# Calculating the minimum and maximum $R_{load}$ for a zener voltage regulator So, I have a generic zener voltage regulator with $$\R_{s}=650Ω\$$, $$\V_{in}=15V\$$, I'm using the zener BZX55C8V2 (datasheet). From the datasheet the test current is $$\5 mA\$$ and the maximum zener current is $$\74 mA\$$. I want to find out the minimum and maximum for the load resistance so that the Zener is still regulating at $$\V_{out}=8.2V\$$ and without the zener breaking.

I'm wondering how exactly I should utilize the datasheet to figure out the values for the load.

• If the output is 8.2V you have 6.8 V across Rs making 10 mA flow. That 10 mA can either flow completely through the zener diode or (almost) completely through Rl. So what does that mean for Rl? You use the datasheet to check that the current and power dissipation in the zener diode do not exceed the maximum ratings. – Bimpelrekkie Jan 29 at 13:02

## 1 Answer

The maximum load resistance of (say) infinity will push 10.5 mA into the zener. This is because 6.8 volts will be across Rs and, as Rs is 650 ohms, there will be a current into the unloaded zener of 10.46 mA.

The minimum load resistance is that value of load resistance that forms a potential divider with Rs to produce 8.2 volts at the output with the zener removed so, RL will be 784 ohms because 784/(784 + 650)*15 = 8.2 volts.

• So in other words, it has no outright maximum load just based on the datasheet, but the minimum load would be 784 Ohms(?) – Grak Jan 29 at 13:28
• If you go lower than 784 ohms, the zener is not shunt regulating anymore. In practice, there is a shady area around this point because a zener diode doesn't stop conducting immediately the voltage falls below 8.2 volts. – Andy aka Jan 29 at 14:26