Laboratory Standards of Emf(Zener controlled voltage sources)

In the past standard, cells were used exclusively as laboratory standards but in recent years. Semiconductor devices such as Zener-controlled reference sources have replaced standard cells in practically all industrial applications.

Silicon diodes have voltage-current characteristics such that an extremely sharp reverse current occurs at a point on the voltage curve known as "Zener voltage". 

The point indicates a breakdown of the diode under reverse voltage application, but the process is reversible if safe current and heating limits are not exceeded. 

The Zener voltage may be controlled over a wide range by the processing techniques used during the manufacture of the Zener diodes.

In the fig, the circuit is of Zener diode in a typical application as a laboratory standard.

• he supply voltage E is usually much higher than the Zener voltage of the diode assuming the breakdown of the diode.
• resistor R usually has a high value and is placed in series with the diode and the battery. It serves to limit the current through the diode during breakdown to a safe value.
• In addition, when the supply voltage varies for any reason, most of this variation is taken up in the voltage drop across R with a very small change in diode current and as a result, the reference voltage ( output voltage) remains practically the same.

        

Commercial voltage standards using Zener diodes essentially consist of :

(i)    A Zener controlled voltage source placed in a temperature-controlled environment to improve its long-term stability.

(ii)    A precision output voltage divider. The temperature is controlled to +0.03 degree Celcius over an ambient temperature range of 0 degree Celcius to 50 degree Celcius providing output stability of the order of 10 ppm/month.

The four available outputs are :

(a) a 0 -1000 microVolt source with 1 microVolt precision,

(b) a 1.00 V reference for potentiometer measurements with ratio box,

(c) a 1.018+(delta) reference for saturated Weston cells comparisons.

(d) a 1.019+(delta) reference for unsaturated Weston cell comparison.