getting power from the RS232 to run the max232 chip?

Nick · Guest

Here is the deal, the max chip is just using to much current and I only need it when its attached to a computer. What if I sourced the power from the RS232 connecter via the computer to power the max232 chip?

Anyone done this before? Does it sound like a viable solution?

Nick

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

There are plenty of products that are powered by the PC's RS232 port. The awkward bit is that the quiescent state of the RS232 transmit lines is negative. So you use diodes to take negative voltage from Tx (pin 3) and the handshake lines (pins 4 & 7) and store it in a cap. Then you use a negative to positive charge pump to get the positive voltage stored in another cap. These voltages run the RS232 driver chip. You either use your own supply for VCC or if you circuit is low enough power you can get that from the RS232 voltages as well. You won't be able to use long cables at high baud rates, but for many applications it is fine.
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Ttelmah · Guest

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

http://www.hut.fi/Misc/Electronics/circuits/rspower.html

Guest · Posted: Mon Sep 27, 2004 8:32 pm

I always use a pair of 2N3904, wich are fed by one pin of the micro.
When need to comunicate, just put that pin high.
You get a lower power consumption, as lower $$$.
Use 1K R both at base, and collector.
Very cheap. Had no problems at any speed.

Gabriel.-