Re: [CAD_CAM_EDM_DRO] Parallel port relay blues

On Saturday 29 January 2005 09:30 pm, Alan Marconett wrote:

> Hi Bert,
>
> Are you tying one end of the relay to +5, and the collector to the other
> end? The output gate of the parallel port is "sinking" current in this
> case.

Not! Collector to relay (with +5 on the other end), emitter presumably
grounded, and the base going to a port pin with a resistor is *only* going
to be active when that port pin is high -- and therefore _sourcing_ current,
something they don't do all that well. If it were a circuit configuration
with say the inversion of the above arrangement, using a PNP transistor
(2N2907 comes to mind as the opposite number of the 2N2222), emitter to +5,
collector to relay coil, other end of which is _grounded_ then the base to
port pin through a resistor, THEN we would have a setup which would be
active when the port pin went _low_, and therefore would be _sinking_
current.

> More current is available then "sourceing" current.
>
> Alan KM6VV
>
> bertmantx wrote:
> > Bought bread board today and went back to drawing board.
> > I was able to get 2222 to fire but had to go to 510 ohm resistor
> > (would not fire with any higher ohm resistor) at 8.2mA to base it
> > will fire relay using 5v supply. When tried to fire with para port
> > only drew 4.2mA and would not fire 2222.
> >
> > Any Ideas what the para port is capable of?
> >
> > Have been reading all the replies and would like to thank all for
> > help.