Intro to Using Zener Diodes As Voltage References
Zener (a.ka. avalanche or breakdown) diodes are p-n junction semiconductor products designed to function in the reverse breakdown region of their VI characteristic curve. By keeping their reverse current within certain limits, the voltage drop over the diode will remain constant. If kept in this manner of operation, the diode will act like a current reference.
Analyze the specification sheet for a generally avalanche diode - Vz is the Zener voltage measured at certain test current Izt. The knee current Izk, is the minimum current through the apparatus to keep a constant Vz. Because the energetic resistance at the leg of the of the response curve is large, if present through the device is below Izk, regulation will be poor. Izm is the maximum Zener current that can be passed without exceeding the maximum allowable power dissipation. Violate the unit and this will turn into smoke before your eyes.
For operation as a current reference, a zener diode must be reverse biased. When the change current is smaller than Vz simply the ordinary diode reverse saturation current is allowed to flow. When forward biased it acts like an ordinary silicon diode - a large forward current moves, and the forward diode current is usually 0.7 V. Avalanche diodes are available with maintaining voltages from several voltages to several hundreds volts and with power dissipation ratings up to 50 Watts.
The temperature awareness of a zener diodes (temperature coefficient) is offered as a percent change in reference voltage per centigrade degree change in diode temperatures.. This amount is typically in the number of /- 0.1 % per deg C. The direction of the change is linked to the device of breakdown (avalanche multiplication versus Zener breakdown). Generally speaking, if the reference voltage is above 6V the coefficient is positive, if below, negative. Some manufacturers have produced temperature compensated reference diodes by merging a positive temperature coefficient breakdown diode with a forward biased, negative temperature coefficient, silicon diode in a single 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). Rather than use a single larger diode it is frequently better to place multiple breakdown diodes in collection when developing a high voltage reference. This combination permits higher voltage, higher energy dissipation, lower temperature coefficient, and lower dynamic resistance (the reciprocal slope of the volt-amp curve in the operating area). Of course, that is a far more expensive option than a solitary diode.
Frequently, noise in signals may trigger unwanted activating of delicate circuits. If the noise voltage (Vn) is smaller than a diode forward voltage drop (Vf) and the transmission voltage (Vs) is bigger, some of similar connected, opposite polarity (cathode of one attached to the anode of the other) diodes in sequence with an appropriate resistor (R) can be utilized to remove the unwelcome part of the input transmission. The result will be zero, creating a dead band of / - Vf around ground level since the noise current is perhaps not big enough to forward bias either diode between signal peaks. When Vs forward bias either diode the output signal current will be (Vs - Vf)
If Vn is too large for ordinary diodes, two zener diode voltage regulator placed cathode to cathode are often substituted. The Zener voltage (Vz) is chosen to be greater than the noise voltage. Once the input signal goes positive, one diode is forward - biased, while breakdown mode is entered by the other. If the transmission is damaging, the roles are reversed. The dead area is /- (Vf Vz), only signals higher than this may be handed to the output.