If it's a very small relay and the transformer is built correctly.

The power for the relay will be the burden on the CT.

It may be hard to get enough coupling unless there is a large load or many turns.

The small cores, loads, and #of primary turns that I played with can just barely light an led.

There are relays designed to pull in with current rather than voltage.
Common on furnaces to run dehumidifier with blower.

 

If it's a very small relay and the transformer is built correctly.

The power for the relay will be the burden on the CT.

It may be hard to get enough coupling unless there is a large load or many turns.

The small cores, loads, and #of primary turns that I played with can just barely light an led.

There are relays designed to pull in with current rather than voltage.
Common on furnaces to run dehumidifier with blower.

For those Google: Aprilaire A50 A51

Ken

 

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?

If you have a sensitive 24V relay it might need 10 mA to pull in. A 100:1 transformer with that relay on its output would need 1A on its primary to pull the relay in and appear as 0.24 Ω in series with your load. That doesn't sound unreasonable. You should probably match your transformer ratio quite closely to the currents you expect if you want this to work well, that makes an adjustable version more tricky but it should work over a limited range.

Essentially use the most sensitive 24V relay you can find driven from a full wave bridge rectifier directly from the transformer secondary. You'll need the zeners but they could substitute for two diodes in the bridge. To my knowledge a.c. coil relays with such low actuation currents don't exist.

 

Home made CT drives a SSR directly.

No points for neatness.

If both the saw and vacuum cleaner are on the same circuit, the starting current of both activating simultaneously would be quite high. A delay between the starting of the saw and the starting of the vacuum would be highly desirable, otherwise a good, simple plan.

 

If both the saw and vacuum cleaner are on the same circuit, the starting current of both activating simultaneously would be quite high. A delay between the starting of the saw and the starting of the vacuum would be highly desirable, otherwise a good, simple plan.

I did that, and posted it on a forum. Six months later someone came out with a commercial version. Available from Rockler or Woodcraft.

Ken

 

I've built L.Chung's version of (Ken's) circuit but used an external 12vdc brick to power the SSD, rather than the "built-in" CT transformer power supply.

I'd like to incorporate the delay circuit proposed as well but I'm not sure wher to plumb it in. I'll eventually figure it out but it would be nice if that's backed up by input from others.

Thanks.

This is the delay circuit:

http://forum.allaboutcircuits.com/attachment.php?attachmentid=2323&d=1204569634


Here is L. Chung's mod:

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

Hi Ken,

I'm sure your original design will work as is but I'm always a cautious man when working with current transformer circuit.

I would add two diodes to your current sensing circuit as transformer T2 is working like a current transformer.

 

I would have used a ACS712 30A type, followed by a OP-amp wich in turn energized a SSR.
To power the ACS712 you could use a LNK304
To piece theese together is a piece of cake, using the datasheets following each of the components.

 

I would have used a ACS712 30A type, followed by a OP-amp wich in turn energized a SSR.
To power the ACS712 you could use a LNK304
To piece theese together is a piece of cake, using the datasheets following each of the components.

Feel free to do so.

 

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

thanks for the design

 

Someone trying CPR on a 6yr old thread?
Max.

 

I'm planning on building something based on KMoffett's circuit in post #6 above but I am planning on replacing the Darlington and the reed relay in the delay circuit with a 2N2222 driving a relay with a coil current of 33mA at 12V, is this approach OK, I am missing anything?

 

Because of the difference in the relays' coil currents and the difference in the hfe of the Darlington and the 2N2222 there may be a significant difference in timing.

The original design was combined from circuits on Bill Bowdw's website: http://www.bowdenshobbycircuits.info/page2.htm

Ken

 

Many thanks for the quick reply Ken, good point about the hfe, I'll have a go at simulating it.

 

I went back and looked at the linked-to circuits. Bill had used 2n2222s.

Ken

 

Yes, I was just looking at that. In the the Power On time delay circuit he uses a pair of transistors connected as a Darlington pair but I guess that is just to drive the relay and in your circuit is replaced by Q2.

 

For smaller loads: (Computer stuff)
"Belkin Surgemaster 6 Outlet Energy Saving Surge Protector"

Google "Energy Saving Surge Protector" (there are multiple brands)

Belkin - multiple sources:
http://www.amazon.com/gp/product/B0..._m=ATVPDKIKX0DER&pf_rd_r=1PTWRYY2SMAY8M7NGHEA

Cheaper at Dell:
http://accessories.us.dell.com/sna/productdetail.aspx?c=us&l=en&orig_s=gen&sku=a3264149&s=dhs

https://www.google.com/shopping/product/11034311207808072706?q=Belkin%20Surgemaster%206%20Outlet%20Energy%20Saving%20Surge%20Protector&prds=paur:ClkAsKraXzObJxb3Xiov5VWX3cK8bIxrK8d57eSD4smlE3WQT_0m1KsirMoz9dgRiiRASDN5bN6YBBsrpIIyDt5luoXvwecEmvw8900ibZ_8NrFLWIOF2GgqsBIZAFPVH70DAg9g8lmPG8CsMVQO1YOMoM-LEQ,scoring:tp&ved=0CBwQpitqFQoTCOL17uyE8sYCFRJZiAodjmYIjw&ei=QkexVeLMM5KyoQSOzaH4CA

 

Tank you very match

I believe that's pronounced Tang Yew Berry Mulch.

ak