temperature controlled hotplate

[redeadnixon]

Okay, just out of curiosity was thinking of building my own hot plate,
(I'm a chemist so it's not the sort used in restaurants),
I was thinking of using a variable resistor to control temp, with a thermocouple connected to the mains;
First of all, I'm not an expert in electronics, I did circuits in high school physics, thats about it.
And the temperature would need to go up to <200C.
I know where to get all the parts, but was wondering whether it would be feasible to just use a couple of strong resistors on the power source, or whether more complex circuitry would be involved.
If so, could someone post ideas for circuits?
Thanks, peace out.

 

[Boncuk]

Depending on the desired accuracy here is a suggestion.

You might also think about using a microcontroller where you can use tendency recognition to avoid temperature overshoots.

Boncuk

 

[Hero999]

You could just use a thermistor, if you can find one with a high enough temperature rating.

Then the circuit becomes as simple as a comparator, a few resistors and a relay.
LM311 Light/dark detector/thermostat

More on comparators:
**broken link removed**

 

[Boncuk]

You could just use a thermistor, if you can find one with a high enough temperature rating.

Then the circuit becomes as simple as a comparator, a few resistors and a relay.

How often to you expect the relay to "click" until it goes up in flames? (I didn't read anything about a snubber circuit for the output, although the load is resistive)

I've seen better posts published by you, dear hero999.

An amplifier is low cost and will help for easy adjustment of the desired setpoint which is max 200deg/F (93.33deg/C = close to water boiling temperature).

Guessing that for chemical experiments the temperature deviations must be small (hysteresis) an amplifier for the thermistor (thermocouple) won't hurt.

Thermistors however won't work precisely at those temperatures, so I suggest to use a thermocouple.

Nobody is perfect, and I'm not either - but this would be my approach.

Regards

Boncuk

 

[redeadnixon]

Thanks guys, I'll give it a go,
I'll try the amplifier method first,
might have to play around a little.
peace out..

 

[AlainB]

I would beleive that a hot plate cannot be made extremely accurate because of the surface involved. An electric stove infinity switch could be all you need. It is a low tech solution but if it is accurate enough for cooking rice, it is accurate enough for many other things.

 

[Hero999]

How often to you expect the relay to "click" until it goes up in flames? (I didn't read anything about a snubber circuit for the output, although the load is resistive)

What can't you use a relay?

As you say it's resistive so no snubber is required.

It's low frequency, < 1 cycle every five minutes.

Mechanical switches such as relays and bimetalic thermostats have been used to control heating elements for many years.

 

[Boncuk]

What can't you use a relay?

As you say it's resistive so no snubber is required.

Relays arc (even with a resistive load) when opening and depending on the power consumption of the heating element contacts burn off pretty quickly. An RC-combination or a VDR across the relay contacts will increase life time of the relay.

It's low frequency, < 1 cycle every five minutes.

A more precise temperature control will be to observe the temperature increase/decrease and pull/insert the plug for the heater.

Mechanical switches such as relays and bimetalic thermostats have been used to control heating elements for many years.

It makes me wonder why even in cheap laundry washers triacs are being used to control temperature nowadays.

 

[AlainB]

Relays arc (even with a resistive load) when opening and depending on the power consumption of the heating element contacts burn off pretty quickly.

What is pretty quickly? A normal north american electric water heater with 3000 or 4500 watts elements at 240 volts working 24 hours a day will normally last about 8 to 10 years before needing service and then most of the times it is the element that is gone. Same for electric heating thermostats and electric ranges controls.

Alain

 

[Hero999]

Relays arc (even with a resistive load) when opening and depending on the power consumption of the heating element contacts burn off pretty quickly. An RC-combination or a VDR across the relay contacts will increase life time of the relay.

I thought it was a purely resistive heating element, with virtually no capacitance?

Sorry but I disagree, lots of heaters still use bimetalic switches to control the temperature and providing they have the appropriate temperature and current rating they'll last for years.

It makes me wonder why even in cheap laundry washers triacs are being used to control temperature nowadays.

That's because they're cheap and can easily be controlled by a microcontroller and has nothing to do with reliability.

Triacs are far more sensitive to voltage spikes on the mains than a mechanical switch such as a relay or bimetalic switch. In my experience, triacs go bad far more often than relays or mechanical thermostats.

Sorry buy I think you're needlessly overcomplicating things.

 

[Boncuk]

I thought it was a purely resistive heating element, with virtually no capacitance?

Sorry but I disagree, lots of heaters still use bimetalic switches to control the temperature and providing they have the appropriate temperature and current rating they'll last for years.


That's because they're cheap and can easily be controlled by a microcontroller and has nothing to do with reliability.

Triacs are far more sensitive to voltage spikes on the mains than a mechanical switch such as a relay or bimetalic switch. In my experience, triacs go bad far more often than relays or mechanical thermostats.

Sorry buy I think you're needlessly overcomplicating things.

OK, that's your opinion about precision and reliabilty. No objections there.

However the OP didn't post the desired accuracy of temperature control - consequently everybody answering in the thread is just guessing.

If you drink whishey every once in a while you will taste the difference between "brew" and whiskey - and probably notice a quality difference of resulting heavy headaches drinking the hot destilled brew.

Considering the desired maximum temperature of the hotplate and the purpose it has to serve (chemical) I just assume that precise temperature must be maintained over a longer time period, which is almost impossible using a mechanical temperature switch (bimetallic switch).

