3.3V power supply

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puddlehaven

Member
Hi,

I'm playing with a WIZ810MJ module that requires a 3.3V power supply. I'm wondering what I need to convert the 5V power supply on my breadboard to the 3.3V that the module needs.

If I was building a 3.3V supply that took power from a wall wart I'd use a 3-terminal regulator, a large filter capacitor, and a couple of small filter capacitors. But I'm not building this supply from a wall wart, I'm using the local 5V supply.

If I build the 3.3V supply to take its input from the 5V supply, what do I need in the way of filter capacitors? Or do I need any?

Thanks for your help,
Chuck

(By the way, level shifting from the 5V Picaxe to the 3.3V WIZnet module shouldn't be a problem, the W5100 chip lets you put 5V on the input pins without causing a problem.)
 
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Steve Joblin

Member
I'm working on the same type of issue (only I am interfacing a 5V PICAXE witha 3V XBEE unit). You will need a regulator, but If you are feeding the regulator from a "clean" regulated 5V source, you don't need any filter caps.

For my 5V supply, I usually use an LM2940 5V Regulator (TO-220) with filter caps on the input and output side.

I wanted a smaller regulator for the 3V supply, but I had a hard time finding one in a TO-92 package. I finally found one at www.kronosrobotics.com (http://kronosrobotics.com/xcart/product.php?productid=16528&cat=284&page=1)

Although you don't need it, you may want to consider using a resistor on the data lines between the 5V PICAXE and the 3V WIZ810MJ for good measure and good practice.
 
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profmason

Member
For my XBee boards I am using a AS2810AT 3.3 (which is a TO220 package voltage regulator.) I have run it straight off the 5 volt line. I use a 47uF cap and a 0.1 uF cap between power and ground. I also use a 4001 diode between input and output. You should run 5 Volt outputs from the microprocessor through a voltage divider (a 10 K and a 22K works great) before they hit the Xbee.

BUT! I have found that the 08M and 14M both run great off the 3.3V supply. This means no voltage divider is required on the outputs. I am now really pushing things by using a single LiIon battery (0.99 cents surplus 4.2 volts 850 mAh) to run the regulator which in turns runs the picaxe + xbee module. I was using 9 Volts, and it worked but the XBee was draining a 9Volt in about 2 hours.

Here is my writeup on using the Xbee with the Picaxe (including the power supply down in the bottom corner.)
http://profmason.com/?p=414

Where is my writeup on the Picaxe based computer vision platform that uses the picaxe and xbee to control a checker finding robot (It discriminates between black and red checkers and only follows the red checkers.)
http://profmason.com/?p=429

have fun!
 
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Steve Joblin

Member
I'm following the schematic I found at KronosRobotics.com It calls for a 1.8K and 3.3K voltage divider on the communications line from the PICAXE to the XBee and a 100 ohm in-line resistor on the communications line from the XBee to the PICAXE.

Profmason - I've been carefully reviewing your site and have found some great info and ideas... keep up the great work!!!!
 
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womai

Senior Member
Watch out - just because the source for your 3.3V regulator is a 5V regulator does not mean you don't need any filter capacitors!!!

First, even though the source may be clean, the load on your 3.3V reg can change quickly, so you definitely need a capacitor on the output side.

But even more important, many regulators will not be stable without capacitors; instead they will oscillate. (This is not dry theory, I have seen this in practice more than once). This is especially true of low-dropout regulators (which can live with a supply only marginally above their output voltage), and this is exactly what you need since your supply is only 5V.

The venerable 7805 regulator is relatively immune against missing capacitor, but it's not a low-dropout type.

Wolfgang
 
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womai

Senior Member
Watch out - just because the source for your 3.3V regulator is a 5V regulator does not mean you don't need any filter capacitors!!!

First, even though the source may be clean, the load on your 3.3V reg can change quickly, so you definitely need a capacitor on the output side.

But even more important, many regulators will not be stable without capacitors; instead they will oscillate. (This is not dry theory, I have seen this in practice more than once). This is especially true of low-dropout regulators (which can live with a supply only marginally above their output voltage), and this is exactly what you need since your supply is only 5V.

The venerable 7805 regulator is relatively immune against missing capacitor, but it's not a low-dropout type.

Wolfgang
 
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BCJKiwi

Senior Member
Timely warning Wolfgang.

Just implemented an LM1117T-3.3 regulator off a 5V regulated supply.

This required a 10uF Tantalum cap on the output side MINIMUM. Placed one both sides (recommended). Cap required for stability and had to be the Tantalum type for it's ESR properties.
Message is - read the data sheet for the device you actually are using - not the other make/model you used last week!
Regulator cost NZ$2.57, Caps cost NZ$1.84 each all plus GST @ 12.5%)!
 
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BrendanP

Senior Member
Fairchild make a 3.3 V reg that outputs 3 amps. Mouser has them for a couple of bucks each. They also have a enable/ disable pin if you need it. Ive used them a lot ,they're great.
 
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