Thermostatically controlled ac outlet

[pcc]

I'd like to buy or build a thermostatically controlled ac outlet. I want to use it on my electric barbecue smoker. So it needs to have a probe on lead, rather than the temperature sensor built in to the outlet. I would place the temperature probe in the cooking chamber.
It must be capable of controlling a 1650 watt load.

It should turn on the outlet when temperature drops below the set temperature, and turn it off when the temperature goes above the set temperature. The threshold temperature should be adjustable (I don't need a read out; I can calibrate it using a thermometer). The tempurature threshold needs to be adjustable in the approximate range of 200F to 300F.

If anyone can help me by referring me to an existing product, or point me to circuit designs, I'd appreciate it. I'm not experienced in electronics or electrical projects, but I'll either figure it out, or get help from some friends at work.

Thanks
Phil

 

[Oznog]

Thermistor and potentiometer hooked up to an analog comparator, and a solid state relay.

Realistically, you probably want some hysteresis in the switching, else when the temp exactly hits with switching point it goes on, off, on, off as minor noise unbalances the comparator reading. I think a high value resistor pulling the set temp (from the pot) a bit higher when the signal to the relay is on and a bit lower when off will cause enough latching to fix this problem.

 

[zevon8]

Maybe an electric oven capillary tube type thermostat would do the job. I realize it's just buying a part, but if you check the price of a solid state relay, along with the rest of the circuitry involved, I bet the cost is close.

A suitable enclosure, a cord, and an outlet would be all you need after that.

Any appliance repair place will stock oven thermostats.

If you found the exact temperature you needed, you could get even simpler with a bimetalic snapdisc type thermostat. It would then just be the snap switch, and an outlet.

 

[pcc]

Could I use a triac dimmer circuit instead?

Could I "dim" the heating element with an incandescent light dimmer? I guess it would have to be rated at 1500W, the approximate wattage of the heating element.

 

[Nigel Goodwin]

Re: Could I use a triac dimmer circuit instead?

Could I "dim" the heating element with an incandescent light dimmer? I guess it would have to be rated at 1500W, the approximate wattage of the heating element.

NO! - you should use 'burst fire control' for heating elements, using a 'phase shift' dimmer would cause excessive interference.

 

[stevez]

Watlow, Partlow and Omega are manufacturer's or sellers of temperature controllers that might have what you need. You might also find some of these at surplus or salvage places. This type of controller is very common for controlling heating elements in small industrial furnaces, ovens, tanks, etc. In selecting the controller you'll need to choose the type of sensing element. You'll also see that you'll have a choice of outputs so that you might drive the heating element directly or use a relay. Some of these combinations can be had at reasonable prices.

If the controller does not have some kind of 'sanity' checking it might be wise to build some kind of backup shutdown system. The 'sanity' check might be to shut the system down should your probe fail to sense that the cooking chamber was warming while power is being applied. This could happen if the probe was accidentally moved or possibly disconnected. A klixon or similar snap disc thermal element might also serve this purpose - to prevent overheating.

In situations like this the heating element can be turned on or off for fairly long periods of time - as little as a few seconds to half a minute - without adversely impacting the cooking chamber temperature. Because of this you can avoid having to control on each cycle as is normally done on inexpensive lighting or heating controls.