How much distance !

[yusuf]

Hello guys !

I have 12v relay which needs 12v dc and 100ma current!!

And I have a logic circuit which switches the relay on/off !

But I have put the logic circuit in my home and the relay (which switches on/off) at my office !
The distance is approx : 2km from my home...

I am just creating this circuit for only learning !
I know We can use RF or wireless signals ...
But Here I want to use normal wire for 2km distance...

Will the logic circuit can switch the relay on/off at the distance of 2km....
or what will be the max distance !

I am using 12v dc power supply !


Thanks in advance !

 

[JimB]

But Here I want to use normal wire for 2km distance...

Will the logic circuit can switch the relay on/off at the distance of 2km....
or what will be the max distance !

Most unlikely, it all depends on the resistance of the wire, which at the moment we do not know.

I am using 12v dc power supply !

To allow for the resistance of the wire, you will need to increase the supply voltage.
Or, you could use very thick and expensive wire!

JimB

 

[Reloadron]

While I do not see this as at all practical you would need to figure the 12 V 100 mA and they start with wire resistivity. You make no mention of the wire you would use? You have a voltage and you have a current so now you need to know the wire or choose a wire with very low resistivity for the wire run. Again, not practical but that would be what was needed to do. Then apply ohms law with the wire resistance. You want 12 volts at the finish line.

<EDIT> I see Jim beat me to getting a post done. </EDIT>

Ron

 

[4pyros]

You would need to know exactly what kind of wire and the temperer it will operate in.
You all so would need to now if you are using two wires or one. With two wires your distance doubles.
Than you can figure the total resistance of the wire.
Here is a chart for copper wire;
http://hyperphysics.phy-astr.gsu.edu/hbase/tables/wirega.html
As the total wire resistance starts to reach the resistance of the relay it may not work and you will have to incress your supply voltage.
You can do it with one wire like the old days of the telegraph. look it up.

 

[alec_t]

If the hold current (20mA?) of the relay is a lot less than the 100mA pull-in current (it is for many relays) you could use a large capacitor at the remote point to provide a pull-in current pulse. That would enable the use of thinner wire than would be needed to supply 100mA continuously.

 

[ADWSystems]

Everyone has it right. You'll need to take into account the resistance of the wire.

Out of cuorisity, I used 4pyors link and assumed a round trip (ie., power supply->wire->relay->wire->ground). 2km=6561 feet, using 30 gauge wire at 103 ohms/1000ft, the wire has 6.561*103=675 ohms resistance. If the relay requires 100mA, then it's equivalent resistance is 120 ohms. Your equivalent circuit actually is +V -> 675 ohm resistor ->120 ohm resistor ->675 ohm resistor -> ground. So for 100mA through the equivalent 1470 ohms (675+120+675) your power supply will need to be 147V.

Of course if you you larger gauge wire, the resistance will be less and thus the voltage required as well. Changing to 20g, changes the resistance to 66 ohms each way and requires only 25V power supply.

Let us know how you solve this.

 

[MrAl]

Hi,


If you dont use an idea like alec provided then you'd need to use number 10 gauge wire. Here is why...

The relay takes 12v at 100ma, which means the coil resistance is 120 ohms. Now to get proper pull in you should provide at least 90 percent of that voltage at the relay terminals. To get at least 90 percent of the voltage to the relay the series resistance of the wire can not be much more than about 12 ohms (one tenth of 120 ohms). To get 12 ohms at 4000 meters (2000 meters both ways means 4000 meters) you need at least 10 gauge wire which has resistance about 13 ohms for 4km.

As a test for reliability, connect a 13 ohm resistor in series with the relay and see if it still turns on. If it does (and it should) then connect an 18 ohm resistor in series with the relay and see if it still turns on. If it does, the relay should operate reliably with the extra 13 ohms from the wire.

To use a capacitor boost, you could construct a voltage doubler at the drive end of the wire. With the voltage doubler boosting the one shot voltage up to nearly 24 volts, that means you can get away with 120 ohms total wire resistance, or 30 ohms per 1000 meters. This would allow number 18 gauge wire to be used which is a lot cheaper than number 10 gauge wire. The cap however has to be sized large enough to provide enough of a current pulse without loosing too much voltage. If the relay pulls in within 100ms (and most do) then a capacitor value of about 10000uf would be required. To test this, use a cap of 5000uf and see if that works, and if it does then use 10000uf. If it doesnt, try 7500uf and if that works then use 15000uf, if that doesnt work try 10000uf and if that works use 20000uf. So test with 1/2 the capacitance, but use the full value in the application.

 

[Reloadron]

you need at least 10 gauge wire which has resistance about 13 ohms for 4km.

For our friends outside the AWG world that would be a wire diameter of .1019 inch or 2.5 mm diameter or 5.26 mm square. That will be a pretty expensive affair. Not to mention it will be heavy.

Ron

 

[MrAl]

For our friends outside the AWG world that would be a wire diameter of .1019 inch or 2.5 mm diameter or 5.26 mm square. That will be a pretty expensive affair. Not to mention it will be heavy.

Ron

Hi Ron,

Yeah i agree totally. I couldnt see doing it that way either because of the expense alone even without considering anything else. This is one reason i liked the cap boost idea originally suggested by alec.

Of course we have the standard approach of using an AC voltage instead, where we use 120vac at the drive end and a transformer and rectifier at the receive end. That should reduce the wire size significantly. What we dont know is what the limits are here, what we are allowed to do and what we are limited to.
For example, is there a power source located at the receive end? Then we might be able to use a RF signal to switch the relay, which would mean of course no wire at all

Another consideration is the longevity and durability of a double wire that is 2000 meters long. Is it underground? Special wire with the right insulation would be required. Is it suspended between poles? Strain reliefs, etc., etc., required. Is it laying on the ground? Might require conduit.

 

[user_88]

There was a similar experiment to this a few years ago ... circa 1850 ... with the first transatlantic telegraph cable ... laid from Ireland to Newfoundland.
The telegraph pulses weren't getting through from one end to the other. They tried ramping up the input voltage, but just burned out the cable.
The problem turned out to be the distributed inductance associated with the cable, a concept that was not known at that time in history. A few years later, the problem was fixed by using capacitors at intervals to compensate for the inductance. The individual associated with coming up with the corrective innovation was Joseph Henry ... an American.

Sorry ... going over the details, it appears that the problem with the first cable might have been distributed capacitance, rather than distributed inductance.
See this link ... towards the end;
https://en.wikipedia.org/wiki/Transatlantic_telegraph_cable

Also, there is an interesting story in a book by Paul j. Nahin ... Oliver Heaviside: Sage in Solitude

All things considered ... at a distance of two kilometers, and a reasonable pulse rate, you might not encounter serious difficulties.

 

[4pyros]

You can use a single wire fairly easily.

 

[ADWSystems]

Also, there is an interesting story in a book by Paul j. Nahin ... Oliver Heaviside: Sage in Solitude

All things considered ... at a distance of two kilometers, and a reasonable pulse rate, you might not encounter serious difficulties.

Hey what a minute I know that name. Oh yeah! Professor Paul j. Nahin of the Univeristy of New Hampshire. He was only my professor for three or four courses.

 

[user_88]

That book was written several years ago ... probably not available today except in certain libraries.

 

[ADWSystems]

That's OK he was my professor several years ago.