Modifying a Hot Plate

[George L.]

I just bought a 1000W Hot Plate for 20 dollars. It is made for cooking food so it has a large coil, which gets VERY hot. I would like to modify this so I could use it for some chemistry projects. The coil gets TOO HOT. If you touch it on the lowest setting you can get burned. I would like to modify it so the heat range is lower and it doesn't get as hot.

I opened it up and found a very simple yet mysterious circuit inside. I can't figure out how it works so therefore I can't modify it yet.
Attached is a schematic of the inside and a pic of a mysterious part that controls the temp of the coil.
Does anyone understand how this works...the regulating device either has 0 resistance or reads as an open circuit???

(The Neon indicator comes on when the coil is "Active")

The regulator device is the biggest mystery...it has two circular contacts that move either closer or further depending on which way you move the dial. They touch throughout 80% of the range of the dial. There are only two contacts on it. What is it???


thanks

George L.

 

[ParkingLotLust]

Variable resistor or variable capacitor maybe?

 

[kchriste]

It is just a Bi-metal strip. When the temperature rises, it bends and opens the contacts shutting off current to the heater. Then it cools down and the contacts close turning the heater on again. And thus the cycle repeats.

 

[dknguyen]

You could use a relay and a thermocouple to shutoff when the temperature gets too hot and turn on when the temperature gets too low or use a triac for burst control of the current (similar to the relay except with the triac you can switch must more frequently and faster without wearing down the relay so you can get finer control with more circuitry). You could probably stick this between the AC line and the hot plate instead of opening the hot plate up at replacing the regulting device with the relay. Just turn the hot plate to maximum so that the internal regulator does nothing and let the device you built do the job instead (connecting/disconnecting power from the hot plate as required). Do it safe.

www.sparkfun.com sells appropriate thermocouples, thermocouple ICs, and relays. Look under the sensors->temperature and the items listed on the thermocouple page.

I'm working on the same thing with a toaster oven to use it to cure conformal coatings and reflow PCBs.

 

[George L.]

Thanks for the explanation and tip on how to modify the hot plate!

Could I just buy a light dimmer for 10 dollars and put it in the circuit where the bi-metal strip is? Would a cheap light dimmer be able to handle currents of 6A or so at 120VAC? (If not...I will probably have to do what dknguyen said about the thermocouple, but that would take a few weeks) Any other quick solutions?

thanks

George L.

 

[dknguyen]

Lights don't use nearly as much power as an oven does, and if it's the kind that just dissipates extra energy so that's a second oven right there.

You can either dissipate the extra energy or turn it on/off to keep the temperature right. And if you keep applying energy to the oven it will continue to heat up if it's not perfectly balanced to the energy being lost to the air- and it would take forever to heat up at that energy level.

 

[kchriste]

Could I just buy a light dimmer for 10 dollars and put it in the circuit where the bi-metal strip is? Would a cheap light dimmer be able to handle currents of 6A or so at 120VAC?

Yes.... If it is designed to handle 1000 watts (8.4A @ 120V) or more. Most cheap dimmers are rated at 500W or so.
You could try bending the bimetal strip in one direction. Turn the control knob from Max to Min and back a few times while watching the bimetal. Bend it so it is more "open" when cold and then test it to see how hot it gets. Maybe you should have bought one with a 500W element instead?

 

[Roff]

You could use a variac (rated at 10 Amps or more), but it will almost certainly cost you more than the hot plate did. You might find one on Ebay.

 

[Hero999]

How hot do you want your hot plate to get?

If it's below 100 degrees C or so then you could use a normal thermistor and a comparator to control a relay or triac or even rectify the incomming current and use a MOSFET.

 

[JimB]

The "unusual temperature controller" is similar to something I saw many years ago called a "Simmerstat".
The Simmerstat was just a bi-metal strip which heated up due to the current flowing through it and then opened the contacts. The strip cooled down and the contact closed.
The idea of this thing was to add a fine(?) external control to a simple electric cooking ring.

In effect it was like a crude form of pulse width modulation, varying the on/off ratio of the power supply. The thermal mass of the hot plate smooths out the temperature variation.

If you feel like experimenting with the control in the $20 hot plate, try to turn the U-shape piece of metal a but further in the low temperature direction. I cant see from the picture, but there has to be an end stop somewhere, if you can just override that end stop, you may get the thing to run not so hot.
Be aware that the temperature of the hot plate will not be constant, it will cycle as the contacts close and open.

