This section is showing you how to calculate the change in voltage across a zener diode when the current into the zener diode is varied, using the zener diode's dynamic resistance defined at a particular current.
This shows you how stable the zener voltage will be when the current varies. Generally, the zener current will vary because the input voltage (to the zener regulator) varies, and this causes a varying voltage drop across the series resistor, which causes the zener current to vary.
Ideally, the zener voltage would be absolutely steady regardless of variations in the zener current, but zeners are not perfect, and this variation in current will produce a small variation in voltage.
The quantities in the formula are:
- delta V is the change in voltage across the zener as a result of the change in zener current, in volts;
- Rdyn is the dynamic resistance of the zener diode at a specified current of 10 mA, in ohms;
- delta I is the change in current that the zener will experience, in amps ("I" in formulas is always in amps, not milliamps or microamps).
In the calculation section of the formula:
- 10 is Rdyn, expressed in ohms;
- 0.1 is the change in current (10%) expressed as a decimal;
- 0.01 is the actual zener current, 10 mA, expressed in amps;
The "0.1 x 0.01" section of the formula is calculating the change in zener current. The change is defined as a 10% change in a 10 mA current, so the actual amount of the change is 10% x 10 mA which is 0.1 x 0.01 amps.
The result of the formula is 0.01V which is shown at the end as 10 mV.
This means that if the zener is operating at 10 mA current and the current is increased (or decreased) by 10%, the voltage will increase (or decrease) by about 10 mV.
If the nominal zener voltage is 5V, can you calculate the error that a change of ±10 mV represents, as a percentage?