current sensing switches

[Nick]

Hi,

I'm wiring up my shop. I have a total of 8 separate wall circuits.
One circuit is solely for the dust collector. Two of them are 220's.
The rest are simple 20 amp circuits. The lights are on completely
unrelated circuits. They will all come into the breaker box via the
bottom. I am interested in wiring them up so that whenever current is
being drawn by any of the seven, the dust collector circuit will
"sense" that a tool is on and then kick in the dust collector.
Finally, it would be nice to make it so that the dust collector
remains running for an additional 10 or 15 seconds after the tool is
shut down. I wouldn't need to have all seven circuits hooked up this
way maybe just four of the 20 amps and one of the 220's. With ample
instructions I think I can handle the job myself. I'm not that
familiar with relays but if you can point me to the parts I would need
and give me some wiring tips I should be able to do this. At first I
was just going to get one of those remote control gadgets from Penn
State or wire in a half dozen 4-way switches scattered around the room
but this solution would be neater and fully automatic.

Thanks,

Nick

 

[John Fields]

Hi,

I'm wiring up my shop. I have a total of 8 separate wall circuits.
One circuit is solely for the dust collector. Two of them are 220's.
The rest are simple 20 amp circuits. The lights are on completely
unrelated circuits. They will all come into the breaker box via the
bottom. I am interested in wiring them up so that whenever current is
being drawn by any of the seven, the dust collector circuit will
"sense" that a tool is on and then kick in the dust collector.
Finally, it would be nice to make it so that the dust collector
remains running for an additional 10 or 15 seconds after the tool is
shut down. I wouldn't need to have all seven circuits hooked up this
way maybe just four of the 20 amps and one of the 220's. With ample
instructions I think I can handle the job myself. I'm not that
familiar with relays but if you can point me to the parts I would need
and give me some wiring tips I should be able to do this. At first I
was just going to get one of those remote control gadgets from Penn
State or wire in a half dozen 4-way switches scattered around the room
but this solution would be neater and fully automatic.

---
That sounds like a perfect application for a current transformer (CT),
like one of the ones at

http://www.crmagnetics.com/com_tran.pdf

What you could do would be to run all the returns from the machines
through the hole in the CT before you connect them to neutral in the
box, and then use the output from the CT to turn on the relay/contactor
for the dust collector.

If you rectified and filtered the CT's output you could use that to turn
on a solid-state relay which in turn could turn on the dust collector's
contactor, or turn on the dust collector itself if the SSR could handle
the current. Also, if you arranged for the filter cap on the input of
the SSR to be part of an RC circuit you could use its decay to give you
the turn-off delay you want. The main thing you'd have to make sure of,
though, is that you can get enough out of the CT with the lowest-current
tool running to do what you want to do, yet with all of the tools
running that its output is made incapable of hurting the SSR's inout.

Interested?

 

[John Fields]

That sounds like a perfect application for a current transformer (CT),
like one of the ones at

http://www.crmagnetics.com/com_tran.pdf

What you could do would be to run all the returns from the machines
through the hole in the CT before you connect them to neutral in the
box, and then use the output from the CT to turn on the relay/contactor
for the dust collector.

If you rectified and filtered the CT's output you could use that to turn
on a solid-state relay which in turn could turn on the dust collector's
contactor, or turn on the dust collector itself if the SSR could handle
the current. Also, if you arranged for the filter cap on the input of
the SSR to be part of an RC circuit you could use its decay to give you
the turn-off delay you want. The main thing you'd have to make sure of,
though, is that you can get enough out of the CT with the lowest-current
tool running to do what you want to do, yet with all of the tools
running that its output is made incapable of hurting the SSR's inout.

Interested?

 

[John Popelish]

Hi,

I'm wiring up my shop. I have a total of 8 separate wall circuits.
One circuit is solely for the dust collector. Two of them are 220's.
The rest are simple 20 amp circuits. The lights are on completely
unrelated circuits. They will all come into the breaker box via the
bottom. I am interested in wiring them up so that whenever current is
being drawn by any of the seven, the dust collector circuit will
"sense" that a tool is on and then kick in the dust collector.
Finally, it would be nice to make it so that the dust collector
remains running for an additional 10 or 15 seconds after the tool is
shut down. I wouldn't need to have all seven circuits hooked up this
way maybe just four of the 20 amps and one of the 220's. With ample
instructions I think I can handle the job myself. I'm not that
familiar with relays but if you can point me to the parts I would need
and give me some wiring tips I should be able to do this. At first I
was just going to get one of those remote control gadgets from Penn
State or wire in a half dozen 4-way switches scattered around the room
but this solution would be neater and fully automatic.

Thanks,

Nick

One method would be to run one conductor from each of those circuits
through a single current transformer. The current transformer output
can be rectified with a bridge and its voltage clamped with a big
zener diode so that a small DC relay (coil voltage same as zener
voltage) can be powered by this regulated output The contact from that
relay can pass power to the coil of a time delay after disenergize
relay that operates a bigger contactor that operates the dust
collector motor. Manual switches could also be tied into this relay
system to force the collector on or off. The design trick it sizing
the current transformer, zener and relay to respond to the minimum
load and not be damaged by the maximum possible load. If the range of
loads is too large, you will need several current transformers,
bridges, zeners and small relays, with all the small relay contacts in
parallel. Ideally there would be one of these current sensing
circuits for each power circuit. But you may be able to get by with
fewer, especially if their currents pass through the current
transformer in a way that half of them cancel the other half. The
chance of having two loads that are so nearly equal that they cancel
is not likely.

At least, this may give you some ideas.