At 11:10 AM 6/26/2004 +0800, CL Low wrote:

>good MAX6675 but difficult to get at my local electronic shop here, any info
>for use 877A produce PWM to a SSR to control the heater ? for temp senser, I
>found application note for LM335 with cold junction compensation "K" or "T"
>type TC wire, VDC reading fed back to the 877A ADC input,

Should be pretty simple, just define a duty cycle, and if cold, increase it, if warm, decrease it. More on-time makes for higher temperature, less on-time makes less temperature.

That being said, in your specific environment, you might have loose coupling between the heater and the sensor, where by the time that the sensor sees "warm enough" in other places, it's "too hot".

You also might want to control the rate at which the duty cycle is incremented or decremented, so as to avoid oscillations..

Have a look at PID algorithms, they are about solving these sorts of problems.

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