How to build a current detector switch circuit for home outlet

Thread Starter

Kirkstuff1

Joined Mar 2, 2008
3
I want to build what I am sure is a simple project. I want to build a circuit that will fit into a standard home single gang electrical box. The box will have installed a standard 20A – 120V outlet.

What I want is to energize only one outlet. The other outlet will be controlled by the circuit I want to build. Such that when current is detected flowing in the energized outlet the non energized outlet will then be energize. And when current is not flowing in the energized outlet the non energized outlet will be deenergized.

My objective is that when I turn on the device plugged into the energized outlet the other outlet will then become energized turning on another device.

Thanks for the help.
Kirk
 

thingmaker3

Joined May 16, 2005
5,084
I want to build a circuit that will fit into a standard home single gang electrical box.
By "standard," do you mean 8cuin, 18cuin, 22cuin, or something else?

What I want is to energize only one outlet. The other outlet will be controlled by the circuit I want to build.
First off, you'll need a "split duplex outlet" instead of the duplex outlets so commonly used. The "ordinary" duplex outlets have common hot terminals. The "split" duplex have independent hot terminals.

Such that when current is detected flowing in the energized outlet the non energized outlet will then be energize. And when current is not flowing in the energized outlet the non energized outlet will be deenergized.

My objective is that when I turn on the device plugged into the energized outlet the other outlet will then become energized turning on another device.
My first thought is to use a relay with 120Vac coil and contacts rated for 20A. You'd never squeeze that into a 22cuin box with a receptacle, though. The NEC allocates 11cuin for your outlet and conductors. (Assuming you don't have any additional conductors going through the box. If there is a line going out as well, then you'll only have 6.6cuin out of 22cuin for your control gizmo. Or 2.6cuin out of 18cuin - depending on your definition of "standard.")

Perhaps the voltage across a current-sense resistor could be sent to a differential amplifier, and the output used (via opto-isolation) to control a triac.
 

Thread Starter

Kirkstuff1

Joined Mar 2, 2008
3
I am no electrician so my terminology will be off. I don’t know what size a wall outlet having two receptacles is. I will use what ever size box will work.

No problem getting split duplex outlets.

Perhaps the voltage across a current-sense resistor could be sent to a differential amplifier, and the output used (via opto-isolation) to control a triac.
That is over my head and sounds like a more complex solution. A relay sounds more direct and simple. How would I wire the relay to the outlet to accomplish the desired result? Or would the relay come with instructions to do this?
 

thingmaker3

Joined May 16, 2005
5,084
Using a rely won't be all so simple either. The relay will need enough current through its coil to energize, but not so much current that it will burn up in your wall.

Do you know how much current your controlling load will draw?

Also, a relay with contacts rated for 20Amps will probably not fit into the box along with everything else.
 

Thread Starter

Kirkstuff1

Joined Mar 2, 2008
3
The load will be a table saw 1.5Hp, 120V, if i am reading the plate on the moter correctly 13A.

When i start the saw I want the relay to trip and enirgize the other outlet which will have my shop vac pluged into it. I could not find the specs for it. but it is a small 1.5 gal. shop vac. Whenb i turn off the saw i want the relay to open so the vac turns off.

I wil get or make a box large enough to fit what ever components are needed.

I really appreciate the help. What components do I need to get?
 

KMoffett

Joined Dec 19, 2007
2,918
Well, this is not going to fit in a single gang box, but it's what I made for my table saw. The output of the small relay, through a small two wire cable, switches an SSR on the other side of the shop, that switches on the shop vacuum. I also have a start-delay on the shop vac so the table saw and vac don't start at the same instant, and a stop delay to keep the vac running for a little while after the saw stops.

Ken
 

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Ron H

Joined Apr 14, 2005
7,014
Here are plans. I think you can buy these ready-made, but a search was not too productive. I have a friend who wanted one to automate his shop dust collector, but i think he wound up buying a remote controlled switch instead.
Some of the ideas I found in the search passed one side of all the loads to be sensed through a toroid, sensing the current.
 

RyanPline

Joined Jul 10, 2009
1
The design that KMOFFETT posted works very well and can be very compact with good circuit design. To conserve space I chose not to use a duplex but rather ran pigtails outside of a standard single gang junction box. This way my power cord is connected to a pigtail that goes into the j-box with all the circuitry. My master plug comes out as another whip and the slave plug as another. I also used a slightly different T1 and rectifying circuit but those are minor details. In the end I made 4 of these for about $30 total, but I had many parts laying around. Perhaps a better alternative for many would be to buy the "i-socket" found on Amazon.com.
 

CDRIVE

Joined Jul 1, 2008
2,219
Two things here. First.. Any standard duplex outlet can split the two outlets with the return common to both. They are made with a break-off conductor on the hot (copper) side. Secondly: Always do something like this built into a pigtailed box. Building this into a wall box will violate more electrical codes than you can count. If anything ever went wrong your insurance company could easily refuse to pay. ... Pigtails Always!

BTW: Automatic dust collection is a good thing. Old wood worker and long time fan of Norm Abram and Roy Underhill. Roy never has to concern himself with wood dust though. ;)
 

someonesdad

Joined Jul 7, 2009
1,583
A friend wanted this same thing, but settled on a remote-controlled switch activated by either IR or RF.

Another approach would be to use the table saw's contactor or switch to energize a simple delay circuit. When the table saw motor gets power, the AC power is used to start a timer (could be a simple RC charging network with a comparator) that energizes a relay that starts the dust collector.

Even simpler, time delay relays are made and you'd just have to wire the thing in; here's a versatile one. Here's another one that could be used to energize a relay on the dust collector.
 

zigan

Joined Oct 14, 2009
1
Dear KMoffet,

I am little puzzled about modifying T2,

"Secondary Replaced with 3T of #12 Stranded"

Could you describe what that's mean??

Thanks
 

KMoffett

Joined Dec 19, 2007
2,918
I used a split bobbin transformer. The style shown in the picture. I removed the secondary (low voltage) winding. In place of the secondary winding, I looped 3 turns of heavy gauge (#12 AWG) wire around the empty bobbin. That's all I had room for. This carries the current to be detected. I left the primary (120VAC) winding intact. This becomes the output winding, and with the burden resistor, produces a voltage proportional to the current through the 3 turns of wire.

Ken
 

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Is it possible to power the relay directly from the induced current of the current sense transformer T2? I'm trying to do something similar, but would like to avoid having a step down transformer. If the relay needs more power then could a Solid State Relay (SSR) be used instead?
 

inwo

Joined Nov 7, 2013
2,419
Is it possible to power the relay directly from the induced current of the current sense transformer T2? I'm trying to do something similar, but would like to avoid having a step down transformer. If the relay needs more power then could a Solid State Relay (SSR) be used instead?
I believe yes to both. Considering the large current draw of a table saw.

I did some tests in another thread not too long ago. Powered an led directly from a CT.

It should be a matter of the right current transformer. A small relay or ssr should not be too big a burden.

Think I still have the test CTs laying around.
 
I'm trying to do something very similar to power solenoids that will open dust collection blast gates for each of my machines. Since I have a couple machines, I want to minimize the cost of this circuit. The solenoids are 24VAC and I want to be able to adjust the current threshold that will trigger the relay.
This is what I came up with: The opposing zener diodes are to shunt excess voltage. Does this look like a good & cheap plan?

 
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