Help - Trigger 24v relays

Thread Starter

Pjota1

Joined Jun 13, 2019
18
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.
4x1,5mm2 or 8x0,8mm2 (4*2) i'll be safe for trigger the 3 relays at the same time for 600 to 800m long? What do you think?
 

Chris65536

Joined Nov 11, 2019
254
4x1,5mm2 or 8x0,8mm2 (4*2) i'll be safe for trigger the 3 relays at the same time for 600 to 800m long? What do you think?
At 800m, I get about 9.2 ohms per leg. So with all 3 relays on together, you'd have 0.28V drop on the 3 supply legs, and 0.83V on the common return. That's the worst case: 1.1V drop. Will the 24V relays work with 22.9V? Probably yes. Is your power supply exactly 24.0 V, or just nominally?
 

Thread Starter

Pjota1

Joined Jun 13, 2019
18
At 800m, I get about 9.2 ohms per leg. So with all 3 relays on together, you'd have 0.28V drop on the 3 supply legs, and 0.83V on the common return. That's the worst case: 1.1V drop. Will the 24V relays work with 22.9V? Probably yes. Is your power supply exactly 24.0 V, or just nominally?
No, the power supply is 27, 6V. It's the power supply from an alarm painel. It supplies other equipment but has an auxiliar voltage output that i'll use. The current consuption of the system is low, só i'll have for sure extra 0,1A or more for the aditional relays.
The relays are remote signals that i have to give (to inform some conditions). The switches are some contacts/outputs from the panel with no voltage, that i will use to send the positive when they close.
The remote relays should work at near 20V i guess, but i'll check.
So by your calculations a cable with 4*2 pairs*0,8mm2 (8 wires with 0,8mm2) in wich i'll join 2 wires to get 4 conductors of 1,6mm2 cross section should do the job, with no issues right?
I'm talking about this kind of cable because its easier to get for me, and have fire resistance wich is important too.
 

Chris65536

Joined Nov 11, 2019
254
So by your calculations a cable with 4*2 pairs*0,8mm2 (8 wires with 0,8mm2) in wich i'll join 2 wires to get 4 conductors of 1,6mm2 cross section should do the job, with no issues right?
I'm talking about this kind of cable because its easier to get for me, and have fire resistance wich is important too.
You surely won't have any problems with 8 wires, but it is overkill on the copper. You could use just 4 of the 0.8mm2 wires (1 per leg), and still have a minimum of 25V at the relays.
 

djsfantasi

Joined Apr 11, 2010
7,189
I’ve always had one thought in the back of my mind in this thread. I quickly scanned through again and didn’t see the following comment.

With only three relays, why are you using a common ground? With three twisted pair, I would wire one wire of each pair to ground and the other to power the relay. Then you don’t have a larger voltage drop per relay that a common ground would create.
 
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AnalogKid

Joined Aug 1, 2013
8,804
24 ga wire has 84.2 ohms per km. 800 m one way is 1.6 km round trip, or 135 ohms. The voltage drop across that run at 30 mA is 4.1 V. So the first level question is - can your relays pull in reliably with only 20 V at the coil?

But wait, there's more. If all three relays are on at the same time, the voltage drop across 1/2 the run (the common return) will be about three times as high. The voltage across the relay coils will be significantly less; you can work out the details with Ohm's Law. The quick solution is to have a separate return for each relay.

ak
 

Tonyr1084

Joined Sep 24, 2015
5,494
Jumping in late - - - I've taken 24VDC relays out of a dishwasher and they switched almost reliably at 13.8 volts. Almost reliably because every now and then I had to give one a tap. (bang on it lightly)

Wife's Hyundai Sonata had an issue with the sun roof. Relays went bad (coils failed). So I replaced them with these 24VDC relays and the sun roof worked just fine. Used one more on a "Start Delay" setup for panel meters in my truck. One of my meters didn't like the voltage drop during starting of the engine. I would have to reset the color nearly every time. So using one of those 24 volt relays I built a four second delay. So when I turned the key I had four seconds to start the truck. Never had a problem there. And the relay would almost always kick in. By the time it kicked in the truck voltage was anywhere from 13.8 to 14.4 volts. And once in a while I'd have to tap on the kick panel to encourage the relay to click in.

So at the very least if you're still getting 14.4 volts to the coils of YOUR relays chances are pretty good they're going to kick in just fine. Of course these relays I used were low current draw. Don't happen to have one in hand but I'll look around and see if I can find one just to give you the numbers.

[edit] FOUND IT!
Sanyou SRD-S-124DM
Pickup voltage at 75% nominal voltage
Dropout voltage at 5% nominal voltage

According to the specs it should pull in at 75% of 24 volts. That calculates to 18 volts. Yet, I had these clicking in at 13.8 volts. Don't know how or why but - - - .
[end edit]
 
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