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Old 05-26-2011, 09:40 PM   #41
shushikiary
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Screwing around with my brew troller I've messed with PWM duty cycles up to 10hz just fine.

The issue becomes the SSR. The element is just a resistor and can switch just fine up to high frequency and still work. The SSR however when you tell it to shut off it doesnt shut off until the zero crossing of the 60hz signal coming out of the wall, so you can only ever get down to 1/120 of a second accuracy for your duty cycle control.

Thus if you're running much under 1hz and using 0 - 100% you will have no change in output power between say 1-5 % and then have a single half wave power change once you cross the proper timing, so your control resolution is reduced due to the SSR once you get much below 1hz PWM period.

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Old 05-27-2011, 06:28 AM   #42
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Quote:
Originally Posted by Walker View Post
gerry... PID and PWM are not one in the same. Some PIDs support a PWM mode in addition to the normal PID mode, so you can indeed just wire up the PID to the SSR if it supports that kind of PWM behavior.

I built a stand-alone PWM to drive the SSR. A couple capacitors and diodes, a small 555 timer chip, and a potentiometer.

Cool! Do you have a PID working with this setup to hit a setpoint and hold that temp?
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Old 05-27-2011, 04:07 PM   #43
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Originally Posted by gerrywalsh

Cool! Do you have a PID working with this setup to hit a setpoint and hold that temp?
PWM is good for controlling power but cannot maintain a temp automatically. I use the PWM to control my boil kettle trength and have a separate PID for hitting and maitaining temps.
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