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Old 05-11-2012, 07:18 PM   #1
bblack7489
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Default PWM frequency question

I know that it's probably a subject that has been discussed and beaten to death, but I've spent the last 2 hours reading long threads about PWM, and I haven't really seen anyone who answered what I'm looking for.

From all of the discussion, it looks like people who are making their own PWM circuits or modifying existing ones are designing them so that they have a 0.25-2Hz base frequency. In some of the recent projects that I've worked on, I've used triacs that did zero-crossing detection and switched on at the frequency of 60Hz. Switching the SSR on at the zero crossing and off somewhere else in the cycle makes for much cleaner power.

Is the common ~1Hz solution just to minimize the mid-period switching, or is it to make it a simpler PWM implementation? Also if you're using a 555 circuit and a pot to control the duty cycle, do you limit the minimum duty cycle so that you're not switching both on and off in one cycle of the power?

I'm using a pretty intense and customizable industrial control system, so I can implement my PWMs however makes the most sense; however as I start to tinker with it though, I don't want to just blindly follow the HBT conventions without knowing some of the background reasons.

Any light on that background would be greatly appreciated



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Old 05-14-2012, 08:14 PM   #2
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I'm using the 555 timer and a pot to adjust the duty cycle. Read the thread "PWM show us how". I built a circuit based on what Walker drew up. I did add a simple switch that allows the ground to bypass the PWM and go right to the SSR so that there is no switching. I use that at the beginning. Once I get a rolling boil, I switch to the PWM and adjust accordingly.



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Old 05-14-2012, 09:09 PM   #3
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Yah, I've read through that thread and two more similarly long ones about PWM. I'm definitely comfortable with the "how," but there's very little explanation of the "why."

Everyone seems to be using an analog circuit (cap and resistor) tuned to give about a 0.25-2Hz switching frequency of the PWM generator. Why was that chosen? Does it cause noticeable brown-outs?

It's really hard on the SSR to switch a load mid-cycle of the AC source, and it's equally hard on the entire circuit unless it's a HIGHLY inductive load. Is the mid-cycle switching ok because the elements are actually that inductive, or is the assumption that we're switching slowly enough that it's not a big problem?

...just looking for more explanation of the reasoning behind what everyone does. I'd rather not blindly follow convention if the convention doesn't really apply to the hardware that I'm putting together.

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Old 05-14-2012, 11:19 PM   #4
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The main reason for using a frequency around 1hz is to maximize the life of the ssr. Switching faster than 1hz or so is unnecessary and needlessly taxes the ssr.

SSRs inherently switch at zero crossings.

For what its worth, My pwm runs right around 1hz. The boils is smooth and I do not notice and pulsing of the boil, even with a small duty cycle.

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Old 05-15-2012, 01:47 AM   #5
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Quote:
Originally Posted by bblack7489 View Post
Yah, I've read through that thread and two more similarly long ones about PWM. I'm definitely comfortable with the "how," but there's very little explanation of the "why."

Everyone seems to be using an analog circuit (cap and resistor) tuned to give about a 0.25-2Hz switching frequency of the PWM generator. Why was that chosen? Does it cause noticeable brown-outs?

It's really hard on the SSR to switch a load mid-cycle of the AC source, and it's equally hard on the entire circuit unless it's a HIGHLY inductive load. Is the mid-cycle switching ok because the elements are actually that inductive, or is the assumption that we're switching slowly enough that it's not a big problem?

...just looking for more explanation of the reasoning behind what everyone does. I'd rather not blindly follow convention if the convention doesn't really apply to the hardware that I'm putting together.
I'm really not sure as what you describe is a little above my knowledge of circuits. I would recommend sending a PM to PJ or Walker. Either of them might answer your question.


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