PSU/RS485 Problem, cant make my mind up.

I am designing a RS485 remote control system, to switch relay outputs
on some 32 terminals, over an area of some 250 Metres by 200 Metres. I
have got a lot of information from the DCI Bus system from Elektor
Magazine (December 2002), the only real difference is I am using a PIC
Chip with a USART and Max 487e chips for the RS485 Drivers/Receivers.
I was intending to use a 3 wire system with two for the RS485 and one
for a common ground via a 100 ohm resistor to each board.

The problem is, how do I power the 32+ terminals. I could if I have
to, insist each terminal has a mains power supply available, so power
supply for each individual terminal was not a problem. But this would
be problematical, it would be easier if only every 6th or so, terminal
had a power supply, and all the terminals shared their power with
adjacent terminals, via a pair of twisted cables I have spare, over
from the RS485 system. I would fuse each terminal, so no one terminal
could short circuit the whole system Each terminal would be fed from a
1N4001, so that volatge differences in the power supplies did not
cause a problem. But what I cannot work out is the grounding. In the
DCI bus I mentioned earlier, the Signal grounds were via a 100 Ohm
resistor to signal ground. But if I have a common power supply
arragement, then I think I cannot use the 100 Ohm "decoupler". I need
any thoughts you have on my proposal, thanking you in anticipation.
Mark in Spain
(remove the X to reply)

 

Run a seperate ground for the power supply? One more wire.

 

You forgot to tell us what power these terminals would take.
1 Watt, 10 Watt, 100 Watts ? If it was just little one could have an
additional wirepair and deliver a couple watts for local DC/DC to setp
down again. similar to power over ethernet.

Rene

 

I need 5v for the logic via a 7805 and I need 8-15V for the relay
coils. I was going to use the spare pair of wires (Cat5 cable) with
12V DC. The current requirement for each terminal, should not be more
than about 200Ma. I was going to daisy chain all the terminals, and
all the power supplies.

Assuming I get the OK with the above idea, I just need clarification
on the seperate ground mentioned earlier. If all my terminals are
supplied from 12V DC then I have the ground on the power supply, going
to each board. Was you suggesting that I have a seperate ground
circuit for the RS485, and does that mean the ground on the MAX 487
goes to the power supply ground or the RS485 ground circuit?

 

Ok, so you have a logic problem. First, those relay coils can be
thirsty, so each terminal might need a lot more current to handle them.
You didn't say how many relays per terminal.

So, typically,you have two jacks on your board, and if using cat-5, you
probably just use RJ45s, so your send is on one pair and the receive is
on the other pair. If you try to just use a single wire for return, you
give up the noise rejection of the twisted pairs. This gives you two
'extra' pairs for power and ground.

I personally would use both pairs, with tip at +12 and ring at ground.
This way, the twisted capacitance helps filter the supplies. I would
have a two pairs of jumpers on each board to connect to each of the
jacks, so you can either connect to the power bus one way, both ways, or
neither. By doing this, you have several options: If this board has a
power supply, it can send out power to other terminals in either
direction, both directions, or neither. If it doesn't have a supply, it
can get power from either direction, or pass it on through down the
chain. They will also come in real handy in trouble shooting power
issues (and they will come up!) without just disconnecting cables.

Charlie

 

For 200 meters?
You are going to need a lot higher than 12V.

 

Thanks Charlie, I was going to use only one pair of twisted for the
comms (half duplex). I like the idea of links on the PCB to give me
different powring options. Yes I can use two pairs for the power.

 

I was proposing PSU's every sixth of so terminal, so a power supply
was never more than lets say 20 metres away. The terminals will have
up to 16 relays, but these are the very tiny "signal" type with a coil
resistance of 700 ohms.

 

Just done the maths on powering 6 X boards X 16 relays X 20 metres,
and I have answered my own question. It was a nice idea, but creates
more problems than it solves, will go back to powering each terminal
from the mains, thanks to all.

 

Maybe run a higher voltage and PWM the relays

At 48V you'd be in the ballpark of 450mA with all the relays energised.
less if you drive them at half power after they close

Bye.
Jasen