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PICList Thread
'[PIC] crockpot control?'
2006\01\11@032228 by Michael Park

picon face
What's a good way for a PIC to control (on/off) a crockpot? (120 VAC, a
couple hundred watts.)
"Good" in this case means simple, cheap, and most importantly, safe. I'm a
bit scared of high voltage, and I consider anything over 5V high :)  I'd
just go with a relay, but I expect there'll be arcing when the contacts
close and that makes me nervous. Better ideas?


2006\01\11@034926 by Shawn Wilton

picon face
Solid state relay if you're just looking to turn it on and off.  Sharp makes
these.  So do other companies.  Look for one with a built in zero-xing
detect.


On 1/11/06, Michael Park <spam_OUTmp__TakeThisOuTspamhotmail.com> wrote:
>
> What's a good way for a PIC to control (on/off) a crockpot? (120 VAC, a
> couple hundred watts.)
> "Good" in this case means simple, cheap, and most importantly, safe. I'm a
> bit scared of high voltage, and I consider anything over 5V high :)  I'd
> just go with a relay, but I expect there'll be arcing when the contacts
> close and that makes me nervous. Better ideas?
>
>
> -

2006\01\11@041619 by Jinx

face picon face
> What's a good way for a PIC to control (on/off) a crockpot?
> (120 VAC, a couple hundred watts.)

Dear scaredy-cat Michael ;-)

I'm afraid you're going to have to come to terms with some voltages
with a lot more ooomph behind them than a 78L05

The PIC will be a timer ?

First thing you have to sort out is the power supply. Something
along the lines of this

http://home.clear.net.nz/pages/joecolquitt/txlesspsu.gif

Unless you want to go with a transformer/plugpack. Which sounds
like you'd be more comfortable doing. And a lot less dangerous too

A transformerless supply has the possible advantage that it might be
physically smaller and can be installed inside the equipment. Although
your PSU load is fairly low, so you should be able to use a quite
small transformer

A relay isn't a bad idea, even if it does arc a little (there are ways
to minimise or eliminate that anyway). But relay coils need power,
a few 10s of mA, so the PSU has to be up to that. As a rough
guide, 1uF at 50Hz will "pass" about 70mA. The 0u68 capacitor
in the diagram -> 0.68 * 70mA = 48mA. But you put in whatever
you need to get the current required

One alternative to a relay is an opto-triac / switching triac combo.
The opto-triac isolates the PIC from the high voltage and also
provides drive for the main switching triac. eg the r-h side of

http://home.clear.net.nz/pages/joecolquitt/ir-rec.gif

The load is your element. As it's resistive, I'd not expect any
of the problems you have with something inductive like a
motor. If the opto is a zero-crossing type, eg MOC3041,
that would be better, but may not be necessary

2006\01\11@042518 by Roy

flavicon
face
part 1 838 bytes content-type:text/plain; (decoded 7bit)

Detect zero cross point for switching relay on or off.
That way there will be minimal arching.
You could use a TRIAC - same as light dimmer circuits.
Jus make sure you wire TRIAC correctly as I have blown up a PIC/RS232
port that way :-(


> What's a good way for a PIC to control (on/off) a crockpot? (120 VAC,
a
> couple hundred watts.)
> "Good" in this case means simple, cheap, and most importantly, safe.
I'm a
> bit scared of high voltage, and I consider anything over 5V high :)
I'd
> just go with a relay, but I expect there'll be arcing when the
contacts
> close and that makes me nervous. Better ideas?
>

--
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.371 / Virus Database: 267.14.16/225 - Release Date:
9/01/2006




part 2 35 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

2006\01\11@045752 by Joe McCauley

picon face
If it is a resistive load, then why not use a solid state relay. They take
care of zero crossing point automatically & the control line is optically
isolated from the AC lines.

Joe

> {Original Message removed}

2006\01\11@072818 by Bill & Pookie

picon face
Thought first of a device that stores have that turn on lights at dusk.
Hook crock pot to that and put light or led next to sensor and tape the
combo light tight.  But that would require led to be on to keep crock pot
off.  Could this be considered a "FAIL SAFE" feature?

Alternative the first would be to get one of the "clap clap it's on, clap
clap its off" clapper thingees for lamps.  Again (sort of as before), place
a speaker from the pic next to sensor and tape it along with maybe some
soundproofing.  Monitor the crock pot with a temperature sensor to insure
that it did go on.