JimB

 

[Techno]

**broken link removed**

A $26 kiln switch.

 

[zevon8]

Not to dismiss the contribution, but the above control listed by Techno is just an electric stove heat control. Works on nearly the same principle, bimetalic strip cycling the circuit on and off. It probably won't have the burner at much lower temperature than what you have now. Turning the dial to all the way "low" on any electric stove I ever used will still burn you if you touch the element.

What you need ( I think ) is one of those plate heaters that go from basically room temperature to say 300 degrees F. They have a flat metal/ceramic plate that you can heat a flask, beaker, pitri dish, etc on.

here's an example: http://www.thesciencefair.com/Merch...Product_Code=RG-70-1016&Category_Code=lab-bur

Check and see if there are any surplus places near you, these kinds of things are pretty cheap.

 

[philba]

build your own "dimmer". use a triac that is rated 2X the current draw. lots of circuits that you can use. You will probably want to add noise filtering. If you want to get sophisticated, use a uC and a thermistor to control the temperature.

 

[George L.]

Cheapest dimmer I saw at the store was 17.99 at 300W. This is the price of the hot plate.

Zevon8...that hot plate you linked is exactly what I want...the problem is the PRICE TAG!!! $149 is not cheap. Most others are even more expensive. It will cost 200 with shipping and tax. I think I will try to bend the bi-metal strip until I get the desirable range.

This seems to be the easiest solution. If not I will try to make my own dimmer as philba pointed out. (Radio shack doesn't sell triacs, I will have to order one)

George L.

 

[stevez]

To cut the power in half to the hot plate you might put a diode in series. That won't give you any more control but is a low cost way of cutting the power back. Make sure the diode has a decent current rating and possibly a heat sink. I'd start with 400 PRV, 20 amps - might or might not be overkill.

 

[zevon8]

The link was to a new one. Surplus places often get "clear-outs" from closing industry and labs, so often they will have say 30 of these in used but working condition.

Just for laughs, I checked Ebay, hows a current bid of $3.25 US for a lab hotplate with magnetic stirrer?

**broken link removed**

practically the same os the new one I linked in the earlier post: $11 current bid.

**broken link removed**

Nothing beats the right tool for the job. Do a search in EBay for "hotplate"

 

[George L.]

Steve...that is a good idea with the diode. Is it just cutting down the AC wave and therefore only putting though half the power? That would probably do...I have instant access to radio shack (I can walk there in 4 min.). They sell 3A 400 PIV diodes for 80 cents each.

Could I just put 6 of these in parallel to meet the 20 amps and maybe a few extra so I don't need a heat sink? I don't know if this solution is possible.

Zevon8...I guess they are cheaper then I thought, but I already have a hot plate for $20, all I need to do is modify it a bit.

George L.

 

[stevez]

I'd avoid paralleling diodes like that. Maybe you can find a heavy bridge rectifier and just use one of the diodes. Can't recall what Radio Shack stocks but I thought they had at least one heavy one.

 

[zevon8]

I would rather see a single diode capable of the load. Diodes don't always share well, even with a small series resistor, since small differences in forward voltage will upset the current sharing. Also, if one fails, it could quickly become a string of firecrackers as the load gets bigger as each successive diode fails.

A single diode capable of this would be in a stud mount package ( 1N3212 for example ) would need heatsinking, and obviously all of this needs to be isolated electrically from the hotplates metal which is surely grounded.

The diode is probably about 5 dollars, you could make a heatsink from some alluminum, or buy one for maybe another 5 or 10 dollars. then you need to build it all, and do it safely. If you need to pay for shipping of any of the parts, it adds to the cost.

Add it all up and maybe selling the hot plate or keeping it as a hot plate, and spending the money on a used lab plate maybe more cost effective.

Plan#2

As suggested, I would try to modify the bimetal switch, and maybe put a metal plate, say 1/4" thick on the burner to add thermal mass. This way even if the burner cycles really slowly, the metal plate will have stored thermal energy to "smooth out" the pulsing in the heat. This could be done pretty cheap.

 

[stevez]

Zevon makes a good point about cost however if you have some friends who might give you what you need then cost is less of an issue. I am constantly trading stuff like this with pals locally.

The diode will only cut the power to the element in half and maybe that coupled with some tweaking of the bimetal control might improve things.

I may be mistaken but I thought a number of light dimmers were available from Home Depot that would handle 1500 watts. The last one I bought was less than $10 but it also might have a lower rating.