In article <6uhlia$77a@sjx-ixn6.ix.netcom.com>,
alex c peper <xxalexx@ix.netcom.com> wrote:
> I would like to know best design to allow only positive
> rs232 transmit signals to switch a transistor.
> Place diode in series or parallel.
> Most designs I have seen used parallel, for some reason
> this does not work for me.
> I am using a IN4007 in series. Would this give better port protection?
A series-resistor between the Txd output and the base of the
transistor and a shunt diode between base-emitter would seem
the better way to go. That keeps the load on the Txd output
symmetrical, plus it allows the negative swing of Txd to smack
the transistor OFF, but clamping the max negative Vbe to -0.7v.
|/ c
Txd-------[10k-ish]--+---|
| |\ e
--- |
/ \ |
--- |
| |
0v------------------+-----+
--
Tony Williams.
Date: Mon, 28 Sep 1998 10:13:38 +0100
Original Subject: Re: rs232 rectify diode