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Reading Zener datasheet

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Full Member level 6
Aug 1, 2009
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I am designing a voltage regulator using zener ofcourse

Specs as follows

Input range: 5.5V to 10V
Output: 3 to 3.5 is fine
Output current needed: 5 to 20-25mA

I am trying to get the following data out of datasheet

Vz = 3.1 to 3.5V (I got that)
Max Power = 250mW @ 25 deg cel and less ( I got that)

1. I would be operating at say 50 deg max and hence need to know deteriorated power (Pdmax - deltaT). Dinot find the power deteoration per degree rise

2. How much is knee current

3. How much is max reverse current Iz max

The datasheet I am referring given in the link below

**broken link removed**

I would be using BZX84-C3V3 (5% variation part)

when Input voltage = 5.5v,output voltage = 3.5v, output current = 25mA, Iz=5mA
you need a resistance about 66 ohm.
when Input voltage = 10v, Izmax = 100mA. Pdmax = 350mW
thermal resistance from junction to solder point is 330K/W ,temperature rising is 0.35*330 = 115.5K
50deg+115.k is over the junction temperature.
so I think the solution is fault.

It's no good idea to implement the regulator with a zener diode only. The series resistor must be chosen for minimal input voltage and will dissipate a lot of power at maximum input voltage. The zener diode power rating would be most likely exceeded. You better use a zener diode/transistor combination.

The datasheet shows that a 5.6V zener diode holds almost the same voltage when its temperature changes but a zener with a lower voltage has its voltage drop when it heats and a zener with a higher voltage has its voltage increase when it heats. Your zener will get extremely hot.
The datasheet shows that a zener diode has a resistance that ruins its voltage regulation when its external resistor has a low value. It should be fed from a constant current source like in a voltage regulator IC.

Well... I now have a input from adapter (say 7.5V)

I am, for time being not considering battery change :) hence adapter voltage can be considered @ 7.5V

and when battery used (2 cell in series): Can vary from about 5.5 (lower cuttoff- Not sure about this, hence using this as reference as of now) to 8.4 (higher cutoff)

With these two voltages possible, I want have a small circuit that can go to 3.3V that will run IO pin (10mA max I guess)

This is where I am trying to use this circuit.

Any other reliable options, do suggest... even if its beyond zener

A 7.5V adapter produces 7.5V ONLY when it has a voltage regulator circuit in it or it has its rated load current. It might be 14V if the load current is low.
We are wondering why you don't use a cool voltage regulator IC instead of a zener diode that wastes power by getting hot.

You talked about a 2-cells battery that is 8.4V maximum that must be a Lithium rechargeable battery. Its voltage drops to about 6.4V when its load should be disconnected or it will be ruined.

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