ohh its switching alright.. just the wrong way..

and the thermistor I have says that its 1,000Ω @ 200°C (231,440Ω @ 25°C), so I think it has less swing in the target area than ± 600ohms

I just need to put the thermistor in a place that increases the power when the Ω's are higher and decreases it when its lower.

Hi,

you have a NTC Thermistor, get a suitable a PTC Thermistor that should reverse the operation. If the variable pot is the device that sets the temp then surely the thermistor should be in series with that and the value of the pot reduced to compensate for the thermistor resistance at target temperature?

Cheers
Andrew

Well.... thats going to be tough Andrew... I've looked on the net for a PTC thermistor that is as small and fast sensing as the NTCs I have and there's nothing i can find to match it.

The smallest PTCs I have found still have a 10 seconds max in still air thermal constant or whatever. Which is too slow..

There's gotta be some way to put this NTC into the loop.....

I have to figure out where in this circuit I can add this thing in. There has to be a place... its all math right? I just need a reversing factor.

If I can make the NTC Thermistor lower the voltage a little somewhere in the circuit when it is @ room temp, increasing the output power, and vise vesra, will it not work?

It's in the right place if you put it where I said to. I took the trouble to build the circuit myself and double-check it on a scope and it behaves just like I said.

Don't know what you're seeing, or why, but if that's really an NTC thermistor (did you measure it?) and you did what I said, it won't be going "the wrong way".

Put a scope on the output of the 555 and look at it.

After a bit of brainstorming, I had a thought.

What if:

I have a heating element .....

and assuming all calculations are made with a constant volume of air flowing,

if I knew the "effectiveness of heat transfer" of the element, could I not then, create a circuit based on an equation that, with out even knowing the exhaust air temp, calculates the power supplied to the element by knowing the ambient temperature (intake temp) and heating coefficient?


Ex. If I tested heater A @ 25 degrees C., with a fixed air velocity, and the exit temp was 100 degrees C. I could then know the heat transfer coefficient right?


If I base a the power control part of the PWM circuit on an equation that measures ambient temp and knows the heat transfer coefficient of the element (at a fixed velocity) could I not just make a circuit to set the power as X when the ambient temp is Y when the unit is on?

Get what I am saying? Given that all my control factors are tight..(air flow, heat transfer coefficient right, etc..)

Originally Posted by Dacr0n

Whoa... I turned the pot to full power and tried it out and the 1MFD Electrolytic cap started smoking....

and the heating element emits a very high frequency sound when the power is low.

Is this changing the pulse frequency or the duty cycle?

That's a power supply cap, sounds like it's too low for your source. Try 100µf. Make sure the voltage rating is at least 16V.

The sound is normal. This circuit changes both the PWM and the frequency. The frequency change won't matter to a resistive load, only the PWM change.


Lefty's right about your open-loop design.

If you are serious about accuracy, it's naive to keep pretending airflow thermodynamics is simple. Look up "Reynolds number" on Wikipedia.

Strangely enough, I'm about to build the exact same thing!

Heated nichrome wire coiled in a ceramic ~1/2in i.d. tube, with a small amount of air blown through. I'm even trying for a similar temp. range, of about 120-240C, and have been looking into similar control circuits.

I'm very curious as to how your project is shaping up.

What Thermistor are you looking at? And how am I supposed to use a pot to select a temperature in a linear fashion when a thermistor's response to temperature is non-linear?

Yeah, I'm not impressed with hydrogen as an energy storage medium, either. Loads of tanks compressed to insane pressures and even then the range is crappy. Conversion loss to go from electricity to hydrogen, another conversion loss from hydrogen back to electricity. Hydrogen seems to be more of a cult than anything.

Hahaha, the market is just looking for ANY physical form of energy that you can be made to pay for at a pump (why rework the entire transportation market right?). As far as I'm concerned gasoline is already a cult...

What's even funnier is that commercials claim that Hydrogen would be a truly zero-emission fuel, completely ignoring the tremendous costs of forming and transporting hydrogen gas.


I guess I missed that post of Dacr0n's where he states he's building a hydrogen generator...

I've been trying to read up on zero cross circuit, but I'm getting confused. Do I need to implement one if I control the triac gate through a smoothed-DC thermostat circuit? Could I also use an "alternistor" or snubberless triac and avoid the need for a snubber (or will the "fourth quadrant" failing of alternistors interrupt proper switching)?