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Pulse transformers and RS232 (2nd try)

Hi Group,
I'm looking for a relatively inexpensive way to transmit information
from one mains switching power supply to up to two other mains
switching power supplies a few feet away. The data rate would be 1200
baud or less. The only catch is the common lines can differ by as much
as 170V, depending on mode of operation. (The common line on each
power supply is normally at about -170V with respect to earth ground)
At first glance, this application screams for an infrared connection,
but this has been discounted because of a stray wire or an object
interfering with the signal. So, I have been looking at using pulse
transformers to fully isolate the wired communications connector from
the -170V. I'll be sending and receiving information through the
pulse transformers using Atmel, ATTiny2313 microcontrollers. Several
companies (Tamura, C&D technologies, etc.) make pulse transformers.
The issue I'm running into is that I have no idea how to drive the
pulse transformer or how to receive data from the pulse transformer.
Does anyone have any thoughts on this? (Application notes where are
you?)
Matt Meerian
www.geocities.com/matt6ft9
 
A

Ancient_Hacker

Jan 1, 1970
0
You can't send RS232 signals thru pulse transformers.

How about optocouplers?

They're a lot cheaper than pulse transformers, plus they can pass DC
signals too.
 
M

matt6ft9

Jan 1, 1970
0
Yes, 'suppose I would have to use Manchester encoding or something
like that.
The reason that I don't believe optocouplers would work is the signal
needs to be compeletely isolated from the -170V. (we can't have a
pullup from the microcontroller on the external wire) Say we have an
optocoupler (transmitter and receiver) in device A and we are wanting
to transmit information to an optocoupler in device B. The LED can be
energized in the optocoupler in device A. But, there is no power to
energize the LED in the optocoupler in device B. (it would be helpful
to have a picture)
Matt Meerian
 
M

Mark

Jan 1, 1970
0
matt6ft9 said:
Yes, 'suppose I would have to use Manchester encoding or something
like that.
The reason that I don't believe optocouplers would work is the signal
needs to be compeletely isolated from the -170V. (we can't have a
pullup from the microcontroller on the external wire) Say we have an
optocoupler (transmitter and receiver) in device A and we are wanting
to transmit information to an optocoupler in device B. The LED can be
energized in the optocoupler in device A. But, there is no power to
energize the LED in the optocoupler in device B. (it would be helpful
to have a picture)
Matt Meerian

If there is no power in device B, how could a transformer coupled
circuit work?
Mark
 
A

Ancient_Hacker

Jan 1, 1970
0
matt6ft9 said:
Yes, 'suppose I would have to use Manchester encoding or something
like that.
The reason that I don't believe optocouplers would work is the signal
needs to be compeletely isolated from the -170V. (we can't have a
pullup from the microcontroller on the external wire) Say we have an
optocoupler (transmitter and receiver) in device A and we are wanting
to transmit information to an optocoupler in device B. The LED can be
energized in the optocoupler in device A. But, there is no power to
energize the LED in the optocoupler in device B. (it would be helpful
to have a picture)
Matt Meerian

and you can't borrow a milliamp from the microcontroller, or the power
supply because....
 
R

Rich Grise

Jan 1, 1970
0
Yes, 'suppose I would have to use Manchester encoding or something
like that.
The reason that I don't believe optocouplers would work is the signal
needs to be compeletely isolated from the -170V. (we can't have a
pullup from the microcontroller on the external wire) Say we have an
optocoupler (transmitter and receiver) in device A and we are wanting
to transmit information to an optocoupler in device B. The LED can be
energized in the optocoupler in device A. But, there is no power to
energize the LED in the optocoupler in device B. (it would be helpful
to have a picture)
Matt Meerian

Is this one-way or two-way communication? Either way, optocouplers are
the way to go - they have isolation up to thousands of volts. Your task
is to figure out how to arrange them. The fundamental thing is that
the driver provides the current to the LED, and the receiver uses the PHT
in its circuit with no electrical connection at all to the driver. The
leads to the LED would have to be insulated to 170V, of course, but most
wire is good for that. :)

You could have one driver drive a bunch of receivers by putting the
LEDs in parallel, each with its own dropping resistor, or you could
daisy-chain them, where each one has a link to the other, and maybe
another link the other direction.

Then, you could use any old encoding you want to - you just have to figure
out what topology you want.

Good Luck!
Rich
 
J

Joerg

Jan 1, 1970
0
Hello Matt,

Yes, 'suppose I would have to use Manchester encoding or something
like that.


You can use any code via pulse transformers as long as there is a
comparator with sufficient hysteresis on the other side. The hysteresis
makes sure that if there was a high transition it stays high after the
signal falls back to zero DC. And vice versa for low. The only issue you
have to deal with is the arbitrary state of that comparator before the
very first transition. IOW it needs to be "primed" after the system is
turned on, for example by sending a dummy sign that is ignored.

