Intro to making use of Zener Diodes As Voltage References

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Zener (a.ka. avalanche or breakdown) diodes are p-n junction semiconductor devices designed to function in the reverse breakdown location of their VI characteristic curve. By maintaining their inverse current within specific limitations, the voltage drop across the diode will remain constant. If kept in this manner of operation, the diode will act like a voltage reference.

Examine the specifications sheet for a typically avalanche diode - Vz is the Zener voltage calculated at a given test current Izt. The joint current Izk, is the minimum current through the apparatus to maintain a constant Vz. Since the dynamic resistance at the leg of the of the reaction curve is high, if current through the device is below Izk, regulation may be poor. Izm is the maximum Zener current that may be passed without exceeding the maximum allowable power dissipation. Break the device and this may turn into smoke before your eyes.

For operation as a current guide, a zener diodes should be reverse biased. If the invert voltage is smaller than Vz only the normal diode reverse saturation current is permitted to flow. When forward biased it acts like a typical silicon diode - a big forward current flows, and the forward diode voltage is typically 0.7 V. Avalanche diodes are available with maintaining voltages from several voltages to several hundreds volts and with energy dissipation ratings up to 50 T.

Change in reference voltage per centigrade degree change in diode temperatures. percent the heat awareness of a zener diodes (temperature coefficient) is given as a. This amount is usually within the range of /- 0.1 % per deg C. The path of the change is linked to the mechanism of breakdown (avalanche multiplication versus Zener breakdown). Usually, if the reference current is above 6V the coefficient is good, if below, negative. Some manufacturers have made temperature compensated reference diodes by mixing a positive temperature coefficient breakdown diode with a forward biased, negative temperature coefficient, silicon diode in a single bundle (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 often easier to place multiple breakdown diodes in sequence when designing a large voltage guide. This combination permits higher voltage, higher power dissipation, lower temperatures coefficient, and lower dynamic opposition (the reciprocal slope of the volt-amp curve in the working region). Of course, this is a more costly solution than a solitary diode.

Frequently, noise in signals can trigger unwanted triggering of delicate circuits. If the sound voltage (Vn) is smaller than a diode forward voltage drop (Vf) and the signal voltage (Vs) is larger, a pair of parallel connected, reverse polarity (cathode of one attached to the anode of the other) diodes in series with a suitable resistor (R) can be used to eliminate the unwanted section of the input signal. Since the noise voltage is not large enough to forward bias either diode between signal mountains the output will be zero, creating a dead band of / - Vf around ground level. When Vs forward bias either diode the output current will be (Vs - Vf)

If Vn is too big for normal diodes, two zener diode voltage regulator put cathode to cathode are frequently replaced. The Zener voltage (Vz) is chosen to be more than the noise voltage. When the input signal goes positive, one diode is forward - biased, while another inputs breakdown mode. When the signal is damaging, the roles are reversed. The dead area is /- (Vf Vz), only signals greater than this will be handed to the output.

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