How to counter voltage drop over bus cables

[bryce1]

Hi guys,

I am wondering how the pros deal with power supply voltage drop in daisy chained systems.
Here is the stuff I can think of:
- higher supply voltage on the bus, take it down on every system in the chain
- boost it up to make up for the drop
- ignore it

I dont like these though  And I know this is a general question with probably very specific solutions, but I still want to ask:
What can you guys think of? What are good approaches to this? Any old-guy knowledge? 

[McBryce]

Hi bryce1 (so it's YOU who took that name before I got here!  ),

what sort of bus are we talking about here? What is the supply voltage and what are the distances involved?

McBryce.

[bryce1]

Haha, sorry for the name thing In my case, I have an rs485 bus with 5V supply voltage (probably 1-25mA draw on every device, not sure yet).

[Simon]

If it's signals then the protocol should deal with it up to the specified length in the protocol. If it's power make the cables thicker. One option is to step power up and then back down again but we need a bit on context here.

[Simon]

Haha, sorry for the name thing In my case, I have an rs485 bus with 5V supply voltage (probably 1-25mA draw on every device, not sure yet).

if power is your problem then each device needs it's own power supply, but a common ground must be kept as well.

[bryce1]

that is what i want to avoid. one cable only.

[Simon]

that is what i want to avoid. one cable only.

 could you be clearer, do you mean one power and data cable ?

[bryce1]

One cable with 4 wires, two signal, two power.

[Simon]

and what is your total current draw, and what is the tolerance on supply voltage ?

[dom0]

If it's rather low-power over longer cables, just put in 7-9 V and use a small LDO at each node.

[retrolefty]

 
probably not applicable to your application, but at the industrial location I worked at they would specify power supplies that had separate voltage sense input terminals so that the supply could regulate from the load end of the wiring distribution. Most of the industrial linear power supply manufacture (Lambda comes to mind) at the time included those terminals to be used or just shorted to another terminal if not used.

 

[sleemanj]

- higher supply voltage on the bus, take it down on every system in the chain

That's the easiest and probably cheapest way,  tiny buck mode dc-dc converter modules are cheap from china.  Or a linear reg of course if current is low.

[T3sl4co1l]

In general, you need enough supply at the head end so that, after all the drops along the way (including the cumulative current loads of them), the last one still has enough supply.

As mentioned, an LDO on each is probably a good idea, so each unit still gets the 5V (or whatever) it needs, while the supply can be anything higher than that.

If higher current is needed, then switching converters and still higher voltages (12, 24?) are desirable.

If still more power is needed, consider an alternate power network, like fanning out 24V (or 48, or 120, or..) through large cables, which may be routed in parallel with the signal cables, but sooner or later must form loops (i.e., a second power supply chain joins in the middle of a signal chain).  Loops in a cable system are bad.  The best way to address that is to use receivers/transmitters with high common mode tolerance (RS485 should be pretty tolerant of this already).  At higher voltages and currents, you may find the transceivers need to be fully isolated!  Keeping such a system quiet for EMC purposes is also a challenge.

If you are using a power fan-out network, be sure to fuse each branch that is not rated to carry the full current capacity of the source.  You might use a 24V, 50A supply to energize an LED lighting installation; but each power supply chain might only need 5A, and might be wired with 5-10A rated wire.  Each chain therefore must be fused for 10A, so that under fault conditions, it does not accidentally carry the full 50A the source is capable of.

Tim

[Simon]

apparently we are only talking up to 25mA but the question is how many devices