Bill

Is there a prize for the first team to use a Pic in the Rube Goldberg
Competition?



{Original Message removed}

2006\01\11@074237 by olin piclist

face picon face
Michael Park wrote:
> What's a good way for a PIC to control (on/off) a crockpot? (120 VAC, a
> couple hundred watts.)
> "Good" in this case means simple, cheap, and most importantly, safe.
> I'm a bit scared of high voltage, and I consider anything over 5V high
> :)  I'd just go with a relay, but I expect there'll be arcing when the
> contacts close and that makes me nervous. Better ideas?

Since you had to ask, keep it simple and use a plain old relay.  Get one
rated for the current, and it should last quite a long time.  Temperature
can't change that fast, so you should be able to get perfectly fine control
and not switch the relay more than once every 5 seconds, probably longer.
When the relay wears out after 10 years, replace it with a new one.

I once made a darkroom water bath temperature regulator that switched a
consumer immersion heater on/off as needed every 64 power line cycles.  It
worked incredibly well and was still working last time I tried to use it,
which was probably 15 years or so after it was built.  By the way, this was
built in 1980 and didn't have a PIC in it.  The logic was just a comparator,
a flipflop, and a 6 bit counter.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\01\11@113816 by Mike Hagen

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face
Forget the PIC, use a $5 wall light dimmer?

----- Original Message -----
From: "Michael Park" <.....mp__KILLspamspam@spam@hotmail.com>
To: <piclistspamKILLspammit.edu>
Sent: Wednesday, January 11, 2006 12:22 AM
Subject: [PIC] crockpot control?


> What's a good way for a PIC to control (on/off) a crockpot? (120 VAC, a
> couple hundred watts.)
> "Good" in this case means simple, cheap, and most importantly, safe. I'm a
> bit scared of high voltage, and I consider anything over 5V high :)  I'd
> just go with a relay, but I expect there'll be arcing when the contacts
> close and that makes me nervous. Better ideas?
>
>
> --

2006\01\11@115222 by Mike Hagen

flavicon
face
I re-read the thread and realized you wanted ON/OFF, try one of the
appliance controllers the plugs into a wall plug.  Some are all electronic
with LCDs others just a motors with wheels running a contact.  They are
usually good for a lot of amps and real cheap at Christmas time at ACE
Hardware.  I guess people buy them to turn tree lights on and off.  They are
usually about $10?

{Original Message removed}

2006\01\11@192355 by Jinx

face picon face
> Found a great Microchip app note:

www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&
appnote=en021266)

BE VERY WARY of Microchip app notes dealing with power
switching. I don't recall if the above is it, but some people have
serious concerns about the safety of at least one


> Hi Jinx, and thanks for the info. In the first diagram, what are the
> squiggly things on the left?

> They looke vaguely coil-like, but I don't think that's what they are.

That's what they are alright, toroidal ferrite coils scavenged from
busted PC supplies. You'd recognise them, about 20mm diameter,
probably around 50-100uH

A few of those components are for spike suppression and could
actually be omitted. The 630V caps, the coils and the 10W
resistor for example. If you're getting them free from discarded
equipment though........  The capacitor is essential though

Working with mains in this situation is dangerous and you should
be using an isolating transformer. And especially if you want to
measure it with anything earthed like an oscilloscope. Connecting
scope E to circuit N is not good

Please be careful

2006\01\11@195703 by Michael Park

picon face
>
>Please be careful
>
No worries -- I'm a scaredy-cat, remember?
This power line stuff is very interesting and educational, but reading about
it might be as far as I go.


2006\01\12@042742 by Alan B. Pearce

face picon face
>> Hi Jinx, and thanks for the info. In the first diagram,
>> what are the squiggly things on the left?
>>
>> They looke vaguely coil-like, but I don't think that's what they are.
>
>That's what they are alright, toroidal ferrite coils
>scavenged from busted PC supplies. You'd recognise them,
>about 20mm diameter, probably around 50-100uH
>
>A few of those components are for spike suppression and
>could actually be omitted. The 630V caps,

It is worth the effort to get capacitors rated for X2 safety standard, as
they are designed for continuous mains use, and if they fail, they do it in
a safe mode.

2006\01\12@045802 by Jinx

face picon face
> It is worth the effort to get capacitors rated for X2 safety standard

It's marked X2 on the diagram, but a point worth emphasising in case
the significance was missed

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