Relay coil voltage: higher or lower?

[a.periz]

Hi!
I'm making a system supplied by a DC 12V bus via a CAT5 cable.
I have many modules connected in series:

Supply -> mod1 -> mod2 -> ... -> modN

Each module has a mcu supplied by an 5v LDO (e.g. 7805) that drive a relay.
My problem is: the bus voltage lower with the distance because of the resistivity of the cable, so, it is better to choose a 12V relay (that works up to 9V) or a 9V relay and supply it with an higher voltage?
I'll use this family:
https://www.farnell.com/datasheets/1019795.pdf

Thank you

Andrea

 

[mahm150]

I THINK THAT THE VOLTAGE DROP ACCORDING TO YOUR LOAD .
YOUR DC 12V MUST BE WITH SUITABLE CURRENT OUTPUT TO BE STABLE WITH YOUR LOAD.
FOR EX IF ANY MOD1 NEED 200ma the supply output current you need is 200ma * n at min.
if u do that the voltage not drop
about relay the first relay is better

 

[a.periz]

Thank you for answering!
The power supply has enough current for all modules.
The voltage drop in the line, not in the power supply, because I need to use some meters of cable because of the distance among modules.
My doubt is: is it better to supply a 12V relay with a lower voltage or a 9V relay with a higher voltage?
I think the second choice is better because it can work from about 7V to 12V, but I don't know if it is dangerous to supply a 9V relay with 12V.

Thank you

 

[mahm150]

i think it is dangerous the coil of relay may be damage

 

[Kral]

One solution is to use a constant current source to drive the coil of the relay. That way, the coil current, which is really the important parameter, will be correct independent of the resistance of the line. This assumes that the voltage compliance of the current source is adequate to supply the line drop plus the rated relay coil voltage.

 

[chuckey]

Use the 9V relay with a couple of S1 diodes in series with their coils, short out the diode as you go down the line.
Frank

 

[a.periz]

Thank you!!
Instead of the two diodes I'm thinking about a resistor Rc:

The 9V relay has a 202Ohm coil resistance, so I think that put Rc=39Ohm is a good choice:
The relay has a max coil voltage of 110% of the rated (9.9V) and a min of 75% (6.75V). So, if the bus voltage is max (12V) I have about 50mA and it works up to 8V.
What do you think?
I can't use 2 diodes because I have a pcb already printed and I can change only traces, otherwise I must pay for a new solder mask.

Thank you!