Re: [CAD_CAM_EDM_DRO] Parallel Port/Relay Problems... more

In a message dated 05-Dec-01 14:20:25 Central Standard Time, KM6VV@...
writes:

> To expand just a little, there are also open-collector buffers, which
> are meant to work into a load with a pullup. They only sink current.
>
>

Good "everyday" examples are the ULN-2003. Seven "transistors" in a 16-pin
DIP (same as an "IC"). Each can sink up to half an amp. Also contains
"anti-backlash diodes" for protecting transistors if driving solenoids, etc.
There are also some "75nn series" mini-DIP's such as the 75477, which can
buffer the parallel-port outputs for driving small relays.

IMHO, one should NEVER contemplate trying to "drive" anything but a printer,
or some IC's gate-inputs with the lines of a parallel-port! If you "smoke"
only some soldered-in IC that drives that port, deep inside your PC, the
labor-costs alone will be $100 or more! It is NOT unlikely that you would
also "smoke" a major nearly-unrepairable portion of your motherboard, if you
fiddle with the port, trying to "drive" even a little DIP-relay directly from
the port!

Example: Even if you find a "sensitive" relay which would operate with the
"advertised" SINK or SOURCE current out the printer-port, if you connected it
withOUT the "backlash diode", you WILL do damage to the PC as soon as you
"release" the tiny current you try to put through that relay's coil!

Recap: Unless you have LOTSA "breadboard experience" and know what "stored
inductive EMF" is (and what it can do) and ALWAYS use "backwards diodes"
across the coils, etc., etc., do NOT monkey with your PC's parallel port!
You WILL be sorry you "got brave"!

Jan Rowland, Ignernt, but experienced old troll


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