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Zener Diode Operation Question
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rcbuck:
I am dropping 12V to a maximum of 3.3 volts to drive an input pin on a micro. I have two resistors setup as a voltage divider to create the 3.3 volt level. I wanted to use a zener diode for extra protection in case the lower divider resistor failed (unlikely).

With just the two resistors (photo 1), I get 3.56 volts at the resistors junction point. I wrongly assumed that when I put the zener from the junction to ground, the voltage would drop to roughly 3.3 volts. However, the voltage drops to 2.94 volts. The zener is a 2% device and 2.9 volts is outside the tolerance range. If I change the top resistor to 820 ohms, the junction voltage then rises to 3.286 volts. This confirms the zener does work.

So why does the voltage drop to 2.94 volts when there is only about 3 mA of current? Decreasing the top resistor value so there is roughly 10 mA of current and the zener works properly.

Does the operation I am seeing have something to do with the zener internal resistance? Does it behave like a resistor before it avalanches? The data sheet shows a internal impedance ZZT@IZT of 1600 ohms at a current of 0.25 mA. However, I would think this only applies to an AC signal? The part number of the zener is Mouser 863-MMSZ5226CT1G.

Last Edit: Changed ZZT@IZXT to ZZT@IZT
T3sl4co1l:
Check the V(I) curve of the diode, and check what current I_ZT it's measured at. :)

Tim
SiliconWizard:
Oh yeah. Real diodes are not ideal.
 ::)
rcbuck:
Tim,

Looking at Fig 9 in the data sheet it appears that any current over about 1 mA is a straight line for any zener voltage above 5 volts. Unfortunately there is no curve for the 3.3 volt part. Below 3 volts the curve is not straight line. Maybe I need to select a different zener.
Tomorokoshi:
It's better to characterize the Zener and then tune by selecting resistors than to select resistors and tune by selecting Zeners. Figure on +/- 2% to 5% for that part.

Then compare the actual range of the circuit with the Zener to the actual High and Low ranges of the processor. 2.94V may be perfectly within the High range.
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