Advice needed!

[schexter]

Switching 120V AC devices with DC circuits?

Hello.
I have a total combined load of 10 A max, each component and the switches must be on very lengthy runs (120VAC). How, if possible and/or feasible, do I make the switching low voltage/low current? I'm trying to avoid having to implement 300-400 feet of 14ga x 4 wire SJO cable. It seems pretty simple - relays? SCR? Any suggestions would be appeciated, thanks!

Schecky

 

[mramos1]

Are you saying you have things 300-400 feet away you need to control?

More details please.

 

[dknguyen]

Hello.
I have a total combined load of 10 A max, each component and the switches must be on very lengthy runs (120VAC). How, if possible and/or feasible, do I make the switching low voltage/low current?

Schecky

Well..you would use relays, transistors, or solid-state relays (triac/diac/transistor-based) to enable low power signals to switch high power signals....

But you are going to have SERIOUS inductances in the lines...both the power lines and the control lines. So a device that can be switched with a lower power signal from a few feet away will not switch from 400 feet away. At 400 feet, the resistance in a 14 gauge wire is comparable (or greater than) a conventional length of 28 gauge wire. PLus, because of all the inductances you will need very massive switches.

Just look at this table:
Wire Gauge Resistance per foot
4 .000292
6 .000465
8 .000739
10 .00118
12 .00187
14 .00297
16 .00473
18 .00751
20 .0119
22 .0190
24 .0302
26 .0480
28 .0764

1 foot of 28 gauge wire = .0764
400 feet of 14AWG wire = 1.188ohms (almost double)

And even if you were a foot away...the current that the 28 gauge wire can carry is probably not going to be enough to control the size of switch you need. If anything, I would think you would need even thicker wire.

Surely, there must be some better way? But not enough is known about what you are doing to try and reorganize it.

 

[mramos1]

RF is an option, but we still have no idea what you are trying to do. RS232 at low speed will also make that trip to control things.

 

[dknguyen]

With RF you would need a power source close to each switch. I guess it depends on how much energy it takes to keep the switch on, and how long you need it on for.

 

[schexter]

switching...

This is a 'WARNING BELL/RED-LIGHT' system for a sound stage. There are several strobe and rotating lights and bells(all 120vac) positioned around the building, each wired with 14 ga THHN. The lights and bells are up to 300 feet apart, and are controlled separately by remote switches, which are themselves on a very long cable (up to 250 ft). What I want is an alternative switching method to control the lights and bells, similar to a light dimmer control board, but without the dimmer function. By alternative, I mean not having to use several hundred feet of 14 x 4 SJO cable because it is expensive and heavy!! And it adds to the voltage drop. The bells are momentary, and draw negligable current. My estimation of 10 Amps is safely high for the combined load. Thanks!

 

[schexter]

RF is an option, but we still have no idea what you are trying to do. RS232 at low speed will also make that trip to control things.

This is a 'WARNING BELL/RED-LIGHT' system for a sound stage. There are several strobe and rotating lights and bells(all 120vac) positioned around the building, each wired with 14 ga THHN. The lights and bells are up to 300 feet apart, and are controlled separately by remote switches, which are themselves on a very long cable (up to 250 ft). What I want is an alternative switching method to control the lights and bells, similar to a light dimmer control board, but without the dimmer function. By alternative, I mean not having to use several hundred feet of 14 x 4 SJO cable because it is expensive and heavy!! And it adds to the voltage drop. The bells are momentary, and draw negligable current. My estimation of 10 Amps is safely high for the combined load. Thanks!

 

[dknguyen]

Perhaps you could build a "box" with and in and out plug. Inside, the box would have the power relay (or whatever switch), a voltage regulator,an RF switch and some electronics to drive the relay/switch. The main cable would plug into the box's input. Inside, the wires would be split away in a Y-section to the relay input and to a regulator that powers the RF & electronics. Then a second main cable could be plugged to the output of the box which leads straight to the relay's output so that when the relay is closed, the power from the main cable is continuous.

Can the cables be sectioned off like this? So that the RF relay box can be the intermediary between two continuous cables?

THis lets you have a RF-controlled relay (no long control wires...which probably have to be thicker than 14AWG anyways due to accumulating resistance) that piggybacks the main cable for power (no batteries).

 

[philba]

so, you have 120 VAC at the locations for the bell/light and just need control? That should be fairly easy with relays and switches. You just run a control wire pair to each location. At the location you have a 12V relay with the coil connected to the control wire and the relay's NO and Com contacts in series with the hot wire to the bell/light. Your control panel would have a 12V power supply and a SPST switch per control circuit. One side of the switch goes to the power supply, the other to one of the control wires. The other control wire goes to the Gnd (or negative) side of the power supply. I would look at landscape sprinkler wire (18 ga, I think and pretty sturdy). If the wire will run through all the bell/light locations, you can share a single wired for Gnd and have only one wire for control at each location. You might have to start with a higher voltage to compensate for the voltage drop of the long runs (say 16V for 12V relays). I wouldn't bother with anything more elaborate like RF.

If you don't have the power, put in one run of 14 ga plus the higher ga control lines.