inductive sensing relay ?

Looking to have a relay close when current passes through a wire that I'm NOT allowed to cut into, or probe.
Basically leaves me with needing an inductive
pickup which ultimately can pull in a N/O relay
Is there an off the shelf solution, or any other ideas ?
Thank in advance!
Glenn
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Hello Glenn

How about a minature split core current transformer providing the current is ac. then rectify the output current and use it to drive your relay. It would only give you a low energy output so you might need to use a reed switch or something like that to drive the final relay. If you have not come across split core transformers "Google".
John
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Hadleigh IPSWICH England
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I have had some luck using a current transformer feeding an OP amp.
This is an example of what you will see
http://gfretwell.com/electrical/CT%20fun /
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     snipped-for-privacy@aol.com writes:

Beware that a current transformer must never be allowed to have an open circuit secondary - that can result in some nasty unsafe voltages.
I can't see any load resistor on the current transformer secondary in your setup. If you are really measuring the *voltage* output of a *current* transformer into a high impedance scope, it isn't giving you a useful representation of the actual current waveform, and indeed the current waveform for the 100W lamp looks wrong. You need to have a relatively low load resistance across the CT secondary, and then measure the current through it by measuring the voltage across it. The value of the resistor depends on the CT turns ratio, CT power rating, and the max current on the primary side.
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Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
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On Fri, 11 Mar 2011 00:18:24 +0000 (UTC), snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

My bad, I didn't talk about the resistor. The packaged unit I came up (device picture) with has a 1k resistor potted in there. That will screw into a 3/4" knockout in a regular US electric box. That was big enough to provide a load without actually crushing the output. It dropped about half with 100 ohms. If you are sensing with an OP amp it really would not matter that much since they are pretty easy to drive into saturation.
I am using LM324s
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I have to agree with Andrew about the waveform which looks a lot like transformer magnetizing current. The thing is that you want the load on the secondary of the CT to be low as it will reflect to the primary. Ct's should , ideally be at the minimum load resistance for which you have a reasonable voltage for your sensor. 1K seems high Do you have any information with regard to the turns ratio? What is the voltage across the 100W lamp with the CT and scope vs without the CT and scope? In other words, does the addition of the CT with its load have any appreciable effect on the lamp load/
Don Kelly cross out to reply wrote in message wrote:

My bad, I didn't talk about the resistor. The packaged unit I came up (device picture) with has a 1k resistor potted in there. That will screw into a 3/4" knockout in a regular US electric box. That was big enough to provide a load without actually crushing the output. It dropped about half with 100 ohms. If you are sensing with an OP amp it really would not matter that much since they are pretty easy to drive into saturation.
I am using LM324s
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wrote:

I was just playing with the CTs. I don't really know much about them I got them for a buck apiece from a surplus outfit.
The 1k did not seem to affect the output much so I went with it I don't think the CT had any appreciable loading effect on a 120v 100w light bulb.
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wrote in message wrote:

I was just playing with the CTs. I don't really know much about them I got them for a buck apiece from a surplus outfit.
The 1k did not seem to affect the output much so I went with it I don't think the CT had any appreciable loading effect on a 120v 100w light bulb.
OK, trying to remember the scope trace information, the resistance reflected to the primary is quite negligible.
Just don't open circuit the CT. as it is a step up voltage transformer.
Don Kelly cross out to reply
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The way I view it, the inductance of the transformer (with secondary open circuit) determines the voltage drop there will be across the primary. That voltage drop is multiplied by the turns ratio to get the voltage on the secondary. CTs generally have significant inductance to be able to drive a secondary current proportional to the primary current. Core saturation can also be a limitation. (But magnetics is not my strong suite.)
For gfretwell's CT, it fits in a knockout, doesn't have much inductance, and probably can't produce much secondary open circuit voltage. I think that is why the 100W (if I remember right) waveform is not a sine wave (or saturation?). The CT should be enough to drive a circuit to do what the OP wanted.
I certainly agree, in general, that you want a low resistance load, or short, on the secondary of a CT.
==============Runway/taxiway lightning at large airports is often series wired with thousands of volts supply. Lights are tapped off with a CT. They must have a voltage limiter or automatic short on the lamp/secondary side for when a lamp burns out.
-- bud--
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at constant current
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Guys,      What im trying to do is to open a powered damper when a commercial range hood is turned on.
No silly stuff here...
Im not sure what the actual voltage will be, or the current the motor draws. All I know is that as soon as the hood is turned on, I need to open the damper. I thought that I could do it easily electrically but it looks like using air pressure switches will be much simpler.
Thanks any way guys
Glenn
On 3/12/2011 6:42 PM, Don Kelly wrote:

