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Thermostat Circuit

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Overclocked

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I am building a hot plate to heat things, I got the heaters from a grill cheese machine my mom was throwing out, so I kept the heating coils. They work, and when you plug them in, they glow bright red. I really wouldnt like to do that, since I fear if when Im heating what I want to heat, it will catch fire.

I measured its resistance (after cooling down to ambient) to be about 6.5 Ohms. I powered it using 120V, since thats what it was used for, but it was in series with another heater. The current required came out to be around 18 Amps! When I calculated power, I came out with something near 2.2kW!!! kw!!

Along with safety in mind, along with Enormous power requirements I decided to make a controller. I need controller to shutoff once it reaches a certain temp, but turn back on once it starts cooling.

**broken link removed**

Does this circuit turn the load On when the temp is below a certain temp and then turn it off when it is above that temp?

**broken link removed**
But whats the difference in function between these 2 circuits?

Would it also be Ok to use a 16 Amps relay even though my load is 18 Amps? Would it make a difference?
 
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6.5ohms ia the cold resistance, when hot it's much higher than that.

Those circuits turn on the device when it gets too hot. You need the reverse, you could swap TH1 and R1 + POT1 round, use a positive coefficient thermistor or use a relay with normally closed contects.
 
Hero999 said:
6.5ohms ia the cold resistance, when hot it's much higher than that.

Those circuits turn on the device when it gets too hot. You need the reverse, you could swap TH1 and R1 + POT1 round, use a positive coefficient thermistor or use a relay with normally closed contects.

Luckly, I found another

**broken link removed**

I dont have a LM339, I do have LM311 which is practically the same thing.
 
just about any comparator should work. did you save the rating plate from the unit? it will tell you how many watts it consumes. from there you can properly size you relay. My guess is that its between 600 and 1000 watts. Probably a 10 A relay would be ok.

by the way, shouldn't that circuit have a base resistor for the transistor? I'd use 2.7K which would work for a relay coil up to about 130 mA. edit: yes, I'm being a bit loose with the numbers...

You could also use a Triac and a moc303x for an all solid state solution. might even be cheaper than a relay.
 
philba said:
just about any comparator should work. did you save the rating plate from the unit? it will tell you how many watts it consumes. from there you can properly size you relay. My guess is that its between 600 and 1000 watts. Probably a 10 A relay would be ok.

by the way, shouldn't that circuit have a base resistor for the transistor? I'd use 2.7K which would work for a relay coil up to about 130 mA. edit: yes, I'm being a bit loose with the numbers...

You could also use a Triac and a moc303x for an all solid state solution. might even be cheaper than a relay.
'

No, I didnt save it. I think it was all crumy or something..

Im building on parts that I have, I'll have to find the data sheet for the relay. The relay Is 12V..Then again, I could probably find a triac in a old computer PSU..


Isnt that what the 2.2k Resistor is for at the base of the transistor?
 
No, that resistor pulls the output of the comparator high. a base resistor would go between pin 2 and the base of the NPN. It may not make a difference but makes sense to limit Ib to the range where it is effective.
 
A temperature controlled switch to serve as a high limit might improve the level of safety. A switch like this is in series so that if the other circuit fails it can still open the circuit.
 
Safety is a prime concern when working with lines voltages and high powers like this. So, take the advice of the previous poster and put a high limit switch in as a final resort in case of faults. Follow the typical practices that you see in other equipment, where all lines voltage connections are made with insulated connectors or are potted so that anyone reaching into the circuitry can't touch a live high voltage conductor.

Also consider that if your device burns your house down, the insurance company probably won't cover it.

You should also beware that you are going to need a great deal of line regulation before your low voltage circuit. When your element switches on and off, the line voltage is going to jump up and down a significant amount due to changes in voltage drop. So you need to put in more line regulation and filtering than you normally would with typical low voltage circuits. Perhaps you should consider a high performance linear regulator and also consider cascading two linear regulators, the first outputting a higher voltage than the second one requires, of course.

One reason that a lot of simple appliances use non-electronic temperature regulating controls, like bi-metal strip switches and the like, is that they are simpler and aren't sensitive to the tough conditions involved and probably more importantly, cheaper than the solid state alternatives. Crude but effective. Electromechanical controls are not sensitive to static, are slow so they don't respond to glitches, and don't have RFI problems. And did I mention that they are simple? For example, a percolator that I opened up recently had only two controls, one being a temperature regulator device that used a bi-metallic switch and the other being a timer device that was simply a variety of temperature sensitive resistor. Now, that's simplicity.
 
philba said:
No, that resistor pulls the output of the comparator high. a base resistor would go between pin 2 and the base of the NPN. It may not make a difference but makes sense to limit Ib to the range where it is effective.
LM339 and LM311 both have open-collector outputs. The "pullup" resistor is the only source of base current for the transistor, so no additional resistor is needed.
 
Overclocked said:
Luckly, I found another
Which involves building a non-isolated transformerless supply which can be hazardous.

Overclocked said:
I dont have a LM339, I do have LM311 which is practically the same thing.
I don't mean to be rude but if you have to ask this question then you probably don't understand how this circuit works. Bulding circuits without really understanding them achieves nothing in my oppinion and it's hopeless trying to fulat find when you don't really know what you're doing.

My advice would be to look at the origional circuit, figure it out, then you'll be able to see how to modify it so it will turn the heater off when it gets too hot instead of the reverse.
 
Hero999 said:
Which involves building a non-isolated transformerless supply which can be hazardous.


I don't mean to be rude but if you have to ask this question then you probably don't understand how this circuit works. Bulding circuits without really understanding them achieves nothing in my oppinion and it's hopeless trying to fulat find when you don't really know what you're doing.

My advice would be to look at the origional circuit, figure it out, then you'll be able to see how to modify it so it will turn the heater off when it gets too hot instead of the reverse.
I think he was talking about the circuit below, which does not (necessarily) have a transformerless supply. I think you were looking at the SCR-controlled one below it.
The LM311 comment was not a question, it was a statement, and, for his purposes, a true one. I would still go out and get an LM393 (dual), and save the LM311s for another project.
 

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I don't know why you'd need a dual anyway sine the one comparator design is more than sufficient anyway.
 
Hero999 said:
I don't know why you'd need a dual anyway sine the one comparator design is more than sufficient anyway.
I thought he had settled on **broken link removed**, which has 2 comparators.
 
Ron H said:
I think he was talking about the circuit below, which does not (necessarily) have a transformerless supply. I think you were looking at the SCR-controlled one below it.
The LM311 comment was not a question, it was a statement, and, for his purposes, a true one. I would still go out and get an LM393 (dual), and save the LM311s for another project.

You sir are correct. I was talking about that circuit. Dont worry, I have a lot LM311's that I dont even know what to do with!

I do know how to make my own PSU, and as always I have safety in mind. Luckly, I happen to have a LM7812 which would be perfect. I could always use a 9V battery to power it, or a computer PSU.

[For some reason I cant copy and paste..] As far as my knowledge goes, This is a window Comparator (or just a comparator) circuit.
 
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