controlling a mains-powered oven

I found my cheap oven very easy to take to bits to check stuff, etc. As it was very mechanical, it was easy to trace all the wires and work out what was doing what. In the end, I simply wired the SSR over the 2 terminals going into the rotary thermostat thingy, so to run it with the Pi, I leave the dial set to zero but turn on the main power and oven selector, or just use the temperature dial to run it in the old way.

Also, the main power knob is the mechanical timer knob - one hour max, so I just wind that up, then if something does go wrong, at least it will turn itself off after an hour.

For your oven - if it's a capiliary tube system, you might get away with bypassing the existing controller by simply buying a replacement tube +

the one on my Beko when I replaced it last year.

However, here's a suggestion - scrap the oven and replace it with a really

Then use the old one as a test-bed to play with, then retro fit the controller to the new oven (or swap them back) when you have Pi control :-)

Good luck!

Gordon

Reply to
Gordon Henderson
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I applaud your ingenuity. That said, if I had to do such a thing, I'd likely just order an industrial panel unit that measures temperature and contains enough "stuff" to control such things as ovens and heaters. In my long ago working days as as a QC instrument and electronics tech, when I ran out of more useful things to do, I'd end up calibrating temperature controllers for molds, ovens, and so forth. The control units always impressed me as to the accuracy and durability.Today, with oven resistive elements, and high capacity MOSFETS I'd likely consider a PWM controller. EMI might be a problem to consider as well. Solid state switches could also be used, and should be the zero crossing type.

Reply to
Charlie

FWIW we just bought one of these:

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which they also sell with a cheap oven:
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We haven't had a chance to get the hang of it yet, but since everything is sold as a kit it seems to work together reasonably well. This is for soldering rather than baking, but the same principle holds. The EUR119 for the controller is less time, hassle and risk of blowing things up than trying to make one ourselves.

The controller output won't do more than 1.5kW, but secondary switching (eg a mains relay) should handle that.

Theo

Reply to
Theo Markettos

I'd be tempted to use any conventional oven temperature control present as a backup do not go above temperature setting. Relay contacts have been known to occasionally fail closed. If I remember correctly, 50hz is harder on contacts than 60hz.

Reply to
Charlie

It becomes a matter of how far do you go? There is *no* kind of disconnector that can't fail with the contacts fused, but in over 50 years industrial experience I've only come across it once, and that was because a fire in the cabinet warped the plastic housing.

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W J G
Reply to
Folderol

It becomes a matter of how far do you go? True, so true!

But, with electric Dryers and Ovens, at least in the US there often is a high temp cutout. Gas Ovens may or may not also have one. We had an older name brand gas oven melt the pot metal gears in the built-in rotisserie. Seems that the electrically operated gas valve stuck open. I ended up doing something that you supposedly are not to do, and repaired the valve. In the US the classic gas valve relies on a spring to close. Eventually, the spring weakens slightly, and stuff from the gas builds up, causing the valve to stick. An old gas company repairman (From the days when the gas company provided free repair) showed me how.

The industrial gas and electric ovens we used in the 70's had a safety temp sensor and lockout. If it tripped, you found out why, fixed the problem, and reset the trip.

One area I had nothing to do with by choice was aluminum casting. They melted aluminum and cast the air tight explosion proof containers used for oil well switch and motor control gear.

Reply to
Charlie

I finally got round to doing a little blog post in this:

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cheers,

Gordon

Reply to
Gordon Henderson

Have you considered that if, you are controlling it electronically, with care, thought and some careful timing, it ought to be possible to arrange the relay contacts to open at a zero crossing?

--
Stuart Winsor 

Tools With A Mission 
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Reply to
Stuart

Never with a mechanical contact. Even if you grab the right time for the cut (only by chance!), the distance between the opening contacts will raise slower than the voltage, causing a nice flashover.

My suggestion would be a Solid-State-Relais.

For maximum security you have to backup the installation with a mains relais controlled by and only by a thermo-switch opening by overheating in the oven. If this relais is allways closed befor the oven is started and normallly only opens after shuting down the oven, it will work its contacts with no load and deliver a maximum lifetime.

--
Have a nice day! 

Kallu
Reply to
Kallu Wiegand

Yes, of course, but you will still generate less of an arc than if the contacts open at Max volts. Zero crossing is easy enough to detect and anticipate so you cut the relay holding current a few milliseconds before zero.

--
Stuart Winsor 

Tools With A Mission 
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Reply to
Stuart

A relay capable of of safely breaking that kind of current takes upwards of

15mS to fire, so you would be (variably) about 3/4 the way though a complete mains cycle.
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W J G
Reply to
Folderol

"Less"??? With a resistive load opening the contacts at the zero crossing will create no arc since current is in phase with voltage and so also zero. I believe the arc is largely created by inductive loads because of the large voltage spike caused by the large dI/dt, no?

--

Rick
Reply to
rickman

A (theoretical) solution would be a propper treshold. If that dammned^hbad mechanical part wouldn't alter its behavier in such a short time! Some thousend switchings and the timing is far from good and ever changing.

--
Have a nice day! 

Kallu
Reply to
Kallu Wiegand

And a practical solution is to use an SSR that does all the zero crossing for you without pissing about with mechanical relays, timings and other rubbish that went out with the arc.

Gordon

Reply to
Gordon Henderson

I'd have thought you'd have a snubber to prevent that :-P

Reply to
Andy Burns

^^^^

Oh very good...

--
the biggest threat to humanity comes from socialism, which has utterly  
diverted our attention away from what really matters to our existential  
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Reply to
The Natural Philosopher

Hmmm "that went out with the arc." :(

The only proviso I'd make is that if you are doing fast switching, use the mains itself as your switching time source. That way you can ensure you don't inadvertently create any DC offsets with odd numbered half cycles.

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W J G
Reply to
Folderol

Absolutely. So that's two of us who understand switching AC loads.

Reply to
mm0fmf

The switch that put the R in AC..

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the biggest threat to humanity comes from socialism, which has utterly  
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Reply to
The Natural Philosopher

1/ No load is PURELY resistive. The other parts may be small but not always neglible with higher power loads. 2/ Contacts take TIME to move apart and most relays that you will see have air gap, and at 1 mm mains voltage can arc, that is why there are creepage and clearance DISTANCES involved with dealing with voltages generally above 50V AC and 36V DC.

Having had to replace many a mains switch or relay due to arced contacts eroding or becoming insulated.

Even power grid switches that are immersed in oil have characteristic pops, bangs and bubbling for switching on or off, causing some oil to avaporate and similar.

--
Paul Carpenter          | paul@pcserviceselectronics.co.uk 
    PC Services 
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Reply to
Paul

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