About Zener Diodes
Zener diodes are for voltage regulation tasks. as discrete components, and also ICs that a reference voltage. Zener diodes (also voltage reference diodes) act like a silicon diode forward direction, but are to break down at a voltage when subjected to a reverse voltage.
All diodes will do this, but at voltages unpredictable and too high for voltage regulation tasks. two effects in Zener diodes … Impact ionisation (also avalanche breakdown)

Zener breakdown
Above . Volts, the zener is predominant, with avalanche breakdown the at lower voltages. no intention to go into , deal of on the Net (See References) for know . the two effects have opposite thermal characteristics, zener diodes at close to .5V have stable performance with respect to temperature.

Zener Diodes
For that I , no on the Net on use a zener diode. Contrary to what , limitations on the usage, and if these observed, the performance worse than expected. Figure 1 shows the characteristics of a zener, but as with all such diagrams omits .

So, what’s missing? The missed is that the slope of the breakdown section a straight line. Zeners have ‘dynamic resistance’ (or impedance), and this is that be when designing a circuit a zener diode.

The actual voltage where breakdown is the knee of the curve, and at this region the voltage is unstable. It varies with current, so that the zener is operated above the knee, where the slope is most linear.

Some sheets will give the figure for dynamic resistance, and this is at 0.25 of the maximum rated current. Dynamic resistance as low as a couple of ohms at that current, with zener voltages – 6V giving result. Note that this coincides thermal performance .

This is all and , but dynamic resistance? the ‘apparent’ resistance be measured by the current. This is explained with an example. Let’s assume that the dynamic resistance is quoted as 10 ohms for a zener diode. If we vary the current by 10mA, the voltage across the zener will by …

V = R * I = 10O * 10mA = 0.1V (or 100mV)
So the voltage across the zener will by 100mV for a 10mA in current. that not like 15V zener , it represents a error. For this reason, to feed zeners in regulator circuits from a constant current source, or via a resistor from the regulated output. This minimises the current variation and improves regulation.

Manufacturers’ sheets will specify the dynamic resistance at the knee and at a specified current. worth noting that the dynamic resistance of a zener be as low as 1. ohms at 25% of maximum current, it over 500 ohms knee, just as the zener to break down. The actual figures vary with breakdown voltage, with high voltage zeners dynamic resistance (at all parts of the breakdown curve) than low voltage units. Likewise, power parts will have a lower dynamic resistance than low power versions (but current to reach a stable operating point).

, to at the maximum current for a zener, and establish a rule of thumb for optimising the current for performance. Zener sheets give the maximum current for voltages, worked out …

I = P / V where I = current, P = zener power rating, and V = zener voltage rating.
, a 27V 2W zener can carry a maximum continuous current of …

I = / 27 = 0.074A = 74mA ( a ‘fudge factor’, and at 25°C)
As noted ‘transistor assisted zener’ app note (AN-007), for optimum zener operation, the current to a maximum of 0.7 of the claimed maximum, so a 27V/2W zener not be run at than about 47mA. The is about 25% of the maximum (17mA close , reference says 18.5mA), as this minimises wasted and ensures that the zener is operating linear of the curve. zener table below, see that the test current is 25% and 36% of the maximum continuous current. The wise reader will that this range has been chosen to show the diode light, and is the operating current .

none of this is , it does show that bit (not so) humble zener diode than beginners (and ) tend to realise. Only by understanding the component can you get performance from it. This only apply to zeners – most (so ) components have characteristics of which are unaware.

a zener is the as a diode, except that it has a defined breakdown voltage far lower than any rectifier diode. Zeners are connected with reverse polarity compared to a rectifier diode, so the cathode (the terminal band on the case) connects most positive point circuit.