Electronic Design reader C.N. Rajendra Simha asks:
"I am looking for some design tips on negative-to-positive voltage converters. I am specifically looking for +5-V generation from negative RS-232 signals (like RTS) for one of our designs."
There is such a wide range of drive capability in RS-232 outputs that you have to be careful to not demand too much current. Desktop vs. laptop output capability can be a problem. RS-232 connections are hard to find in laptops these days, especially the low-end ones. They are all USB. USB is safer to use as a power supply, since it was designed to supply current to the attached device.
B&B Electronics Tech Library has some info on use of RS-232 outputs. For example: http://www.bb-elec.com/techlibrary_request.asp?SpecifiedLocation=tech_articles/port_powered_converter_tips.asp
You have to register for access.
Use an opto-coupler such as a 6N137 or faster. You get the benefit of isolation also. I had a similar project and it works perfect.
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Readers seeking help with their own design problems are encouraged to e-mail Lisa Maliniak, eMedia Editor, for posting in future editions of Electronic Design's Helpline.