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salahaldeen

5 volt to 3.3 volt

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5V isn't a high enough input voltage for an ordinary 3.3V regulator. There might be a low dropout 3.3V regulator available.

Make a voltage divider with two resistors to get 4V. Then use an emitter-follower transistor to have an output of 3.3V at up to 200mA. The output voltage will change a little by the amount of load. Connect a resistor to the output so it always has a load.

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I think a couple of diodes wouldn't be bad.

You can convert logic levels if your stuck with a 3.3v logic device. Just power that device alone with a AA cell.

But you have to watch your logic IC's when appending transistors and resistors. Often you have to source and sink within 10mA, or just source less than 10mA if it's MOS. You can also sink with a MOS.

It's kind of funny with logic outputs, they are more typically designed to work with logic inputs, nothing else. If I add a weird load to the output, I can't easily determine the current at a particular voltage, or whether it will source, sink, or both. And you'll fry low power logic if not careful.

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A 3.3V zener diode doesn't regulate very well. A 6.8V zener diode regulates 11.5 times better.

The  diode in series with a zener diode reduces the amount of regulation.

A 3.3V zener diode's voltage decreases as it heats, the same as the diode in series with it. So the total voltage changes with temperature change. A zener diode with a voltage of about 5.1V is temperature stable.

A 3.9V zener would regulate better and wouldn't need the diode. The temperature effect on the zener diode would be cancelled by the temperature effect on the transistor if the transistor doesn't heat from the current very much.

A voltage regulator IC regulates much better than a zener diode and doean't waste as much power if the load current is reduced.

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A 3.3V zener diode's voltage decreases as it heats, the same as the diode in series with it.

Increasing diode current increases diode heat and diode voltage. If I apply heat to the diode, I think the current will be higher, which means the voltage will be lower. Applying heat is different from dissipating heat. If I char a resistor with a heat gun, it doesn't mean I'll develop a lot of voltage across it.

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The 3.3V zener diode has only 7.3mA of current so its 24.1mW of power dissipation won't heat it much. I was talking about an increase in the ambient temperature causing the zener's voltage to drop. Heat from dissipation is the same as heat from the ambient.

Here is a graph showing that the dynamic impedance of a 3.3V zener diode is poor, and another graph showing the temperature coefficient of zener diodes with different voltage ratings:

post-1706-14279143279053_thumb.png

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This is a static type application (regulated 5V to 3.3V), so exactly how does a 6.8V zener regulate 11.5 times better than 3.3V zener? Please explain why you think "dynamic impedance " comes into consideration here?

The magic number is 5.1V for a zero temperature coefficient. The series diode in this case hurts  regulation as  both zener and diode forward voltage are both going down... use the series diode when the zener has a positve temperature coefficient (i.e. >5.1V).

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Kevin,
Manufactures make registered (1Nxxxx) zener diodes with exactly the same spec's.
A 1N5226 3.3V zener diode from Fairchild has the same specs as one from Motorola (now ON Semi).
The way a zener is made (doped?) determines its voltage. Its voltage also indicates if it increases or decreases its voltage with temperature changes or is temperature stable and how well it regulates.

Indulis,
The graph of impedance I attached before shows that the dynamic impedance is 10 times better for a 6.8V zener diode than for a 3.3V one at 10mA and is much better at lower current.
Here is another graph that shows how bad is a low voltage zener diode. Its voltage changes so much with current changes that you can't even see its voltage rating.
The 6.5V and 7.3V ones show excellent voltage regulation when their current is changed.

post-1706-14279143279736_thumb.png

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Audioguru

Before I reply, please give me your definition of “static and dynamic” circuit behavior, and into which catagory you feel this particular circiut falls. Again, the 3.3V is being derived from a regulated 5V.

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There seems to be a problem with arun's circuit: It produces a regulated output at 2.42V and not 3.3V!! This is because the 3.3V zener used (1N4728) shows a voltage drop of 2.43V instead of 3.3V; maybe someone can explain this

post-1706-14279143280272_thumb.jpg

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Audioguru

Before I reply, please give me your definition of “static and dynamic” circuit behavior, and into which catagory you feel this particular circiut falls. Again, the 3.3V is being derived from a regulated 5V.

Arun's circuit gives the zener diode and transistor plenty of current so changes in load current don't cause changes in zener current.
The input voltage is fixed so it doesn't change the zener current.
So the zener should provide pretty good voltage regulation (as shown by Zeppelin) if its temperature doesn't change.

Here is a list of 3.3V zener diodes:
3.3V half-watt zener diodes are rated at different currents:
1N4684= 50uA
MZ4620= 250uA
1N5988= 5mA
1N746 and 1N5226= 20mA
Plus many European types.

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Audioguru

While you didn't exactly answer the question directly, your reply kinda has the answer if you "read between the lines".  OK, now let gets back to why you think  "dynamic impedance " comes into play in this particular circuit, as in your reply, you basically stated that the circuit DOESN'T have "any" (virtually non-existent) dynamic properties? Now if the regulated 5V feeding the circuit wasn't so regulated... that would be a different story as there would now be some "dynamic" circuit behavior.

I misspoke earlier in regards to not using the series diode with zeners below 5.1V. The diode also does a "good" thing!! The diode in series with the zener diode has the effect of cancelling changes due to Vbe changes with  temperature. 

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I misspoke earlier in regards to not using the series diode with zeners below 5.1V. The diode also does a "good" thing!! The diode in series with the zener diode has the effect of cancelling changes due to Vbe changes with

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Audioguru

First you said

A 3.3V zener diode doesn't regulate very well. A 6.8V zener diode regulates 11.5 times better...


Then you brought "dynamic impedance " into the picture for some reason (NEVER applied to this circuit) and now your saying...

The zener diode isn't regulating anything because the input voltage is regulated...



If the zener isn't regulating anything, then what is controlling the output voltage??? Whether the load is changing or not in this circuit is irrelevant. The point was that because the input voltage was regulated and the typical Hfe of a small signal transitor is high enough to not affect the the current in the zener current limit resistor, ergo no dynamic behavior!!!!

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The 2nd reply was from Gazza who recommended a simple voltage divider.
The next reply was from me adding an emitter-follower transistor to the voltage divider.
Another reply from somebody recommended a lousy zener diode that is not needed and it was tested to perform wrong.

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The 2nd reply was from Gazza who recommended a simple voltage divider.
The next reply was from me adding an emitter-follower transistor to the voltage divider.
Another reply from somebody recommended a lousy zener diode that is not needed and it was tested to perform wrong.



Hi Audioguru,

We regard you very high for the strog fundas you have and demonstrate. while so we feel some times let down by expressions

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