120V heating element control

Archiver

Hello fellow Stampers,

I would like to use the BS2 to "autosense" a 120VAC heating element being
plugged in and activate a relay to supply power to the wires feeding the
element until the heater is unplugged, at which time the relay would drop
out and the power would be disconnected. In other words, monitor the feed
going to the heater receptacle and supply power when a heater is there, and
shut off the power when it isn't.

My first thought was to somehow sense the resistance of the element as it's
being plugged and unplugged, but I'm not sure if that's even possible with a
120V source present. If so, how would you interface it to the stamp's input?

My second thought, while not as sexy, involves pushing a button after the
heater is plugged in to activate a holding relay and using a current-sensing
relay to detect when the heater has been unplugged to drop out the holding
relay. I'd rather not go this route because I can use off-the-shelf parts
which means I don't get to play with the stamp ;~)

Any thoughts or suggestions would be very much appreciated!

TIA,
Tom



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Archiver

You can try winding a few turns of copper wire around the the 120V (AC I
guess!)
powerline somewhere near your circuit and use some sort of simple circuit to
turn the induced ac (small amplitude, small number of turns) in to some form
recoconizable by your BS2. This way there is no electrical contact between
120V circuit and your stamp circuit.
Basically it is a current sensor. There is current only when you plug the
heater.
May be you can get readymade transducer of this type available somewhere. I
know the clip-on current meters are available and they are in expensive, but
I don't know if you can easily modify them for your use.

Hope this helps.
Vetrivel
UCSF

Archiver

At 04:28 AM 1/6/01 +0000, Tom S wrote:
>Hello fellow Stampers,
>
>I would like to use the BS2 to "autosense" a 120VAC heating element being
>plugged in and activate a relay to supply power to the wires feeding the
>element until the heater is unplugged, at which time the relay would drop
>out and the power would be disconnected. In other words, monitor the feed
>going to the heater receptacle and supply power when a heater is there, and
>shut off the power when it isn't.
>
>My first thought was to somehow sense the resistance of the element as it's
>being plugged and unplugged, but I'm not sure if that's even possible with a
>120V source present. If so, how would you interface it to the stamp's input?

Sounds like its gonna take a two pronged attack (pun not intended). First,
you have to sense that the heater has been plugged in - by your definition,
120Vac is NOT present at that time. So - you can use a DPDT relay to
disconnect the heater outlet from the 120 Vac source and instead connect it
to a circuit that simply looks for continuity. Once you have detected
continuity, you cause the relay to operate and apply power to the outlet.

Now that you have power on the outlet, you need to detect when the heater
has been disconnected. You would use your current sensing relay for that
purpose.

The controller for all the above could be a stamp, but it seems like major
overkill to do so. The continuity detector is pretty simple - I personally
would use a 555 connected as a flip flop to directly drive the relay. In
essence, use one input to sense when there is continuity. The current sense
relay drives the other input to reset the FF. Grab a block diagram of the
555 and think on it for a bit.

There are many other techniques as well. But I don't see anything that
justifies the expense of a stamp.

dwayne



Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax

Celebrating 16 years of Engineering Innovation (1984 - 2000)

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Do NOT send unsolicited commercial email to this email address.
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commercial email nor is intended to solicit commercial email.

Archiver

> >Hello fellow Stampers,
> >
> >I would like to use the BS2 to "autosense" a 120VAC heating element being
> >plugged in and activate a relay to supply power to the wires feeding the
> >element until the heater is unplugged, at which time the relay would drop
> >out and the power would be disconnected. In other words, monitor the feed
> >going to the heater receptacle and supply power when a heater is there,
and
> >shut off the power when it isn't.

I came across this just a few hours before I read this note, and here is
a nifty device that I'm surprised hasn't been introduced sooner!

http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR2530
They've got others similar to this one, too.

Interface this with a phototransistor and you have all you need. All
this really is in a nutshell is a current transformer, some limiting
circuitry (unlimited voltage is very high otherwise!) and an LED! Like the
kewl factor, too.

Archiver

Thanks for the reply, Dwayne. You've hit the nail right on the head because
there's one thing I forgot about - the thermostat! If I was to use just the
pushbutton/current sense/holding relay approach and the thermostat reached a
point where it was satisfied it would turn the heater off and NOT TURN IT
BACK ON! So I definitely need to use both methods.

My next problem is this: is it posssible to have a sensing circuit that
would give me a window of about 6-10 ohms? The elements are about 8-9 ohms
and I would like to be able to differentiate between a heater and a person.
I know the stamp doesn't have the capability to measure such a precise
resistance, but maybe there's a circuit that can give it a go / no-go
signal?

I'm not hell-bent on using the stamp for this. I just love using them!! :~)
If I have to go with straight electronics I will, but I just like the fact
that the stamp is so versatile that if I need to change something (like in
this case, maybe adding a time delay before the power is switched on) I can
usually do it with just a few lines of code. I guess you could say it's the
"tweakability" of the stamps that makes them so darn cool!


Thanks for the help!
Tom



>Sounds like its gonna take a two pronged attack (pun not intended). First,
>you have to sense that the heater has been plugged in - by your definition,
>120Vac is NOT present at that time. So - you can use a DPDT relay to
>disconnect the heater outlet from the 120 Vac source and instead connect it
>to a circuit that simply looks for continuity. Once you have detected
>continuity, you cause the relay to operate and apply power to the outlet.
>
>Now that you have power on the outlet, you need to detect when the heater
>has been disconnected. You would use your current sensing relay for that
>purpose.
>
>The controller for all the above could be a stamp, but it seems like major
>overkill to do so. The continuity detector is pretty simple - I personally
>would use a 555 connected as a flip flop to directly drive the relay. In
>essence, use one input to sense when there is continuity. The current sense
>relay drives the other input to reset the FF. Grab a block diagram of the
>555 and think on it for a bit.
>
>There are many other techniques as well. But I don't see anything that
>justifies the expense of a stamp.
>
>dwayne
>
>
>
>Dwayne Reid <dwayner@p...>
>Trinity Electronics Systems Ltd Edmonton, AB, CANADA
>(780) 489-3199 voice (780) 487-6397 fax
>
>Celebrating 16 years of Engineering Innovation (1984 - 2000)
>
>* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
>Do NOT send unsolicited commercial email to this email address.
>This message neither grants consent to receive unsolicited
>commercial email nor is intended to solicit commercial email.
>
>

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Archiver

Thanks, Carl! It's PERFECT!! And at less than a third of the current sensing
relay I was looking at in Grainger!


Date: Sat, 6 Jan 2001 21:39:12 -0800
> From: "Carl McIver" <cmciver@m...>
>Subject: Re: 120V heating element control
>
> > >Hello fellow Stampers,
> > >
> > >I would like to use the BS2 to "autosense" a 120VAC heating element
>being
> > >plugged in and activate a relay to supply power to the wires feeding
>the
> > >element until the heater is unplugged, at which time the relay would
>drop
> > >out and the power would be disconnected. In other words, monitor the
>feed
> > >going to the heater receptacle and supply power when a heater is there,
>and
> > >shut off the power when it isn't.
>
> I came across this just a few hours before I read this note, and here
>is
>a nifty device that I'm surprised hasn't been introduced sooner!
>
> http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR2530
>They've got others similar to this one, too.
>
> Interface this with a phototransistor and you have all you need. All
>this really is in a nutshell is a current transformer, some limiting
>circuitry (unlimited voltage is very high otherwise!) and an LED! Like the
>kewl factor, too.
>
>
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