Help - Trigger 24v relays

Thread Starter

Pjota1

Joined Jun 13, 2019
18
Hi,
I need help to figure how to trigger 3 or 4 24vdc relays at 700m distance. Probably is too easy but... I have a 24vdc 1A power supply and 3 switches. Each switch should trigger a 24v relay but they are at near 700m distance. I'm unsure what cable to use (number of cores and sectional area)... This cable will run in cable trays with other cables. Can someone help me please?
Ty in advance!
 

MaxHeadRoom

Joined Jul 18, 2013
21,601
Power supply is a little light at 1A.!
There is 4 pair cat3 stranded pairs twisted.
Most PS can be adjusted up slightly to account for voltage drop, relays are not that voltage sensative, the other alternative is a PS with a voltage sense pair to the final destination.
Max..
 

Thread Starter

Pjota1

Joined Jun 13, 2019
18
Show us a block diagram indicating how you propose connecting things.

What is the relay coil resistance?
I was thinking to use one core to negative and three cores for positive, one for each relay. Each switch, when closed, send the positive to trigger the relay. Ill draw a diagram later to show.
Im not sure about the relays core resistance but they are basic 24v/10a (single or double pole) Finder relays so i think the coil resistence should be near 30mA.
 

Thread Starter

Pjota1

Joined Jun 13, 2019
18
Power supply is a little light at 1A.!
There is 4 pair cat3 stranded pairs twisted.
Most PS can be adjusted up slightly to account for voltage drop, relays are not that voltage sensative, the other alternative is a PS with a voltage sense pair to the final destination.
Max..
Well i have only a 1A 24V power supply, but if i need to have cores with a big sectional area to overcome the voltage drop for 700m distance, maybe i can see the cost to buy another psu, like 2,5A...
 

MaxHeadRoom

Joined Jul 18, 2013
21,601
Well i have only a 1A 24V power supply, but if i need to have cores with a big sectional area to overcome the voltage drop for 700m distance, maybe i can see the cost to buy another psu, like 2,5A...
Is it adjustable? But most 24v relays will still pick up with a 3v to 4v drop.
You don't really need a regulated supply for relays, a door bell transformer with just a bridge should do it.
Max.
 

Thread Starter

Pjota1

Joined Jun 13, 2019
18
Is it adjustable? But most 24v relays will still pick up with a 3v to 4v drop.
You don't really need a regulated supply for relays, a door bell transformer with just a bridge should do it.
Max.
No the psu its not adjustable...
Im not sure how to choose the core's sectional area to get less than 3v or 4v voltage drop with the 1A 24 vdc psu that i have. 0,8mm2 for each core is enough?
 

drc_567

Joined Dec 29, 2008
912
... Thinking that it may be possible to use the metallic shield layer of some type of coaxial cable as the DC current conductor between the switch and the relay coils. The idea is that the collective shield cross section will yield a minimum DC voltage drop. Even so, that is a considerable length of cable to purchase. ... No numbers at this point, so just a guess. The cost will likely be the determining factor.
 

Thread Starter

Pjota1

Joined Jun 13, 2019
18
If the coils only draw 30mA, CAT5 (or similar) cable should work. I'd put some snubber diodes across the switches just to be safe.
Ty for your help.
Yes i'll put diodes in parallel with the relay coils.
Anyone know how to calculate the voltage drop for that distance, just to be safe and know for sure what is the minimal wire cross section for that distance?
 

MaxHeadRoom

Joined Jul 18, 2013
21,601
What is the resistance of the coils? The Omron I have here have a 24v 628ohm coils.
There are several calculators out there that you just plug the numbers in and shows the result, I see it as around 2v drop, if you use the transformer and bridge method, the open circuit voltage will compensate.
Home depot has the 24v transformers.
Voltdrop calculator.net.
Max.
 

Chris65536

Joined Nov 11, 2019
237
No the psu its not adjustable...
Im not sure how to choose the core's sectional area to get less than 3v or 4v voltage drop with the 1A 24 vdc psu that i have. 0,8mm2 for each core is enough?
I assume that by "core" you mean the cable wire? Here's a wire gauge calculator:

https://www.inchcalculator.com/wire-gauge-size-and-resistance-calculator/
It looks like 1400m of 0.8mm^2 wire (1.0mm Dia.) would total about 30 ohms. If your 30mA guess for the coil is correct, the voltage drop would be about 0.9V.
 

MaxHeadRoom

Joined Jul 18, 2013
21,601
BTW, the transformer and bridge method is far more reliable long term than the SMPS version. ;)
If using a calculator don't forget the return path, the one I listed takes it into consideration.
Max.
 

Thread Starter

Pjota1

Joined Jun 13, 2019
18
I assume that by "core" you mean the cable wire? Here's a wire gauge calculator:

https://www.inchcalculator.com/wire-gauge-size-and-resistance-calculator/
It looks like 1400m of 0.8mm^2 wire (1.0mm Dia.) would total about 30 ohms. If your 30mA guess for the coil is correct, the voltage drop would be about 0.9V.
Yes, i've checked the relay specs and the current drawn by the coil is less than 30mA.
And yes, the core is each cable wire. Like i draw in the diagram, 4 wires. Btw, why do you say in your post 1400m of 0,8mm2? Its 700m for positve and 700 for negative?
 
Last edited:

Chris65536

Joined Nov 11, 2019
237
Btw, why do you say in your post 1400m of 0,8mm2? Its 700m for positve and 700 for negative?
Yes, you need to use the total wire length. Technically, half of the voltage drop will be on the positive wire, and half on the negative. But the total drop is all we're really concerned with. Also, if you trigger more than one relay at a time, the voltage drop will be higher, because the current in the common return wire will be higher.
 

Thread Starter

Pjota1

Joined Jun 13, 2019
18
You should be OK with something like cat3 24g, are all on at once at all?
Max.
Yes i'll need to sometimes trigger the 3 relays at the same time. I'll need near 0,1 mA to trigger them at the same time. For that distance maybe a cable with 4 wires/cores with 1,5mm2 cross section (or 8 with 0,8mm2) should be better to be safe, dont you think?
 
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