Posted by Temporary voltage or current spikes, or transients, can cause circuits to experience anything from little malfunctions to complete breakdowns. The magnitude and duration of a voltage transient can range from nanoseconds to hundreds of milliseconds, and they can be anything from a few millivolts to thousands of volts. While ESD episodes are more irregular, inductive ringing in a motor produces recurrent transients.
Zener Diodes
The breakdown voltage of a Zener Diode 50W is lower than that of a regular diode because of its unique construction. Current flows through these diodes to maintain a voltage near the Zener breakdown voltage, which displays a controlled breakdown. This quality is why Zener diodes are great for making reference voltages.
A common purpose for Zener diodes is to shield circuits from electrostatic discharge (ESD) and other overvoltage transients.
Zener Diode Transient Suppressors
Because they function normally when biased forward but malfunction and conduct when biased backwards, more zen diodes are useful for protection on DC supply (unidirectional). Therefore, the reference or clamping voltage level can be the reverse breakdown voltage (VZ) of a Zener Diode 50W.
Upon inversion and at voltages lower than their breakdown voltages, VZ Zener diodes show low leakage current and high supply impedance. The Zener begins to break at voltages higher than its Zener voltage, allowing a very low impedance path to the over-voltage transient through its conduction, which grows gradually with rising voltages across it.
Zener Transient Suppression
The Zener diode is “invisible” when connected to a supply or the protected components until a transient voltage shows up; it has a high impedance below its reverse breakdown voltage and a low impedance above it.
During the breakdown mode of operation, the diode immediately clamps the excess voltage to limit the spike to a safe level. Once the transient voltage drops below the Zener voltage, VZ, the diode resumes its usual operation, effectively suppressing the transient. Clamping voltage (VC) is thus equivalent to the reverse breakdown voltage (RBV) of the Zener Diode 50W. The zener diode’s clamping properties make it useful for suppressing transients by diverting harmful currents from the protected load.
Approximately proportional to the junction area of the 50w Zener diode is its power capability and surge current. Most zener diodes are optimize for use with very low voltage and power. Avalanche diodes are a type of Zener diode that can withstand bigger surge currents and function at higher voltages without damaging themselves.
As we mentioned before, the forward biassed diode properties of a single zener diode limit its employment to transient suppression on steady-state DC supply. Two Zener Diodes connected “back-to-back” allow us to harness the clamping characteristics of a bidirectional AC supply.
Zener Transient Suppression
Two Zener Diodes connected in series allow us to shield the positive and negative halves of the cycle from overvoltage transients in turn.
When the reverse breakdown voltages of two zener diodes are identical, a transient voltage of either polarity can be clamp at the same zener voltage level because one 50w zener diode will be in reverse bias mode and the other will be in forward bias mode.
A transient voltage suppressor (TVS) device is excellent for AC power applications since it has opposing junctions incorporated into a single device, as opposed to using two back-to-back zener diodes, which is one method for transient suppression of an AC supply. A wide range of voltage and power levels is available for bidirectional avalanche diodes.
Zener Crowbar Transient Suppressors
Adding a 50w zener diode to the basic crowbar circuit to detect an excess voltage condition improves its performance and transient sensing.
Zener Crowbar Clamping Circuit
This Zener crowbar circuit is an upgrade over the standard voltage divider network, but it has a sluggish turn-on since the Zener breakdown voltage is curved, not sharply rising. The original crowbar circuit may be tweaked and made better by using an operational amplifier (op-amp) to boost the voltage of the detection and triggering circuit.
To counteract transients and surges in voltage in both directions, thyristors equipped with an over-voltage trigger have been developed. For example, the RCA SK9345 series of integrated circuit crowbars is made to safeguard 15-volt power supplies, the SK9346 series protects 112-volt supplies, and the SK9347 series safeguards 115-volt supplies.
A Zener diode, transistors, and a solid-state rectifier (SCR) are all incorporated into one circuit. The MC3423 single-integral crowbar sensing circuit may detect voltage spikes when coupled with an external crowbar SCR.
Conclusion
Clamping the voltage to a safe level is the primary goal of a voltage suppression circuit. Metal oxide varistors (MOVs) and Zener Diodes are the most popular types of voltage clamping devices. While zener diodes work well with smaller, lower-energy DC sources, MOVs are better suite to protect bidirectional AC power supplies.