Doing some preliminary design of a project that will be using RS-485 for serial communications over moderately large distances (10-100m, possibly more) and I am trying to come up with the best way to power the remote devices. I would like to use CAT5/6 and RJ45 plugs because they are readily available. The RS-485 would be half-duplex, so it would only require one pair, and I don't need it to be particularly fast. I am not able to find a power transmission solution that I like though, here are the ideas I've had so far:
48V
I have considered 48VDC through the remaining pairs. This is a good option to combat voltage drop over longer distances, since the highest voltage I will need to supply at a remote device would be 12V for the RS-485. This would require more complex/expensive voltage regulators at the remote and host devices as I need to have 12V and 5V or 3.3V for logic devices. Using 3 wires each for power and ground would mean I need to balance higher loads across all 3 wires, again increasing complexity. This doesn't seem to satisfy enough of my design criteria, so I kept looking.
Mixed 12V and 24V
I also considered having one pair supply 24VDC and ground to power the remote devices, and another pair supply 12VDC and ground for the RS-485 transceiver. This would make voltage regulation simpler, but increases concerns about voltage drop, especially for the 12V. This also didn't seem like the right solution.
Isolated AC
It occurred to me that I could use lower voltage (18-24V say) AC to be able to get the power farther and avoid the voltage drop suffered by DC. I would have a transformer at the host reducing and isolating the 120VAC mains down to something more reasonable, and transmit that down a single pair of the CAT5/6 cable. Each remote device would have a small rectifier and SMPS to get 12V/5V/3.3V as required.
Now we are into things that I know less about, so here is what I'm hoping you will be able to help me with:
Having serial data in the same cable as AC raises some concerns though, would 60Hz AC impart much interference into a differential signal like RS-485? If so, are there easy ways to combat this?
Is there an easier way to do any of this that I might have missed?
Thanks in advance for your help,
Mike
48V
I have considered 48VDC through the remaining pairs. This is a good option to combat voltage drop over longer distances, since the highest voltage I will need to supply at a remote device would be 12V for the RS-485. This would require more complex/expensive voltage regulators at the remote and host devices as I need to have 12V and 5V or 3.3V for logic devices. Using 3 wires each for power and ground would mean I need to balance higher loads across all 3 wires, again increasing complexity. This doesn't seem to satisfy enough of my design criteria, so I kept looking.
Mixed 12V and 24V
I also considered having one pair supply 24VDC and ground to power the remote devices, and another pair supply 12VDC and ground for the RS-485 transceiver. This would make voltage regulation simpler, but increases concerns about voltage drop, especially for the 12V. This also didn't seem like the right solution.
Isolated AC
It occurred to me that I could use lower voltage (18-24V say) AC to be able to get the power farther and avoid the voltage drop suffered by DC. I would have a transformer at the host reducing and isolating the 120VAC mains down to something more reasonable, and transmit that down a single pair of the CAT5/6 cable. Each remote device would have a small rectifier and SMPS to get 12V/5V/3.3V as required.
Now we are into things that I know less about, so here is what I'm hoping you will be able to help me with:
Having serial data in the same cable as AC raises some concerns though, would 60Hz AC impart much interference into a differential signal like RS-485? If so, are there easy ways to combat this?
Is there an easier way to do any of this that I might have missed?
Thanks in advance for your help,
Mike