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did you try Google? particularly current sensing relay humidifier current sensing relay current relay current switch
Does it have to be UL listed?
some possibilities from a simple search http://www.automationdirect.com/adc/Overview/Catalog/Sensors_-z-_Encoders/ Current_Sensing_Transducers?source=google&keyword=current%20sensing %20relay &type=search&gclid=CPOE5s_FzqcCFcHrKgodHClVGQ recombine into single line
may be the same http://www.automationdirect.com/adc/Overview/Catalog/Sensors_-z-_Encoders/ Current_Sensing_Transducers;jsessionid^30fb06835232301eddd329453e7b19673a ?source=google&keyword=current%20relay&type=search &gclid=CJGo043FzqcCFZFoKgodU27dFQ recombine into single line
(I believe automation direct tends to be spendy. Finding their source is probably cheaper.)
<http://www.grainger.com/Grainger/current-sensor-relays/relays/ electrical/ecatalog/N-8ek>
<http://customer.honeywell.com/honeywell/ProductInfo.aspx / 32001754-001#ProductSpecification>
<http://www.time-mark.com/SearchResults.aspx?categoryID=3
<http://energycontrol.com/Current-Monitoring-Sensors.aspx ? gclid=COLLu4_FzqcCFRG4Kgod0VH1Cw>
<http://www.crmagnetics.com/products/CR-9321-P11.aspx
If a sensor is a donut/window type with your wire going through it, a 10A sensor with a singe wire through the window is a 5A sensor with the wire looped 2 times through the window.
alt.hvac would be good but the inhabitants tend to work in field and often don't like mere mortals
sci.electronics.design might be good. State if UL is important.
alt.home.repair might be good
-- bud--
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On 3/14/2011 2:01 PM, bud-- wrote:

Jack POT !
http://cgi.ebay.com/Generalaire%2fAprilaire-50-Current-Sensing-Relay_W0QQitemZ130496445207QQcmdZViewItem
I looked for every other term I could think of !
THANKS !
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GH wrote:

Use a thermostat.
--
You can't fix stupid. You can't even put a Band-Aid on it, because it's
Teflon coated.
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So just come off the switch for the range hood and run it to the damper motor. If the voltages are different then run it to a relay or a transformer which will in turn control the damper. It's not that complicated.
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Owner does not want 1 wire touched in the hood. They dont want any warranty issues.
All I have is the house wiring going to the hood. And yes, because of this, it is a little more complicated.
If I could, I could grab power from the power light, pull a relay.
If I could I would make the on button open the damper, and when fully open and only then pull a relay that powers the fan...
There are a ton of ways, I know.
But do it NON destructively ?
Still looks like pressure switch is the only way, but the problem with that is grease..
On 3/13/2011 8:06 PM, Rich. wrote:

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How are they turning the hood on? Since it's a commercial hood, there should be a contactor powering the hood so that the fire suppression system has a means for turning the hood off. Right?
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On 3/13/2011 11:27 PM, Rich. wrote:

Commercial hood retrofitted for residential being place in elite home(s) No fire suppression. I know, its better by the min...
Im thinking of getting one of the circuit testers that light up when placed near a live wire and seeing if it can power a reed...
Again, I cant believe that something isn't already available.
Maybe I should try looking into remote house management or alarm system control's...
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GH wrote:

Is the current AC or DC? At what level? Clamp-on ammeters are available for both cases. It's then just a matter of amplifying the output to the meter enough to actuate the relay.
--
VWWall

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On 3/9/2011 3:29 PM, VWWall wrote:

AC and its to a variable speed exhaust fan, so I I would imaging from 20 to 120V.
Again, we are willing to spend a few bucks for off the shelf solutions...
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