Intro to making use of Zener Diodes As Voltage References

Zener (a.ka. avalanche or breakdown) diodes are p-n junction semiconductor products designed to work in the reverse breakdown region of their VI characteristic curve. By keeping their inverse current within specific limits, the voltage drop over the diode will remain constant. If kept in this mode of operation, the diode will act as a current reference.

Analyze the specifications sheet for a normally avalanche diode - Vz is the Zener voltage measured in certain test current Izt. The joint current Izk, is the minimum current through the device to keep a steady Vz. Since the dynamic resistance at the leg of the of the response curve is large, if current through the apparatus is below Izk, regulation may be inferior. Izm is the maximum Zener current which can be passed without exceeding the maximum allowable power dissipation. Break this and the unit may turn into smoke before your eyes.

For operation as a current guide, a zener diode must be reverse biased. If the reverse voltage is smaller than Vz simply the ordinary diode reverse saturation current is allowed to flow. When forward biased it behaves like a typical silicon diode - a large forward current moves, and the forward diode voltage is commonly 0.7 V. Avalanche diodes are available with sustaining voltages from several voltages to several hundreds volts and with power dissipation ratings around 50 T.

Change in reference voltage per centigrade degree change in diode temperatures. percent the temperatures sensitivity of a zener diode voltage regulator (temperature coefficient) is given as a. This number is usually within the number of /- 0.1 % per deg C. The course of the change is connected to the system of breakdown (avalanche multiplication versus Zener breakdown). Usually, if the reference current is above 6V the coefficient is positive, if below, negative. Some manufacturers have created temperature compensated reference diodes by combining a positive temperature coefficient breakdown diode with a forward biased, negative temperature coefficient, silicon diode in one package (i.e. the generic 1N829, a 6.2V reference diode with a temperature coefficient of /- 0.0005 % per deg C over a range of -55 to 100 deg C). Instead of use a single larger diode it is usually better to put multiple breakdown diodes in sequence when developing a high voltage research. This mixture permits higher voltage, higher power dissipation, lower temperature coefficient, and lower dynamic opposition (the reciprocal slope of the volt-amp curve in the working area). Of course, this is just a far more costly option than a single diode.

Frequently, noise in signals can cause unwanted activating of delicate circuits. If the sound voltage (Vn) is smaller than a diode forward voltage drop (Vf) and the transmission voltage (Vs) is larger, a pair of simultaneous connected, opposite polarity (cathode of one attached to the anode of the other) diodes in series with an appropriate resistor (R) can be utilized to eliminate the undesired section of the input signal. The output will be zero, creating a dead band of / - Vf around ground level since the noise voltage is perhaps not big enough to forward bias either diode between signal peaks. When Vs forward bias either diode the output voltage may be (Vs - Vf)

If Vn is too large for normal diodes, two zener diodes placed cathode to cathode are often replaced. The Zener voltage (Vz) is chosen to be more than the noise voltage. Once the input signal goes positive, one diode is forward - biased, while the other enters breakdown mode. Once the transmission is damaging, the roles are reversed. The dead zone is /- (Vf Vz), only signals better than this may be handed to the output signal.