Sense Power Line Cord, Turn-on Fan

Actually, I need to look in the garage... I probably have one in my junk box.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

There is a product that is the electrical current equivalent of a mechanical limit switch, usually called a current switch. You pass a current carrying conductor through a hole in it, and either a powered or unpowered contact closes at a preset or adjustable current. E.g.

I read in sci.electronics.design that Jim Thompson wrote (in ) about 'Sense Power Line Cord, Turn-on Fan', on Wed, 17 Aug 2005:

Pass *one* conductor of the mains lead through a high-permeability (low frequency) ferrite toroid and wind say 50 turns of thin wire on the toroid. That will give you enough signal (roughly sinusoidal pulse waveform at mains frequency) to do with what you will, and I can't think you need to be told how to process it to operate a solid-state relay.

I don't know of one here, but you are elsewhere.

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The "Mini Power Minder" is shown on

as available from several different vendors for around US$ 15-20

Sears sells an auto-switch designed for power tools. I'm not sure how well it will work with your TV.

--
--Steve

Hello John,

That's the way to do it. #77 material comes to mind which is quite ubiquitous (Amidon etc.).

You can buy them, look under current transformers:

But by the time you have finished the order you'd be done rolling your own. If you have the toroid handy, that is. No idea if Radio Shack carries hi-perm toroids. It used to be all #43 material there but even that should work in a simple application like this.

Or slaughter a switcher and pick it out of there.

Regards, Joerg

Radio Shaft used to have a power strip that turned the monitor and printer, for example, on when the PC was switched on. It sensed the current on the master outlet and switched the others on. It wasn't cheap, tho. There are still other companies that sell a similar outlet strip, probably much cheaper than RS. Check around, on the 'net and at Home Depot, etc.

Radio Shack (atleast *used to* carry them),and probably other local places (Circuit City,etc)sells power/surge strips for home entertainment systems with a "sense" outlet or two,and the rest were switched on/off depending on the load sensed on the "sense" outlet

Turn your TV on,and it senses it,and turns on the stereo/vcr/dvd player for you.

[snip]

But aren't switch supplies using ferrite mat'l that's going to do okay at 80kHz or more, but would saturate easily at 60Hz? I guess it's no big deal if it saturates, since it just passes the current thru easier. As long as there is enough out of the sense winding to turn on the relay driver.

(it

One thing that I don't believe anyone has yet suggested is a heat sensor. The fan comes on when it gets hot. and then turns off when the sensor gets cool. Trick is to get the sensor near the point where it can reliably sense heat. Another thought is to use a simple light sensor on a power LED of the nmaster equipment.

Hello Jim,

While you are at it look at any transformers flying around. Small ones. If you find one where there is enough space to loop a decent 12 gauge around it once or where you could pry off the secondary to do that you'd be in business.

Another source are those "oh s..t" toroids that get clamped onto computer gear cords after they blew the first EMC test. If you can get it off. Sometimes they are snap-on.

Regards, Joerg

"John Woodgate" schreef in bericht news: snipped-for-privacy@jmwa.demon.co.uk...

Googling for 'power slave switch' turned up this:

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Thanks, Frank.
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Look at the "Smart Strip" at

It has pretty much all you need to run a complete entertainment center from a single device. Probably overkill for a single fan, but perhaps you could use it anyway. Save some construction time. They have a number of outlets, both controlled and always hot. Surge protection and EMI/RFI filter. Adjustable sensitivity too. I have a couple of these units and from personal experience, I can recommend them. They have worked flawlessly for over 3 years. Sure beats pushing all those darn switches.

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Out of idle interest (since you've gotten lots of real answers already): what for? Is the fan to cool the TV (why does it need cooling?) or for something else?

As already pointed out, ferrites designed for the KHZ range make lousy

50-60 cycle current sensors.

Cheap and easy to add a few turns to a transformer and use one of the existing windings to sense current. The "flat pack" style cores on small 60 HZ power transformers are easier to use in that respect (square iron structure with separate bobbins for primary and secondary). A hacksaw can easily remove one set of windings without damaging the other.

I've used them to do things like turn on a horn when a warning light fails, or turn on the fan when some chemist in the lab powers up the an ultrasonic cleaner in an un vented enclosure - at work. So simple, I never drew the schematics. Basically a current sensing transformer made from a square core power transformer. Usually the 12 VAC winding works best to drive a solid state relay through a rectifier bridge and small filter cap.

These things where all the rage in the 50-60's. and were used to turn off the vacuum tube amplifier when the last record in a stack had played - a circuit or two appeared in Poptronics at the time. (and a lot more complicated with no solid state relays)

I built a little gizmo for my water heater. Sounds a buzzer for a few seconds when the heating element turns on or off, and blinks an LED while the element is on.

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You can roll your own cheaply with either cat# VT-27 or VT-5 from

Ed

Some of the kits were a low voltage relay in series with the load, and had back to back clamping zener diodes to limit the current flow. SOme used a full wave bridge in line with the load to reduce the voltage drop, and used an opto-isolator to turn on a relay. Heathkit sold one, and several hobby magazines had construction projects. You might turn up the Heathkit schematic online if anyone has the model number.

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The TV is tucked under a kitchen cabinet, the contents of which are getting hot... coffee beans, vitamins, daily medications, etc. ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Might be a good excuse to replace it with a modern LCD TV receiver! :-)