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Old 04-25-2013, 10:02 PM   #1
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Default Can A PWM Deliver True 100% Duty Cycle?

I'm wanting to control my SSR with a PWM based on Walker's design:



But the more I read about 555 circuits, the more I see they are not actually capable of delivering 100% duty cycle (always on). Is this true, or is it modified by the specific circuit?

Could I just add a bypass switch to run the DC directly to the SSR when I wanted a 100%-on signal to my element?

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Old 04-26-2013, 03:06 AM   #2
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I used that circuit for my boil control. In the off position, I notice a very slight flash on. It has to be dark and doesn't even light the led fully it is so quick. I remember discussing this before and I believe it was concluded that the reaction time of the SSR is too slow to respond to such a quick pulse. So for all practical purposes, this is 0 to 100%.

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Old 04-26-2013, 03:10 AM   #3
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Why do you want to use the 555 timer? Microcontrollers are better.

https://github.com/Fasrad/brewtroller/tree/fancy

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Old 04-26-2013, 04:50 AM   #4
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I'm not really interested in programming at any level for right now, maybe in the future. I just want to control a boil, I'm not mashing with my heating system.

So can that PWM deliver 100%? I'm not worried about turning it off, I have a kill switch for that.

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Old 04-26-2013, 05:33 AM   #5
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You probably won't get a true 100% with a 555 timer PWM circuit, but you are probably looking at something like 99%. You won't ever notice the difference. The timer will reset and then immediately pickup again. If you are that worried about it use a microcontroller or just simply wire a switch straight from your dc source to the SSR. When you want 100% simple flip the switch.

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Old 04-26-2013, 09:05 PM   #6
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Quote:
Originally Posted by thomashp View Post
You probably won't get a true 100% with a 555 timer PWM circuit, but you are probably looking at something like 99%. You won't ever notice the difference. The timer will reset and then immediately pickup again. If you are that worried about it use a microcontroller or just simply wire a switch straight from your dc source to the SSR. When you want 100% simple flip the switch.
Look closely at Walker's circuit. It's a bit different.
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Old 04-27-2013, 12:00 AM   #7
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A 555 will never truly get 100%
That being said, depending on your potentiometer, it may be so close to 100% that's it's a negligible difference

If it can adjust to 1k/99k (the two sides) you'll get 99% on time.

you could also add a switch that just holds the output high for on.

search for 555 calculator and you can figure an answer out.

R1 and R2 will add up to ~100k, and C1 is 33uF

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Old 04-27-2013, 05:05 AM   #8
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Quote:
Originally Posted by Shockerengr View Post
A 555 will never truly get 100%
That being said, depending on your potentiometer, it may be so close to 100% that's it's a negligible difference

If it can adjust to 1k/99k (the two sides) you'll get 99% on time.

you could also add a switch that just holds the output high for on.

search for 555 calculator and you can figure an answer out.

R1 and R2 will add up to ~100k, and C1 is 33uF
Notice that the circuit above has no fixed resistors. That is the key.

Here is a link with some explanation and another link.

http://www.homebrewtalk.com/f170/pwm...rouble-229000/
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Old 04-27-2013, 05:16 AM   #9
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Quote:
Originally Posted by lschiavo View Post
Notice that the circuit above has no fixed resistors. That is the key.

Here is a link with some explanation and another link.

http://www.homebrewtalk.com/f170/pwm...rouble-229000/
While true, not all pots will go fully to 0 ohms / full rating. If they don't you won't hit 100%. but for this application, it's pretty dang close.
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Old 04-27-2013, 05:27 AM   #10
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Quote:
Originally Posted by Shockerengr View Post
While true, not all pots will go fully to 0 ohms / full rating. If they don't you won't hit 100%. but for this application, it's pretty dang close.
Exactly. That is why I get that little tiny pulse when set to 0% and probably a tiny pulse also when set to 100% (which I cannot notice). Using this circuit to control an SSR for boiling it becomes negligible.
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