Need help on heating element by PWM

[delphes123]

Dear All,


I need to understand few unknown point in this schematic in order to modify with 12 Volts power.

The main purpose is to heat a Nichrom wire on J5 (resistance at 20 °C = 4 Ohms) and to reach a certain temperature and stay at this temperature until stop.

What I understood :

1) VR4 allows to define target temperature with a voltage divider R35, VR4 and R38.

2) U6B op amp is an inverting comparator, it gets reference voltage on + (pin 5) and variable voltage on - (pin 6)

3) R32 is a shunt in order to measure current on heating element, voltage accross R32 is proportional to current, as soon as heating element will increase temperature,
resistance will also increase and current will decrease.

4) U6A op amp is also an inverting comparator, it gets reference on voltage divider R33, R34

System should start and stop with the pin Heating_PWM (PWM signal) on optocoupler U7

What I did not undestand

1) The purpose of R37 C15 ? act as filter ?

2) The purpose of C16 D10 and R36 ?

3) How system can start, run and stop with signal Heating_PWM ?

Form me ( I am may be wrong) system can start with PIN2 voltage of U6A is less than Pin3 so the pin 1 in this case put U5 ON on his gate.

The final purpose of my understanding is to switch to 12V and need to know where apply changes.

Thanks in advance

[inse]

1) correct
2) D10 decouples PWM from feedback signal, C16 makes sharp edges to the PWM, R36 disables the output (PWM off phase) by making the neg input higher than the pos input.
I wonder why there is a need for PWM anyway, the circuit is a on-off regulator that will maintain the current in the wire to the value set by the potmeter.
But there is no hysteresis, so it would probably oscillate around the setpoint.
With 10k as gate resistance only rather slow PWM is possible without creating too much losses in the MOSFET. The imbalance of 10k vs 100 Ohms in the gate drive is rather unusual.
to make it work at 12V you would have to remove the gate voltage divider.
Will you be able to reach the desired temperature at 12V with the same wire?
Don’t forget: P=U^2/R

[delphes123]

Thanks for the first answer, could you clarify how system start and can be stopped with the signal heating_pwm ?

[inse]

Sorry for posting like this but I do this on my phone and keep switching from schematic to message.
If PWM signal is low, the opto coupler is off and the neg input of the second comparator is pulled high by R39 disabling the output.
If PWM is high, the optocoupler shorts the pull up resistor to GND and the first comparator rules.

[delphes123]

Thanks again for answering from your phone, any idea which resistor I should update by using 12 Volts instead of 24 Volts, R38, R33, R39 ?
or none ?

rgds

[inse]

Try to think this through by yourself.
It difficult to give advice without knowing the entire system.
NiCr wire has a low thermal coefficient, so PTC like self regulation is not a factor.
With 12V you only have 1/4 of the heating power than at 24V.
If the PWM had been at 25% before, it would have to be at 100% then to get the same heat/power.
Measure the resistance of the wire and calculate the current.
Then calculate the shunt voltage and the adjustment range of the potmeter circuit.

[inse]

I am starting to doubt some of the things I wrote in the second post, let me think..
Is the circuit proven working?

[inse]

Initial condition (all assumptions): setpoint at pin 5: 1V, no current flowing-> voltage at pin 6: 0V -> output pin 7: 24V
Pin 3: 21V, pin 2: 24V -> output pin 1 0V, MOSFET remains off.

[delphes123]

So it was main concern how system can start at the very first stage ?

[inse]

I didn‘t get that part of the question. But obviously you were right.
Had a closer look at the input polarity too late.
Let’s continue with the initial condition:
A PWM signal will allow pin 2 to be dragged to 0 for a short time allowing the output going high, turning on the current.
Now let’s think about turning off again
If the first pulse created a load current above the setpoint, the circuit will remain ON - if it was below, it will shut off again until the next PWM pulse wakes it up.
I hope I got it right this time though it doesn’t seem to make sense

[delphes123]

Dear

I tried the same schematics in real life with 12 volts and the system immediately start to heat with 0 Volt on Heating_PWM signal.

I have 7,5 Volts on pin 2 of U6 and I have 9,3 volts on pin 3 increasing up to 9,5 volts with temperature (why since it is only a voltage divider)

Should I increase R33 ?

[inse]

I calculated 10.9 V on pin 3, what is U6b doing?
What is the setpoint voltage?

[delphes123]

I have 0,7 Volts on pin 7 of U6 until the temperature reach the set point and 9,45 volts after but temperature continue to increase with no control (I stopped power supply before firing lab)

[inse]

What‘s pin 5 and 6 doing?
What is the cold resistance and hot current of the wire?

[delphes123]

Cold resistance = 3,9 Ohms
Hot resistance when reaching set point around 360 °c = 9 to 10 ohms (difficult to measure since I have to disconnect and temp decreasing)

On pin 5 I have 94 mV, on pin 6 starting around 150 mV and decreasing to 88 mV ( I stopped due to temperature)

Power supply draw 1,3 Amps at starting and 0,82 Amps just before stopping

Remember I have no control on ON/OFF it start immediately when power supply of 12 volts is ON.

[inse]

Ok, I didn’t think NiCr had that much temperature coefficient.
I thought it was like Constantan.
That changes things, let me think about it.
Btw, you only have to read the current consumption and we would be able to calculate the resistance.

[delphes123]

I think the U6B part is doing the job, the most problem are around U6A (R33, R34, C16, R39, D10 etc), I can try on 24 volts to see how it works.

[Roehrenonkel]

Hi,
what potentiometer (value) are you using?

Good luck

[inse]

What happens when you set the potmeter to a higher voltage?

[delphes123]

This is 1Meg pot

I started with 24 Volts and it works as expected, I mean No signal on heating_PWM do nothing, applying PWM (20 Hz DC = 20%) start the system, as soon as it reach the set point it turn around final temperature. Until I stop everything.

So How to change somethings in order to work with 12 volts as expected ?