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

formatting link

Reply to
matt6ft9
Loading thread data ...

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.

Reply to
Ancient_Hacker

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

Reply to
matt6ft9

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

Reply to
Mark

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

Reply to
Ancient_Hacker

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

Reply to
Rich Grise

Hello Matt,

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.

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.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Matt,

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

"matt6ft9" schrieb im Newsbeitrag news: snipped-for-privacy@m73g2000cwd.googlegroups.com...

Reply to
Ulrich Bangert

X10 maybe ?

greet> Hi Group,

Reply to
vasile

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

formatting link
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

Reply to
matt6ft9

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.

Reply to
DJ Delorie

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

Reply to
nospam

Hello Matt,

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.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

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

Reply to
jasen

Capacitively couple a 1200 baud modem.

joseph2k

Reply to
joseph2k

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.