Proportional Control Circuit w/ Thermocouple

[Dacr0n]

Don't worry blueroomelectronics, I won't.

You might want to take temp readings several diameters downstream of the heating element.

Lefty

Lefty

I was thinking that...

So I get the actual air temp and not temperature simply radiating from the element.


Ill let you know how it goes next week.

Thanks.

 

[blueroomelectronics]

So you are building an HHO generator. You do know they are very inefficient.

 

[Dacr0n]

No.

I am building a heater. A very smart heater.

 

[duffy]

Have you tried cow magnets?

 

[Dacr0n]

Cow magnets?

No whats the application and purpose of those?

 

[duffy]

Cow magnets have been proven in many, many tests to increase the power of HHO generators using magnetic power to couple zero-point energy from dark matter. You have to set the cow magnets perpendicular to a ley-line and use a crystal in a pyramid, though. Do you have a moon mirror and a divining rod with a witness well handy?

 

[Dacr0n]

Ehh.... I have no idea what an HHO generator is so that doesn't really concern me but Ill pass the info on.

Im just the heater/heater control system guy.

 

[duffy]

If you don't know what an HHO generator is, maybe you're legit. We get a bunch of cranks on here, and one of the favorite topics is some kind of overunity scam involving hydrogen.

 

[duffy]

Uh-oh, deafening silence...

 

[Dacr0n]

Still waiting for my kit. :|

 

[Dacr0n]

Okay... More news for those who are interested..

It works.... The PWM driver heated the element alright and in the few bench trials that I have done, i find that I need a bit more control... It's all working like I thought, but it's going over my target temp...

As of right now, the setup is purely a PWM circuit controlled by the turn of a POT. So.. im obviously having to sit there with my hand on it and tune..

Ideally... PI control would be great but I am not so sure I need to go that far...

So a quick refresher.... The conditions in which this will be applied will be very mild... No extremes, hot or cold... So once I can have the system deliver the target temp i should be okay and not have to worry about radically changing variables.

Back to what I was saying about the over temp..

I've been monitoring the final air temp with a thermocouple and, with my selected preset on the potentiometer... I am hanging pretty close to my desired range... The circuit has no temperature feedback but thats where I was thinking that I could add something in, that is fairly simple, that cuts off or shortens the pulse width in order to prevent overshoot.

As I mentioned before I have a very fast responding thermistor that is designed specifically for reading fast change in air temperature.

As a form of overshoot protection:
I am just speculating, but ... If I found out what the resistance of the thermistor was at my target temp + 5 or 10 more degrees, is there a way that I could throw that into the system and have it cut off the power or decrease the signal once is goes over that?


Another thing that I have really been thinking is that I need to start designing a new control circuit with the TL494 that you mentioned duffy..

Would it be possible for me to design a circuit with that TL494 in a electronic circuit design/simulation program?

Otherwise... what would be my best first step? Is there any PWM tech literature online that can read to help me design it? or anyone with some tips?

 

[duffy]

You could certainly simulate it if the program has the model for a 494. It's a pretty common chip, and not terribly new, so the odds seem good.

If no, you could still model it off the functional block diagram on the spec sheet using comparator blocks, flip-flops, an oscillator, three voltage references, two drive transistors and a handfull of gates -

- but that's going to be a bigger pain than just building it. Let me see if I can find a suitable example online....

 

[Dacr0n]

Could this be made to work like the 555 PWM circuit? But with more flexibility and options?

 

[duffy]

That's pretty close, except that transistor is only rated for 7A, and your thermal sensor would connected to pin 1 instead of that voltage sensing arrangement.

Do you have a spec sheet for the thermal sensor you decided to use?

You said you got a PWM kit, and it works - how about posting a schematic? Simplest thing to do would be to connect whatever thermal sensor you want to use to that.

 

[steveB]

Ideally... PI control would be great but I am not so sure I need to go that far...

I was wondering if I might ask a question about this comment?

If you don't use PI control, what will you use? Do you mean to use proportional control only, or do you mean to use a thermostat type on/off control? Or, is it something else?

 

[duffy]

Yes, a little illumination as to what it's actually for might help us determine how complex the control loop really needs to be.

 

[Rolf]

Overkill..........

{snip}
It works.... The PWM driver heated the element alright and in the few bench trials that I have done, i find that I need a bit more control... It's all working like I thought, but it's going over my target temp...
{snip}
QUOTE]
IMHO using PWM driver is a complete overkill, that is bore out by the fact that you have temperature over shoot anyway. What you need is an heat anticipateor similar to ones used in house thermostats.
An ordinary house thermostat has no problem keeping its environment at set point +/- 1°F.
You problem is not much different except for scale.

 

[Dacr0n]

The schematic to the Kit i am currently using us on page #2 of this thread...

Here is a link to the data sheet for the thermistor that I have. HERE

Its the "1,000Ω @ 200C" Model: GB2315R-3-200


SteveB: it must be some sort of proportional control. ON/OFF is unacceptable....


Rolf: Its not overkill and quite necessary as one of the requirements is that the heating system is that it needs to turn on, be able to deliver the desired heat(with decent precision) with in 5 seconds of operation signal, maintain the target temp range while the flow rate of the air changes slightly, and immediately begin cooling when the operating signal stopped. I highly doubt a house thermostat can do that..


Also... I want something thats fairly simple and cheap that is home made. I'd like to be at least semi original.

So far I've accomplished a few of my requirements. My current model simply runs off of the PWM circuit and I've adjusted the power so that the once the air is flowing, the circuit is powered on, the target temp is maintained only because the air flow rate is constant, and when I power it down, the little element cools quickly.

What I have in my mind is just setting the power a little higher than i need to get to target temp.. then using the thermistor somehow to bring the power down to when the temp goes above 'target'. So I would have to add some kind of loop with the thermistor that, as the resistance decreases (increase in temp), the pulse width decreases.

So... what do your expert minds think?

 

[Leftyretro]

The schematic to the Kit i am currently using us on page #2 of this thread...

Here is a link to the data sheet for the thermistor that I have. HERE

Its the "1,000Ω @ 200C" Model: GB2315R-3-200


SteveB: it must be some sort of proportional control. ON/OFF is unacceptable....


Rolf: Its not overkill and quite necessary as one of the requirements is that the heating system is that it needs to turn on, be able to deliver the desired heat(with decent precision) with in 5 seconds of operation signal, maintain the target temp range while the flow rate of the air changes slightly, and immediately begin cooling when the operating signal stopped. I highly doubt a house thermostat can do that..


Also... I want something thats fairly simple and cheap that is home made. I'd like to be at least semi original.

So far I've accomplished a few of my requirements. My current model simply runs off of the PWM circuit and I've adjusted the power so that the once the air is flowing, the circuit is powered on, the target temp is maintained only because the air flow rate is constant, and when I power it down, the little element cools quickly.

What I have in my mind is just setting the power a little higher than i need to get to target temp.. then using the thermistor somehow to bring the power down to when the temp goes above 'target'. So I would have to add some kind of loop with the thermistor that, as the resistance decreases (increase in temp), the pulse width decreases.

So... what do your expert minds think?

PID control with carefully adjustment of the tuning variables under actual operating conditions. It's the method the control pros use and anything else you kludge to work will still be a kluge.

Lefty

 

[duffy]

We still don't know what this is for. If he's heating a platinum catalyst, a proportional controller is probably enough. If he's growing crystals, it probably isn't.