Electronic – Zener diode as voltage regulator

voltage-regulatorzener

I've been trying to make a voltage regulator using a zener diode.

I used the BZX79 3.3V Zener diode.
I connected the diode as the following:

Schematic

Data Sheet. I assumed that as as most of them it operates at around 1mA to 10mA safely in the break down region.

I connected my supply which was 5V. The resistance was chosen to be 220 ohms. This will make the max current of the zener around 7mA. Which should be safe.

Now, when the zener was unloaded, I got a smooth 3.3V as an output.

But, when I loaded the zener diode, the voltage across the diode dropped to 2.4V.

Why did that happen?

My assumption is the following:

The load which should operate at 3.3V, is consuming way too much current that the zener current is below the min current. This caused the zener to get out of the break down region.

Is my deduction correct? And what are available solutions, taking in note that I've used probably the smallest resistance possible that is 220 ohms. (At least of what I have).

I also have a supply of 8.4V. Will it work if I connected it directly to the supply?
At the end I will have more current available to the zener even after loading it.

Edit:

The data sheet states that the zener is fine for 6 Amps. That is a huge and weird number isn't it?

Datasheet excerpt

Best Answer

I connected my supply which was 5v. The resistance was chosen to be 220ohms. This will make the max current of the zenor around 7mA.

This means that if your load requires more than 7 mA (I actually calculate 7.7 mA, not 7 mA), the resistor will drop Vout below 3.3 V. In that case the zener will no longer be regulating.

But, when I loaded the zener diode, the voltage across the diode dropped to 2.4v.

Why did that happen?

Your load was taking more than 7.7 mA.

Given that the output voltage dropped to 2.4, we can calculate that the load current was (5-2.4)/220 = 11.8 mA.

You could make this work by reducing your series resistor. If the load is fairly stiff, reducing the resistor to 110 ohms or so should bring you back into regulation. This assumes the load current doesn't increase to more than 15 mA when the supply voltage is raised to 3.3 V.

If it's possible to have an open-circuit condition at the load, you need to choose a zener that can take all the current that would normally go to the load.