5 volt to 3.3 volt

[salahaldeen]

I would like to know any ideas of the simplest circuit to convert 5 volt dc into 3.3 volt dc.

 

[audioguru2]

You forgot to say how much current.

 

[Gazza1]

I would like to know any ideas of the simplest circuit to convert 5 volt dc into 3.3 volt dc.

You should try using a voltage divider.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html

 

[salahaldeen]

I want it for minimum 70mA, and yet doesn't get affected much by loading effect.
Many Thanks

 

[audioguru2]

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.

 

[Kevin Weddle]

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.

 

[nura100]

Hi Salahadeen,

i have drawn a circuit that might be of help to you,

best of luck !

arun

View attachment 40298

 

[Kevin Weddle]

Nice circuit. How good do you think it regulates?

 

[audioguru2]

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.

 

[Kevin Weddle]

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.

 

[audioguru2]

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:

View attachment 40299

 

[Kevin Weddle]

I don't see what makes 4v to 7v zeners inherently better than the others. It may be that this particular manufacturer focuses on 4v to 7v zeners, letting the other ones they make just be as they are.

 

[indulis]

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).

 

[audioguru2]

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.

View attachment 40306

 

[mvs sarma]

I want it for minimum 70mA, and yet doesn't get affected much by loading effect.
Many Thanks

Hi, see the TL431 applicationView attachment 40311

 

[indulis]

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.

 

[audioguru2]

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 thisView attachment 40310

 

[audioguru2]

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.

 

[mvs sarma]

Hi AudioGURU,

Yes. i couldn't size it properly--rather i had to learn how to . let us see the utility aspect of theis element for the pupose.