[ARC5] [Milsurplus] Dear Smart People: RS-232 Com 1

[Kenneth G. Gordon]

On 8 Feb 2020 at 12:02, Gordon Smith wrote:

> Hi David,
> Did a quick google search. According to this thread 
> (https://forum.allaboutcircuits.com/threads/rs232-output-current-rating.64294/ 
> ) most RS-232 driver chips were specified for 45 mA drive, but they 
> give no basis for their answer. However, if you look at this spec 
> (https://ecee.colorado.edu/~mcclurel/dan83.pdf ) the maximum load 
> seen by the driver chip is specified as between 3kOhm to 7 kOhm , 
> which gives you a maximum drive amperage of 5 mA at 15 VDC. So I 
> think the short answer is "no". But I am NO expert on this, just 
> telling you what I found.
> 
> 73, Gordon KJ6IKT

RS-232 driver chips have commonly been used to drive those small PC-board mounted 
relays which require 5 VDC for years.

Assuming the same "power" is available as what your PDF mentions, 5 mA at 15 V, all 
things being equal (which they seldom are) that would mean 15 mA at 5 VDC, which is more 
than enough to drive one of those small relays.

In addition, if one didn't want to, or couldn't, drive a relay directly, a small transistor could be 
used as a "current amp". Say a ubiquitous 2N2222 or two. Feed the base with the output of 
your driver, and connect the other leads appropriately.

As I remember it (very suspect at my age), RS-232 drivers need "pull up" or "pull down" 
resistors on the outputs anyway, so a relay coil could suffice.

Ken W7EKB