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Old 09-26-2012, 07:34 PM   #1
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Default Simple SSR PWM power control

Got started with my electric brewing and yes, bought the temp controller without the manual option to adjust the boil , so had to come up with this circuit. I know this is out there by the zillions, just wanted to share. You simply install this between your controller and the SSR and let you adjust the power (R7) with PWM from about 10% to full power. The control cycle is adjusted by R2. The on/off switch is optional, you could modify R6 from 47K to about 39K or 27K and lower your minimum power from 10 to 0%.







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Old 09-27-2012, 01:16 AM   #2
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Is your plan, to set the sv temp to the max and then control the ssr output with the secondary controller?



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Old 09-27-2012, 05:31 AM   #3
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Is your plan, to set the sv temp to the max and then control the ssr output with the secondary controller?
That is correct, I'll set sv at say 215F and control the boil manually.
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Old 09-27-2012, 01:42 PM   #4
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This may seem like a silly question - but if everything is already wired up and being controlled by your existing PID, why not simply replace the PID with one that has the manual control option?

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Old 09-27-2012, 01:58 PM   #5
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Interesting, curious on SSR's, when you remove the control voltage, when does it actually open the output? Does it open immediately, or does it open at the zero crossing? Maybe I am thinking of triacs though.

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Old 09-27-2012, 02:21 PM   #6
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This may seem like a silly question - but if everything is already wired up and being controlled by your existing PID, why not simply replace the PID with one that has the manual control option?
Yes, that was my first thought too, just didn't want to return the controller and wait for a new one. Also, I like the dial to adjust the power, very intuitive.
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Old 09-27-2012, 07:38 PM   #7
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Interesting, curious on SSR's, when you remove the control voltage, when does it actually open the output? Does it open immediately, or does it open at the zero crossing? Maybe I am thinking of triacs though.
most common SSRs are zero crossing. if they are non-zero crossing it will normally specify that somewhere. non-zero crossing puts extra wear and tear on the device, so they are normally built a little better and are accordingly a little more expensive.
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Old 09-27-2012, 08:19 PM   #8
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Oh ok thanks, will it work to try to PWM a zero crossing SSR then? It seems like the PWM signal would make the SSR close at almost random points during the line frequency wave, but then not open until the zero crossing, resulting in a lower but unpredictable average power to the heating element?

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Old 09-27-2012, 08:44 PM   #9
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Oh ok thanks, will it work to try to PWM a zero crossing SSR then? It seems like the PWM signal would make the SSR close at almost random points during the line frequency wave, but then not open until the zero crossing, resulting in a lower but unpredictable average power to the heating element?
If the PWM circuit command the SSR to connect at little after a zero crossing of the 60Hz wave (worst scenario), it will hold till the coming zero crossing to actually activate the load. That is 1/120s=8.33ms. If the duty cycle of the PWM is high, like 2s, the lost power represents only 0.007%. So for a 5500W element, you get max of 5499.6W. Hope my math is correct
Note.: My understanding is that the zero crossing feature is intented to improve the device's life for inductive loads only, to avoid damage caused by a flyback pulse. For purelly resistive loads, it would not add any benefits.
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Old 09-27-2012, 11:22 PM   #10
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Right, I don't think the zero crossing shutoff benefits resistive loads. I don't think there is much if any power wasted. It just seems like the PWM signal will trigger the SSR at a random point in the line AC waveform, and then even if the PWM signal opens during that half cycle the SSR won't actually open until the zero crossing. Maybe I'm just not understanding how the circuit works. Roughly what would be the frequency of the PWM signal there?



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