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I have a general question regarding the power-supply of the electronic components, that are attached to an Arduino.

If I have a device that operates at voltages between 3,3V and 5V (e.g. this RS232 to ttl converter), what is the best way to connect it with the Arduino?

Connect 3.3V or 5V power supply between VCC and GND

I know, that you can not draw as much current out of the 3,3V Pin like on the 5V Pin. But what if I don't need much current anyways (like in the linked example)?

What ware the advantages / disadvantages of connecting it to 3.3V and 5V?

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Using the MAX3232 data sheet , which is what the TTL Converter has on it's board :

The main difference between using 3.3v or 5v is on the receiver data line "R" (or Rout as it is called on the MAX3232).

Simply put, you want to power the TTL converter from the same voltage as your Arduino is using (3.3v or 5v). Meaning the voltage that your I/O pins will put out and accept.

The received data line R will go between 0 volts and Vcc.

You would NOT want to power the TTL converter at 5 volts if you are using the converter for a 3.3v Arduino, as the received data line would go to 5 volts when high. Actually Vcc - 0.1 volts (typ.) (5 - .1 = 4.9) as per this specification :

enter image description here

If you power the TTL converter from 3.3 volts, the received data line (Rout) switches between 0v and 3.3v as shown here :

enter image description here

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  • @Marla You didn't finish explaining what happens when you connect a 5 volt signal to a 3.3v Arduino pin (I know, it's obvious). – linhartr22 Feb 11 '16 at 21:45
  • @linhartr22 Arduino says, "Owww", and then won't behave afterwards (permanent damage). Some devices are tolerant to over voltage, and mention it in their specifications) – Marla Feb 11 '16 at 21:51

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