> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Olin Lathrop
> Sent: 06 October 2009 13:49
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Cheap, low power, isolated DC-DC converter design
> 
> Michael Rigby-Jones wrote:
> > I have a requirement for monitoring multiple RS232 lines (19600bps)
> > connected between some instruments and a PLC in a factory
environment.
> > I have to ensure full isolation between all the channels, I must not
> > add any significant load to the RS232 line and I must stick as
> > closely as possible to the standard RS232 logic level thresholds
> 
> How much load is "not significant"?  Is there any power at all
available
> on
> the driving side?  If these are truly RS-232 lines, then they should
be
> capable of a few mA at least.

That's a fair point, I don't want to be waving dead fish around but
there are rather a lot of unknowns at this stage, and since the factory
is a significant distance from me (several hundred miles) the first trip
is going to be with a prototype and some test equipment to get some
field measurements.  

Until then I want to minimise any chance of the prototype not working,
or far worse, preventing the existing system from working or making it
unreliable.  The instruments are not all the same model of manufacturer,
and the link lengths vary from 10 to 30 feet so I'm not sure what
assumptions I can make about the current capability of the link at this
stage, hence I would prefer something that was as "invisible" as
possible to the existing devices.  This is clearly far from an ideal
scenario, I would very much like to have solid customer supplied specs
written in a document but that's just not going to happen since they
don't have much of a clue at this level.  Ultimately once we have a
working and characterised design I will write the specs and the customer
will sign up to them.

 
> 
> You can make opto couplers work at a few mA.  One trick is to use two
of
> them, each with a floating phototransistor output.  A high input level
> lights one LED and a low the other.  Then arrange the two
phototransistors
> in a totem pole on your side.  That will be good enough to drive a
CMOS
> logic input, and you can go from there.
> 

The most of the common RS232 drivers seem to have an input impedance of
around 5k  (i.e. 3mA at +-15v) so if I can reliably drive an
opto-coupler at the bit rate required then this looks like a much
simpler/cheaper solution than the isolated supplies + drivers.

Do you have suggestion for a suitably sensitive opto-coupler?  I made an
isolated I2C interface many years back with standard 4N36 devices and it
was incredibly slow, it could barely manage 10kbit/s and then only with
the LED current cranked well up.  With hindsight some of the speed
issues were likely due to saturation of the photo-transistor, I guess a
simple schottky clamp would be effective here? 


> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Dwayne Reid
> Sent: 06 October 2009 22:52
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Cheap, low power, isolated DC-DC converter design
> 
> 
> There's a pretty good chance that it will work with Ethernet
> transformers (I haven't characterized one of those yet) and it for
> sure will work with cheapo audio transformers.  I've run the
> converter as low as 10 KHz and as high as 500 KHz.
>

Ethernet transformers were one of the first things I thought of
(ab)using since they are so cheap and easy to find.  However, the TX and
RX sides appear to have significantly different turns ratios which is
slightly annoying.

Regards

Mike

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