatures. This may be compensated by using copper (which has +ve temperature coefficient) for the construction of a small portion of the series resistance, the remainder being constructed of eureka. In case of large temperature variations, the instrument should be enclosed in a temperature-controlled box.

Since the rectifying action depends upon the frequency to some extend due to capacitance effect and rectifier instruments show lower readings. The instrument readings may be in error by as much as 0.5% decrease for every 1 KHz rise in frequency.

The rectifier instruments sensitivity is lower on a.c. operation than that on d.c. operation. In case of full rectification the sensitivity on ac. is 90% of that with d.c. and only 45% of t that with d.c. while using half-wave rectifier.