Heating a Small Plate with Manual Controls

I don't see any way to control temperature with no feedback. The heating requirement will vary, and if you want to control the temperature in a narrow range, then feedback of some sort will be necessary. A thermostat with a remote sensing element to control voltage according to load would be my first choice or you could try an infrared controller.

https://www.micro-epsilon.com/temperature-sensors/thermoMETER_CS/index.html

The energy dissipated as heat in the electric current passing through a conductor is proportional to the resistance of the conductor, the square of the current intensity and time , E = RI2t This equivalence is known as Joule's law.So you need to adjust amperage not voltage using a rheostat (Coil resistance cursor) connected in series with the heater.

An other law says that I= U/R and if you change U implicitly change I. In general it easier to control U.

In his case not energy counts but power since he has to establish a balance between input and output power at 50°C. If input is too high then system will balance at a higher temperature and the contrary if input decreases.

Joule wrote his law in this way E=R x I²x t (1*)

It is true that when you combine the two laws Ohm and Joule get this : E = U²/R x t (2*)

If we calculate the same values ​​we obtain .

( 1*) E=6 x 5²x 1 = 150 W/h (2*) E=30²/6 x 1 =150 W /h equal time 1 hour

But let's see what happens when we reduce current intensity with 1A and then voltage with 1V .

(1*) E=6 x 4²x 1=96 W/h

(2*) E=29²/6 x 1=140.16 W/h

This result shows that the current intensity produces heat conductors in much greater extent than the voltage.

In reverse , Joule's law is taken into account when transporting energy from the power plant to the consumer at very high voltage and not at high amperage rating.

OK, I accept from your demonstration ONLY that the regulation SENSITIVITY appears to be higher with current than with voltage variation, but this is related to the circuit resistance it gives the differences. This you can also see from a simple derivative of the 2 equations. In fact the law is

P= dE/dt= U*I.

This is a general equation stipulating that POWER = POTENTIAL*FLOW it is the same in all other departments as hydraulics , pneumatics aso where a "flow" is generated by a "potential difference" across a resistance.

Ohm's law allows the 2 ways of writing it which are THE SAME.

Please explain how you can control the current when you have a source which usually is at constant voltage.

You will answer that one has to put in series a resistor but in this way you build a voltage divider with a lower voltage on the heating resistor and a lot of power losses in the series resistor. In fact the 2 cannot be separated one from the other a change in voltage leads to a change in current and the DRIVING force is the VOLTAGE.

Modern electronics allow a voltage/current regulation without such losses by a fast on-off of the supply.

I agree that the first condition is to have a voltageVoltage measured in units of electric potential: volts, or joules per coulomb is the electric potential difference between two points .Amperage, or current, is a measure of the amount of electrons moving in a circuit.Voltage is a measure of how much force those electrons are under. Of course no current flow without a potential difference between two points, must exist a path for the electric charge particles to go through It comes the second condition to appear current : an electrical conductor that opposes current flow resistance .Now we have an electric circuit .Under pressure of voltage , amount of particles (current) goes from one pole to the second pole of the source passes through the conductor which has a resistance .The thermal effect (also known as Joule-Lenz effect ) is the dissipation of heat in a conductor traversed by an electric current. This current is due to the interaction of particles (usually electrons ) with the atoms of the conductor, the first interaction that gives them the last of their kinetic energy , helping to increase thermal mass agitation conductor.For this reason the law talks about current that produces a thermal effect and not voltage .Putting a rheostat in series will have a voltage drop and current drop, fewer particles will pass and their effect is lower heating.For a small heater may not be economical to use a temperature controller with feedback or other expensive solutions . A simple coil of resistive wire with sliding contact is the right solution, but not very good results and no stable.I think too much has been written on behalf of a small heater.Please forgive my poor English, but is a little hard to think in one language and write in another. If I'm wrong please correct me.
Respectfully

"an electrical conductor that opposes current flow resistance"

a conductor yes but it is not compulsory for it to have a resistance to oppose the voltage, do not forget all what is done for super conductor to have as little resistance as possible.

You manage very well.

Decreasing the current by 1A and the voltage by 1V is not comparing like with like. Decreasing the current from 5 to 4 amps is a reduction of 20%. Reducing the voltage by 20% brings it down from 30V to 24V, and the sums (V²/R) result in the same power (96W) as the 1A current reduction.

By "U" you mean "V" right? I often use E instead of V. Don't know why they have different names for the same stuff, but electronics is rife with them.

