5Volts from serial port

[johnsmith123]

Hello everyone,

I am designing a small circuit that uses 5 volts for power. I need to keep the component count low, also there is not much room on the board for components.

I wanted to take 5V from the com port but I am concerned whether this is a good idea, and whether or not it will cause damage to my serial port and/or my board.

The components will consist of a 24LC256 eeprom, an avr micro controller, a MAX232 and two LED's.

Here is a circuit I tested and it works.

Just looking for some advice.

 

[Sceadwian]

Try it, it may work, if your circuit draws less than bout 100ma of current. Depends on the Rs232 port it's connected to, it's a very 'fuzzy' standard. I'm not sure I would trust it and the DTR DSR and RTS lines would have to be held high.

 

[johnsmith123]

Try it, it may work, if your circuit draws less than bout 100ma of current. Depends on the Rs232 port it's connected to, it's a very 'fuzzy' standard. I'm not sure I would trust it and the DTR DSR and RTS lines would have to be held high.

I tried it, and it does work. However I can only try it on my own PC's.

I just wondered if there is a possibility of damaging the serial port with this type of circuit.

 

[Sceadwian]

Possible yes. Likely, I'm going to say no. The output drivers should be able to handle constant short circuit conditions. The biggest hazard you create is from your circuit itself somehow feeding back a high voltage into the serial port, or it's ground.

 

[HiTech]

I may be incorrect but I think you can safely rely on up to 500ma. current at 5v.

 

[Sceadwian]

From what I read on the Rs232 specs I looked up it was close to 200ma per line, but that was only to reproduce a 2v signal on the receiving end through something like a 300 ohm load. Your mileage may vary =)

 

[mneary]

One of the popular RS232 chips is the MAX202e, so I use it as a benchmark. Each output driver is guaranteed to give you +/- 5V into 3k ohms (that's 5 volts at 1.3 mA). Typical output is +/- 9V into 3k. It can drive 300 ohms at +/- 2V.

Just about everything else is stronger, but the MAX202 was the prototype for a lot of laptops.

Since the '202 has 2 outputs, I figure I can count on about 5 volts at 3 mA.

 

[Pommie]

If you look at this thread, you will see that I powered that circuit from the RS232 port. It works OK but you can see the voltage dip when the DS is converting. The DS draws 1.5mA during conversion!

Mike.