Author Topic: Why is the voltage across my load so far off from the 3V zener diode “regulation  (Read 488 times)

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Offline renzoms

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I used a 3V .500W Zener diode 1N5225B for it's voltage regulating abilities. Here is a link to a 47s video including schematics, results, and explanations in voice.
The circuit is Vin, a resistor in series with the Vin (330 ohm), and then the 3V Zener diode and a load (resistor) in parallel.
The Vin ranges from 5V to 9.6V. The load ranges from 330 ohm to 3300 ohm to 33000 ohm. The maximum expected voltage "differentiation" (how far the measured results are from the expected results) is .39V.
For the 3300 ohm and 33000 ohm load the expected result is 3V at 9.6V Vin and 2.61V at 5V Vin. The measured results are 3.01 V, 2.58 V, 2.99V, and 2.61V, respectively. Great!
For the 330 ohm load, the results are 2.81 V and 2.16 V.
Why are the measured results so far from the expected results for the 330 ohm load?

Offline T3sl4co1l

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Low voltage zener diodes (under 5V) have a very soft characteristic.  You need to run it at a lot of bias current to stay close to the rating (note it's measured at 20mA).

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Offline blueskull

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At below around 5V, the Zener effect is dominant. At above around 5V, the avalanche effect dominates. The so called higher voltage "Zener" diodes are actually avalanche diodes.

A Zener diode works by using high electric field to excite tunneling effect. The field may be strong (due to thin depletion region, thus higher E field), but the voltage is low.

An avalanche diode uses Zener current or other stray current as initialization source, accelerated by a high voltage, the the accelerated carrier (electrons) knock more carriers out upon impact, creating more carriers, thus the name avalanche.

Therefore, avalanche effect happens only at higher voltage -- the energy after acceleration by the voltage must be high enough to knock electrons from silicon.

Because Zener effect doesn't multiply, its resistance kinda stay constant (after breakdown). Since avalanche effect knocks out more carriers as current increases, its resistance decreases, thus maintaining a lower resistance as current increases, maintaining a constant voltage.

They can be compared with Schottky diodes and PN diodes in forward mode.
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Offline Gyro

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You might find that you get better regulation with a series chain of 4 forward biased silicon diodes instead.

Alternatively, you could go the whole hog and use a 3V low-dropout linear regulator. It would be more power efficient than a zener shunt too.

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Offline w2aew

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As @blueskull and others have mentioned, low voltage zener diodes have a fairly soft knee in the IV characteristic due to the zener effect.  Higher voltage zeners are actually much sharper due to the avalanche effect.  I did a video on zeners that you might be interested in.  In particular, check out the curve tracer results starting at about 11:45 in the video:

« Last Edit: October 14, 2019, 04:22:17 pm by w2aew »
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Offline SeanB

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A cheap reasonably good 3V zener is a blue or white LED, though it has a rather soft knee it is a lot sharper than most zener diodes at low current, and also makes a handy power on indicator, though you are best keeping it shadowed from light, as it also can be a photoelectric generator as well.
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