"experiment which needs 50 degree celsius temperature for the process"

As an experiment, you'll probably only need to do this a few times.

Stick a thermometer in whatever you're trying to control. Then you control the "button heater" with the on off switch. With a little practice, I'll bet you can manually control to the desired temp within a degree or two.

"but the temp continuously shoots to 80 to 90 degree.."

So, he has feedback.

"I don't see any way to control temperature with no feedback..."

I agree with SolarEagle.

Wire a resistor like an iron in the circuit with the button heater and use the rheostat temperature control on the iron to control the flow of current to the heater. It dims light bulbs in physics demonstrations so it should vary the heat in the circuit.

Putting a step-down transformer between the variac and the heater will let you control the heater voltage down to much lower values. Something like 230:15V @ 150VA or bigger. I'm suggesting the high VA rating in case someone turns it up to full voltage at some time - burnt transformers smell 'orrible!

What is the DC resistance of the heating element?

Is there some sort of thermostat inside the unit that you do not know of? Look!

Is there a control of some sort on the unit? A knob or similar?

Can you supply photographs?

You can only attach fotos here, and you must "draw" them out and make them as large as possible by clicking on the foto, holding say the bottom right hand corner block with the mouse LH button held down, and moving it as far to the right as possible.....

Even then some fotos are basically rubbish.....Get good clear fotos made as near to the "bit" as possible, that will give you the best chance.

Fotos made too far away with too much other stuff also in are basically a waste of time.....

That's like trying to pump up a bicycle tyre with no knowledge of what the pressure might be, or trying to cut a piece of wood to length without using a measure.

Until the problem has been better defined then any WAG will do as a response.

If you have access to an AC Drill speed controller (you don't see them as often today sadly, but there are circuits on the web to make), try that to control the heating.

I searched on google using "ac power drill speed control circuit diy" and found this amongst many others:-

ac+power+drill+speed+control+circuit+diy

Searching with "ac power drill speed control unit" found some built units. Search in electronic stores where you live for one.

Best of luck.

@Andy Germany,

About 3 decades ago I made the 2nd circuit to control a ceiling fan speed. But it did not work well at all (I don't know the reason). But when an inductance was added as your 1st circuit it worked well & ran for next few years till the TRIAC was burnt up. Would you please explain why without an inductance it failed to work correctly though the circuit was collected from a hobby book? This is just for curiosity. Thanks

Sadly, I haven't built one myself, I just wanted to show that it was possible to DIY the problem....maybe that circuit was not such a good idea....

What sort of fan motor was it that you used, if an spinner or stack motor I could possibly understand the problem as the circuit is mainly used on Universal motors.....those with brushes - as in mains Drills and the like....just a thought....

Try PTC heaters. Find one the right size. There are dozens on the internet. They are self regulating.

We had some success with the product in early tests but had put the idea to one side because there was a risk the heater could fail if it got wet when in service.

Current I is a function of E/R. (Energy (Volts) / Resistance) or E/Z

The only way to vary I is to vary either E (the voltage applied) or R (The impedance of the circuit)

It is possible to have E without R or I.

It is not possibe to have I without E.

Most electrical heater elements have a constant R value that varies slightly as the temperature rises until maximum output temperature is reached.

The only way to vary and control the current (I) and therefore the temperature of the heater element by a significant amount is to vary the applied voltage (E).

The chemical and metal composition of the heater element determines the linearity of the output temperature curve.

While calculations will get you close, the only way to accurately map the heater output temperature curve is by trial and error application.

Once the temperature control range vs applied voltage values is identified, you will need to obtain or build a voltage controller that utilizes temperature feedback to keep the correct voltage level applied to the heater that is required to maintiain the desired temperature without suffering runaway.

After you get the voltage value(s) correct, I would think a small battery operated digital HVAC home thermostat would work well for your application.

A simple thermostatic or bi-metallic switch may serve your purpose? Just mount the thermostatic switch on the heat plate, close to the heating element. Select the unit that is rated to trip and interrupt the power at about 50 degree celsius. You may encounter some lag time for it to reset back but response / sensitivity was not a necessity per your query!

Dear all ,

Thank you so much for all your Suggestions.

I used programmable benchtop DC power supply to set the current and voltage.

And set the voltage to 4 V and the current to 0.6 A .

It heated the plate to 50 Degree and slowly went up to 60 Degree in 15 to 20 minutes. (Actual voltage was 3.5 and 0.7 A)

my process requirement is 50 to 60 degree..

This worked .. Yes i too agree we cannot control the temperature without any feedback .. Thanks again to all for the support ..