Lab guide
Why a zener regulator collapses under a heavy load
Working out the minimum load resistance and the zener and resistor heat from 1N4734 simulation results.
A zener regulator makes a steady voltage from one resistor and one zener diode. Load it too heavily (too small a resistance), though, and the output suddenly drops. Here is why, from a PSpice simulation with a 10 V supply, a 100 Ω series resistor and a 1N4734 (5.6 V) zener.
1. A zener turns on over a very narrow voltage
| Reverse voltage | Current |
|---|---|
| 4.0 V | 90 nA |
| 5.0 V | 29.4 µA |
| 5.2 V | 95.2 µA |
| 5.3 V | 171 µA |
| 5.4 V | 313 µA |
| 5.5 V | 1.75 mA |
| 5.6 V | 44.9 mA |
Below 5.4 V less than a milliamp flows, but from 5.5 V to 5.6 V, just 0.1 V, the current rises about 26 times. So the voltage hardly moves while the current changes a lot, which is what makes it a regulator.
2. Output versus load
| Load RL | Output | Load current | Zener current | R current (Vs − Vo) / R |
|---|---|---|---|---|
| 100 Ω | 4.999 V | 50 mA | 29.1 µA | 50 mA |
| 470 Ω | 5.584 V | 11.9 mA | 32.3 mA | 44.2 mA |
| 1 kΩ | 5.592 V | 5.59 mA | 38.5 mA | 44.1 mA |
| 3.3 kΩ | 5.597 V | 1.7 mA | 42.3 mA | 44 mA |
| none | 5.599 V | 0 A | 44 mA | 44 mA |
The current in R, (Vs − Vo) / R, stays near 44 mA, and the load and the zener share it: a light load leaves most to the zener, a heavy load takes most of it. With RL of 470 Ω or more, the load current ranged from 0 to 11.9 mA while the output moved only 15 mV (5.584 to 5.599 V), an output resistance of about 1.26 Ω.
3. The minimum load resistance
If the load resistance is too small, the divider formed by R and RL alone is already below 5.6 V, so the zener never turns on. The boundary is:
RL,min = R · Vz / (Vs − Vz) = 100 Ω × 5.6 / 4.4 ≈ 127 Ω
That is why the 100 Ω load pulled the output down to 4.999 V with only 29.1 µA left in the zener. A real design also keeps a minimum zener current (10 to 25% of the datasheet test current) so the zener stays firmly on. The zener resistor calculator works out the series resistor and the minimum load resistance under that condition.
4. No load is the hottest case
- Zener: with no load all of R's current goes through the zener: 5.599 V × 44 mA ≈ 246 mW, inside the 1N4734's 1 W rating.
- Resistor: (Vs − Vo)² / R ≈ 194 mW. That is 77% of a 1/4 W part, so use 1/2 W or more.
5. In your lab report
- Check that the load current plus the zener current equals (Vs − Vo) / R. In this data every row agrees to within 0.1 mA.
- When regulation fails, quote RL,min alongside it; it makes the discussion concrete.
Check divider voltages with the voltage divider calculator and resistor power with the Ohm's law calculator.