Adding Temp Control to a Lab Stirrer/Hot Plate

Varying the current? A simple thermostat to replace the function of SW2 would suffice!

Why is F1 in the neutral? Very odd.

Thanks PW!

As I said - very cheap. Probably made in ... need I say it.

But wouldn't you need a temperature measurement from the stuff you're heating? Or are you thinking of something like a Simmerstat from an electric hob?

"Simmerstat from an electric hob"
If that's English, could you please translate?

Don't know why #4 came as Anonymous

Perhaps you call them something different over there. It's for adjusting the output of eg a radiant ring on a cooker.

It has a small heater which warms a bimetallic strip and opens the main contacts. Heater is fed from downstream of the contacts so when they open it cools down and the cycle repeated. Biased by a spring so the contacts stay closed longer the more you turn the knob, and fixed closed at max setting. There is no feedback from ring or saucepan temperature, but it works OK.

Thanks Codemaster,

That sounds like just what I need. I could hack an old crock-pot or electric fry pan for the thermostat and try it at various distances from my heating element until I get reasonable results. Does that sound workable to you?

In the US, on commercial cooking equipment, that control is called an infinite control. Available at commercial equipment parts places. Need to know line voltage. The thermostats your mention need contact with the heated surface to read. If the bulb is not in contact with the heated surface, it will never shut off. -- JHF

Unless it's a Simmerstat

The easiest method would be to just use a PID temperature controller to read the thermocouple and drive a solid state relay to control the AC power of the heater. This will not give the finest temperature control possible but it will probably be sufficient for most applications.

Hi redfred,

(unless I am the victim of some very dry wit)

As I said:

"I do have a temperature controller using a "J" TC that I could incorporate into the circuit. It has a 24VDC output PWM that I could use, but the controller itself is about 2.5"W x 3"H x 5" L. I would also need a SS or Mechanical relay. This will add a lot more footprint that I would like to minimize."

I should add that it doesn't need to have feedback control - just something to variably decrease the power to the element.

My point in my time constrained (pre-coffee) reply was that the number of ON/OFF cycles the PWM of any controller will probably quickly roast a mechanical relay.

If you don't want closed loop control (temperature sensor, feedback loop) and can settle for manual control, perhaps a triac lamp dimmer would suffice. It needs to be rated to drive at least 400W load.

Thanks Rix,

That is a great idea!

Between that and the thermostat idea from Codemaster, I believe this one sounds better (as I don't have an old crock pot laying around).

I'll stop by Home Depot tonight.

If I find something, I'll let you all know how it goes.

To your #11, I think it would work OK, but I don't see what try it at various distances from my heating element has to do with it.

Ref the temperature controller and solid state relay method - the solid state relay looks cool, but doesn't the temp controller need an input from measured temperature, or is that your "J" TC? Perhaps I'm missing something.

In redfred's link no size given for the controller, but it doesn't look small. A Simmerstat is about a 1.5 inch cube, plus the knob.

Re: try it at various distances from my heating element has to do with it. - Doesn't the bimetallic strip sense the heat of the element? Maybe I'm missing something here!

Yes, the controller I have has a Type J TC input (and I do have a type J TC and wire to go with).

The bimetallic strip in the Simmerstat only sees heat from the heater in the device. It doesn't get any feedback from the ring or saucepan. It doesn't matter where you put it. It's OK as you soon get an idea what setting it needs to simmer after your veg has come to the boil, usually 1 - 2 out of 5.

If you can live with the physical size, your existing kit seems OK. Presumably you will attach the thermocouple to the hotplate somehow. Make sure it's fixed firmly as if it falls off it will overheat!

Perhaps a potentiometer like this one inserted in the heater circuit. Use a thermometer to dial in the temperature you need.

http://www.newark.com/multicomp/mcwwpt-10-502-lf/trimmer-potentiometer-5kohm-000turn/dp/05N1633

At 3A and 120V, the power that the pot would have to handle would be 360W.

Here are the search results for a 300W rheostat on DigiKey:

http://www.digikey.com/product-search/en/potentiometers-variable-resistors/rotary-potentiometers-rheostats/263488?k=potentiometer&k=&pkeyword=potentiometer&pv2=129&FV=fff40004%2Cfff80540&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

I really don't want to spend over $200 for this fix!

Still cheaper than buying a stir plate that does what you want ;)

You could probably use the triac controller off a 400 watt aquarium heater....

http://ww1.microchip.com/downloads/en/AppNotes/00958A.pdf

If you don't need precise temp control, then Codemaster's simmerstat sounds like a good fit.

If you do need precision then: http://www.amazon.com/Inkbird-100-Thermostat-Temperature-Thermocouple/dp/B01489R8RG/ref=sr_1_1?ie=UTF8&qid=1464975088&sr=8-1-spons&keywords=inkbird+temperature+controller&psc=1

might be a good fit ($40.49). Size is ~2'x2"x3". The SSR is 1.75"x2.25". The 25A rating is a lot more than you need, but that means you won't need to use the included heatsink.

Amazon has lots of cheap controlers.

Try a skeleton bi-metal thermostat similar to ATC Semitec type TY23A with two 4.5mm holes drilled into the stirrer plate 18mm apart. One hole to mount the thermostat, the other to expose the adjustment screw. Switches 10A at 250vAC, 100,000 cycles life. There are lots of manufacturers for this type of stat, some with much smaller footprints, this was just the first one I found on the web (no recommendation implied)

Any old iron, any old iron? No not the song but the type to smooth clothes. Old ones had a bimetal switch of the simmerstat type. One leg/contact is bimetal and wrapped with a nicrome? heater element to make it open then when it cools it closes etc. Adjusted by the setting knob for cotton, wool etc. pushing on a spring to make it harder to open for more heat.

Jim

I just came across this and thought of you;

http://www.ebay.com.au/itm/AC-220V-10000W-SCR-Voltage-Regulator-Motor-Speed-controller-Dimmer-Thermostat-/121478453500?hash=item1c48ae18fc:g:7FAAAOSwzOxUWI2f

Jim

A 400W light dimmer? Maybe, I've never looked.

Your temperature controller PWM output can drive a commercially available Solid State Relay (SSR). An SSR has no moving parts. Its output switches AC current, its input is designed to be driven by a temp controller output. For your app, a 10 amp SSR will easily handle the 3 amp heater load.

Terminal 1 and Terminal 2 are wired in place of the connections for Switch SW2. (remove SW2)

Terminals 3(+) and 4(-) wire (plus to plus, negative to negative) to the PWM output terminals on the temperature controller, where polarity makes a difference - you need to look up which is positive and which is negative.

The J thermocouple wires to the temperature controller and the other end is the sensor end. Whatever temperature the loose end senses is the temperature that the temp controller sees and reports. If you stick it in whatever you're heating, then control to that temperature. If you stick it on the plate itself, then you control to that sensed temperature.

If you're in the US, the thermocouple red wire is negative (really). If wired backwards, the controller will show an decreasing value for an increasing temperature (swap the wires).

Reading a temperature controller manual is a challenge. The writers all assume you are conversant in process control theory and convention.

An SSR generates some heat and heat kills electronics. It is expected that the SSR will be fastened to a heat sink that will dissipate the heat it generates. Find an old PC motherboard with a CPU aluminum finned heat sink, clean it off and use some thermal paste to fasten the SSR to it.

F1 might be the thermal fuse that opens when the device overheats. Yeah, in the neutral ? ?

Claimer: If you buy a $22 SSR from Omega you will make me rich beyond my wildest dreams so I can retire, buy a luxury yacht and cruise the world. Please. Please.