Use a mechanical temperature switch e.g. in an incubator will result in two possible results: fried eggs or zero hatch rate.

So I further suggest to be on standby until the OP tells us what he really wants.

Kind regards

Boncuk

 

[Boncuk]

What is pretty quickly? A normal north american electric water heater with 3000 or 4500 watts elements at 240 volts working 24 hours a day will normally last about 8 to 10 years before needing service and then most of the times it is the element that is gone. Same for electric heating thermostats and electric ranges controls.

Alain

Hi Alain,

I understand "pretty quickly" giving any part a live time of not more than three months.

I also experienced high current switches activating 140KW fan motors making the sound of a frying pan preparing a Wiener Schnitzel after switching on the fan(s). The noise regularly seized after 15 minutes.

The switches were replaced very quickly (this time 10 minutes) after I warned the technical maintenance boss to refuse adjusting the fans (variable pitch types) if this sparkling and tremendeously loud light show goes on.

you will admit that a water heater with 3000 to 4500Watts heating power is just a toy.

In Germany we use three phase 20 to 25KW heaters and if the power contactors fry together (not opening anymore) it's advisable to extremely quickly search for cover, because the water pipe system will react like a steam bomb.

This happens quite often and opening the "healthy" heater you can distinctively see how the contacts arc heavily when disengaging.

Needless to say that none of the power switches has ever "seen" a snubber circuit.

If you have read thoroughly you'll know my interpretation of "quick".

Last not least: Proper Planning Prevents Piss Poor Performance.

Boncuk

 

[AlainB]

My "guess" is that a hot plate cannot be controled with extreme precision, with or without electronic controls.

Alain

Edit: Hi, Boncuk, you posted your last reply as I was writing this one so this was a reply to your previous post. Now, about your last one, I don't know nothing about switching 20 to 25 KW but please let me guess that the OP is not looking to build a hot plate of that size.

 

[Boncuk]

My "guess" is that a hot plate cannot be controled with extreme precision, with or without electronic controls.

I designed a motorbyce handle heater circuit with a max temperature deviation of 0.2deg/C, of course employing an MCU.

The "intelligence" comes from the programmer putting his knowledge into the chip by designing the software to look ahead (tendency recognition).

It does work!

Alain

Edit: Hi, Boncuk, you posted your last reply as I was writing this one so this was a reply to your previous post. Now, about your last one, I don't know nothing about switching 20 to 25 KW but please let me guess that the OP is not looking to build a hot plate of that size.

So what's the difference?

Controlling lousy 100W might be more a challenge than controlling 1000W. (Temperature losses due to environmental reasons - you can't achieve 200deg/F using a 100W heater in a -80deg/F environment.)

Boncuk

 

[AlainB]

You desing a circuit that will react at a temperature deviation of 0,2deg/C. But it does not mean that the handle will stay precisely at the set temperature. While riding, there will be some air flow around the handle that will affect the accuracy and I suspect probably by a few degrees. If yous system is enclose somehow and cannot be affected by the surrounding air temp. then we are not talking about the same thing.

Alain

 

[AlainB]

In Germany we use three phase 20 to 25KW heaters and if the power contactors fry together (not opening anymore) it's advisable to extremely quickly search for cover, because the water pipe system will react like a steam bomb.

Not safe! Not safe!!

With our "toys" we use a safety valve that, if ever needed, would open and evacuate the hot water before it gets dangerous.

Alain

 

[Hero999]

Triacs seem to have a tenancy to fail on more than conductors do and contactors can be specifically designed never to weld shut.

Anyway, you're right about frequency and that if a very fine control is desired a relay might not be able to keep up, if this is going to be a problem then an opto-isolator (such as the MOC3041) and triac could be used.

 

[redeadnixon]

OP here, the hotplate needs to have a temperature deviation of no more than 2deg/C,
And i was thinking of something portable, ie, size of a CD case with depth of no more than 7cm,
I'm not sure whether this is possible as i don't know the tools in this trade as i have said before,
If it is, could someone give me a rough idea of how much i will be spending on this??
thanks again

 

[Boncuk]

You desing a circuit that will react at a temperature deviation of 0,2deg/C. But it does not mean that the handle will stay precisely at the set temperature. While riding, there will be some air flow around the handle that will affect the accuracy and I suspect probably by a few degrees. If yous system is enclose somehow and cannot be affected by the surrounding air temp. then we are not talking about the same thing.

Alain

The individual temperature sensors are built into the handles and react on air flow around the handles if the driver takes a hand off for a prolonged time period, observing the temperature slope and controlling heating power according to that slope.

The control panel is mounted a way to enable the driver to set the desired temperature using a thumb. Driving a two wheeler under icy conditions it is advisible not to take hands off the steering bar anyway.

BTW, humans are more sensitive to temperature variations than commonly is assumed.

A temperature difference of 0.5deg/C decides about feeling comfortable, too hot or too cold.

 

[Boncuk]

OP here, the hotplate needs to have a temperature deviation of no more than 2deg/C,
And i was thinking of something portable, ie, size of a CD case with depth of no more than 7cm,
I'm not sure whether this is possible as i don't know the tools in this trade as i have said before,
If it is, could someone give me a rough idea of how much i will be spending on this??
thanks again

Hi redeadnixon,

you forgot to mention some details:

- AC or DC powered and supply voltage
- maximum mass to be heated (litres)
- heater area in square cm (for the proper container size)

Boncuk