MAX232 with 3.3v supply

[elmediel]

I've seen a design of Max3232 being feed with 3.3v (usually 5v).

Is this design makes the Tx/Rx also have an output of 3.3v??????

I need 3.3v for my xbee.

 

[lockman_akim]

if im rite (if im wrong tell me) xbee is already have uart converter..u just need to figure out how to change (+15V and -15V) to (+3.3V to 0V)..because serial port send data using (+15V and -15V)..so it will blow up your xbee if you directly connect it..but i think max 232 is not necessary to use for xbee..

 

[Colbhaidh]

The MAX 3232 has an on chip charge pump so if supplied at 3.3V the Tx-OUT will be +/-12V (to feed the RS232 signal) and the Rx_IN will tolerate +/-25V input to the chip. This wil give a Rx_OUT and Tx-IN to the low voltage circuits at 3.3V.
So essentially the chipconverts 3.3V logic to +/-12V and converts up to +/-25V signals to 3.3V logic.
However the MAX3232 is not the MAX232. The MAX232 can only operate (reliably) at 4.5V to 5.5V.

 

[elmediel]

oh its not max3232. i mistyped. i was talking about max232... max3232 is not available here.. i only have max232...
see attached

 

[betwixt]

Your circuit will not work reliably with the MAX232, even if you give it a 5V supply you would still have to drop it's TTL level output to 3.3V and convert the Xbee up to 5V for it to work.
I'm afraid you have to change to the MAX3232 or an equivalent. Nothing else needs to be changed, they are pin compatible.

Brian.

 

[Colbhaidh]

If you just want your 3.3V logic to communicate over RS232 then this can be done with 2 transistors and a few other components without the need for the MAX ICs. I can provide a simple schematic if required but it is jusr a simple level shifter.

 

[nandhu015]

if im rite (if im wrong tell me) xbee is already have uart converter..u just need to figure out how to change (+15V and -15V) to (+3.3V to 0V)..because serial port send data using (+15V and -15V)..so it will blow up your xbee if you directly connect it..but i think max 232 is not necessary to use for xbee..

No, you are wrong. Xbee cant tolerate 5v.

Here you can find a list of level converters

**broken link removed**
(Scroll down to find the list)

 

[elmediel]

I only have max232 and it's difficult here to find something else. I need to use a component that is easy to find that can convert 5v to 3.3v, that is the tx/rx 5v to tx/rx 3.3v vice versa... something that is fast enough to handle square waves.

 

[betwixt]

OK, try this:
Run the MAX232 from 5V.
Output pins "R1OUT" and "R2OUT" should be wired with 10K in series with them and 15K from the Xbee side to ground, this will drop the 5V to 3.3V
On input pins "T1IN" and "T2IN", connect a 4.7K resistor to +5V.
Add an NPN transistor to each input, E to the Xbee output, B to +3.3V through 10K and C to the junction of the MAX232 input pins and the 4.7K resistors. This will convert 3.3V to 5V.

So you need 2 x 10K and 2 X 15K to drop the 5V to 3.3V and
2 x 4.7K and 2 x 10K and 2 x NPN transistors to convert 3.3V to 5V

That's the simplest way of doing it.

Brian.

 

[elmediel]

you mean like this? see attached

 

[betwixt]

Not quite. Sorry I can't send images at the moment.

The Emitter of the transistor goes to the Xbee output pin.
The Base of the transistor goes to 10K then to +3.3V
The Collector of the transistor goes to the MAX232 input and also to the 4.7K to +5V.

The idea is that when the Xbee output is low, the emitter is lower in voltage than the base so the transistor turns on. This sinks collector current into the Xbee and makes the collector voltage low.
When the Xbee output is high, the transistor emitter and base are at the same voltage so it does not conduct. The collector is then pulled up to +5V by the 4.7K resistor.
It's a common base amplifier configuration with the transistor driven in and out of saturation.

Brian.

 

[Archerman]

No, the RX ant TX will not be 3V. The function of the MAX232 is actually a level converter no matter what technique you use to make the 5V supply for the chip.