# AC current detector

Discussion in 'Electronic Basics' started by [email protected], Mar 4, 2006.

1. ### Guest

A google search shows this topic comes up repeatedly. The previous
posts are all over my head. I would prefer to find something off the
shelf, but have had no luck. At one time you could buy a power strip
that had one socket that would detect current flow and switch on the
other sockets. I took one a part many years ago and I recall it looked
real simnple. I can not find one of these any longer. I want to plug
in a vacuum and a tool (sander, router, bandsaw) and have the vacuum
come on automatically when I switch on the tool. Fein makes a vacuum
that does this but it costs \$300!
I found a circuit on the web:
http://ourworld.compuserve.com/homepages/Bill_Bowden/page8.htm#aclatch.gif
It looks to me like this might do what I need, except I'm confused
about how the relay coil is powered. Do I need to supply the 12VDC
separately or does it come from the magnetic pickup?
Also, is there a simpler circuit to do what I need?

Thanks,
Mark

2. ### kellGuest

Yeah, that circuit runs off low voltage DC that you have to supply
somehow.
But there must be something simpler. I have an idea.
Let's use round numbers and say your router pulls ten amps and you
happen to have a relay in your junk box that engages when 0.1 amps runs
through the coil. You could use a current transformer with slightly
less than a 100:1 ratio. It would mean winding 80 or 90 turns of fine
wire around ONE (not both) of the conductors running to your router.
The fine wire would put out a little more than one hundredth of the
current your router pulls. That would be enough to engage the relay,
with a little margin. You will of course get AC at the ends of the
winding, but that might work to keep the relay engaged. If not, just
use a bridge rectifier.
Beware that if you leave the ends of the current transformer unloaded,
you will get extremely high voltage. So don't run your current
transformer without the relay coil (or some test load of equal or
lesser resistance) connected to it.
Doesn't make much difference for this application whether you use a 12
volt or 24 volt relay, or whatever. You just want to know how much
coil current it takes to engage. Calculate the current by dividing the
relay voltage by its coil resistance. Example, a 24 volt relay with a
300 ohm coil would engage with .08 amps of coil current.

3. ### kellGuest

I just thought of something that didn't occur to me in my initial