I have a 12 volt relay. It draws 15ma. Is it safe to drive this via rs232 dtr line? Are protection diodes needed? Do I also need a diode on the coil side to keep it from going to negative voltage? Thanks.
Lyle

 

I would not drive a relay directly off of the dtr line, you will most likely fry the usart ic....

I would drive a transistor with the dtr line to trigger the relay, have the relay powered from a separate 12 volt source...

B. Morse

 

What if I used the parallel port and a 5 volt relay? I could power it directly from here? I would drive it from 2 lines. Would this be safe? Are protection diodes needed?

 

Hello,

If you want to use the parallel port for interfacing, take a look at this page:
http://www.epanorama.net/circuits/parallel_output.html

This is a link from this ePanorama page on PC-projects:
http://www.epanorama.net/links/project_pc.html

Bertus

 

What if I used the parallel port and a 5 volt relay? I could power it directly from here? I would drive it from 2 lines. Would this be safe? Are protection diodes needed?

!
Either way you do it, you should still use some kind of buffer between the port and the relay, never connect a relay directly to a parallel port pin or to a serial port pin, use some buffer IC's such as the 74367 TTL Hex buffer/Inverter IC or similar Newer IC to buffer the port pins....

Unless you have money to replace the motherboard after you toast its ports....

B. Morse