The reason that I don't believe optocouplers would work is the signal
needs to be compeletely isolated from the -170V. (we can't have a
pullup from the microcontroller on the external wire) Say we have an
optocoupler (transmitter and receiver) in device A and we are wanting
to transmit information to an optocoupler in device B. The LED can be
energized in the optocoupler in device A. But, there is no power to
energize the LED in the optocoupler in device B. (it would be helpful
to have a picture)


Consider some cheap transformers where you get more than one in a pack.
I have used multi-line ISDN common mode chokes for that (only for low
voltage differentials), for very high isolation (5kV and stuff like
that) we wound our own using certified wire. John Larkin recently did an
isolated power transfer via a LAN transformer. It's rather simple: You
drive one side with a nice 50:50 duty cycle clock and then rectify and
filter on the other side. Just like in a normal transformer-driven power
supply except that the diodes have to be faster and the caps would be
much smaller because you'd certainly run at tens of kHz and not at 60Hz
(LAN transformers couldn't do that anyway).

LAN transformers can be had cheaply four or more to the pack. You could,
say, use two for the RS232 and the other two to create 9V, 18V or
whatever you need on the other side from a +5V supply. Or just use one
and live with about 4V. If your application requires some of the other
RS232 lines you'd need another set but they cost very little because
they are found in every LAN router.

Make sure it's a good brand and the isolation rating is ok. I have seen
some rather questionable varieties. With reputable manufacturers like
Murata you would be quite safe. IIRC that's the brand I used.

One word of caution: Mind the common mode range of the comparator.
Devices like the LM339 or LM324 can't be driven more than 200-300mV
below their negative rail or they'll do a tarantella dance.
 
U

Ulrich Bangert

Jan 1, 1970
0
Matt,
to transmit information to an optocoupler in device B. The LED can be
energized in the optocoupler in device A. But, there is no power to
energize the LED in the optocoupler in device B.

you seem to be thinking that you need to connect TWO optocouplers AMONG EACH
OTHER, don't you? That is NEVER the case. The led side is powered by the
transmitting party, device A in your case. The receiver side is powered by
the the receiveing party. device B in your case. If you have two way
communiction you need a second optocoupler for the opposite direction. Look
at things like the HCPL-2300. They are fast and very easy to use due to
their low led driving current of 0.5 - 1 mA.

Regards
Ulrich
 
M

matt6ft9

Jan 1, 1970
0
The communications would go only from device A to device B. (one way)
Yes, if opto isolators were to be used, there would be one in device A
and one in device B. A rough schematic/system diagram was put up at
http://www.geocities.com/matt6ft9/s.gif
All voltages shown are with respect to earth ground. Note the
"common" line of device A is at -180V and the "common" line
of device B is at -165V. We do not want the -165V or -180V on
the two conductor cable.
X-10 tends to be expensive for production units.
Joerg, thanks for the great pointers.
Matt
 
D

DJ Delorie

Jan 1, 1970
0
How about fiberoptic? A simple LED and detector with a fiber cable
between gives you optical isolation with no electricity at all on the
cable. With fiber audio common in home theaters, it shouldn't be too
much of a problem getting the parts.
 
N

nospam

Jan 1, 1970
0
All voltages shown are with respect to earth ground. Note the
"common" line of device A is at -180V and the "common" line
of device B is at -165V. We do not want the -165V or -180V on
the two conductor cable.

So use a cheap TOSLINK fiber optic cable and something like a Toshiba
TOTX173 TORX173 pair.

--
 
J

Joerg

Jan 1, 1970
0
Hello Matt,

The communications would go only from device A to device B. (one way)
Yes, if opto isolators were to be used, there would be one in device A
and one in device B. A rough schematic/system diagram was put up at
http://www.geocities.com/matt6ft9/s.gif
All voltages shown are with respect to earth ground. Note the
"common" line of device A is at -180V and the "common" line
of device B is at -165V. We do not want the -165V or -180V on
the two conductor cable.


That concept looks feasible, provided that each unit has it's onw power
supply and the "Amp and Driver" section is a current source controlled
by U2. I always thought U2 was a rock band ;-)

It might be a good idea to reference the cable to ground. Leaving it
floating invites charge build up and there always comes a point where
that charge reaches a "bzzzt" limit.
 
J

jasen

Jan 1, 1970
0
If there is no power in device B, how could a transformer coupled
circuit work?
Mark

Sounds like He's got live boards at both ends but doesn't want
the input or output terminals at either end to be live.

basically this means he needs to use passive isolation or provide
a separate supply to power the link.

transformers would do it, optic fibre is another option, or maybe something
could be done using capacitors.

Bye.
Jasen
 
J

joseph2k

Jan 1, 1970
0
matt6ft9 said:
The communications would go only from device A to device B. (one way)
Yes, if opto isolators were to be used, there would be one in device A
and one in device B. A rough schematic/system diagram was put up at
http://www.geocities.com/matt6ft9/s.gif
All voltages shown are with respect to earth ground. Note the
"common" line of device A is at -180V and the "common" line
of device B is at -165V. We do not want the -165V or -180V on
the two conductor cable.
X-10 tends to be expensive for production units.
Joerg, thanks for the great pointers.
Matt

Capacitively couple a 1200 baud modem.

joseph2k
 
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