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Old 01-25-2010, 07:08 PM   #1
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Default PWM for Boil Kettle Control

When I was starting the design of my brew rig, I went looking for a way to control my electric boil kettle without buying another PID. I came across this website.

http://home.highertech.net/~cdp/boilnew/boilnew.htm


I thought to myself. This should be easy. I have built PWM circuits before and I probably have enough spare parts laying around to make it. So that is what I did.

Well yesterday i finally got around to testing the circuit properly. So Plugged a DC motor into the SSR output and turned on the power. I started turning the potentiometer knob expecting to see the speed slow down. Nothing.......... Well that is strange. I tried a few combinations of wiring on the knob. Nothing. It just turns the motor off at certain spots when you turn it down. So now what?

I get out my volt meter and connect it. 12v when turned all the way up. Fine. Start turning the knob "down", and still 12v. I hesitate for a moment and the power goes off. I wait a few more seconds and the power goes back on. So i keep turning the knob and counting the second between on and off. Sure enough it changes when I turn the knob.

This is the first time I have come across a PWM circuit like this. It turns the output full on, or full off for set amounts of time. No variance in voltage and no quicker or slower pulsing that I am used to.

So I guess my question to all of you is and others who may go this route. Would a normal PWM still work? I am thinking that a normal PWM circuit might kill a SSR if it pulsed on/off too fast.

Also if anyone is interested in one of these circuits. I have an extra one that I built. I can also make more boards pretty easy. I could make them in two different configurations. All in a box complete with dial and inputs, or just the board and terminal blocks like the picture below.



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Old 01-25-2010, 07:42 PM   #2
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I used a PWM kit from Bakatronics #FK804, cost $8, and swapped a capacitor to reduce the cycle time to 1Hz (also calculated the cap value for 1/2 Hz but deemed it unnecessarily slow). Hooked the output directly to the SSR and it worked great in my new 5500W brew kettle.

My understanding of SSRs is that the faster you cycle them, the hotter they run. That and how much current they're handling.



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Old 01-25-2010, 07:46 PM   #3
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That makes sense to me. My only experience with PWM has been with motor controls and light controls. I guess I was expecting the same results.
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Old 04-26-2010, 10:22 PM   #4
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did you figure out what was going on with the controller?
I am thinking to build it from CD's schematics...

Any feedback/recommendations?
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Old 04-26-2010, 10:54 PM   #5
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The circuit works great. It just was not what I had thought when I started building it. I thought the pulses were going to be faster. Like the speed controllers I have used in the past. I love the way it works for me.

I say go for it. I am using a 5500w element with an SSD. I use a pot to control it and also put an status LED on my panel so I know when it is on and off.
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Old 08-10-2010, 04:50 AM   #6
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Freddyb, What value capacitor did you settle on? I'm looking at buying that same kit, but am not sure how to calculate the cycle time.

Thanks.
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Old 08-10-2010, 09:46 AM   #7
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I used the following equeations to calculate charging and discharge times along with frequency with the following equations:

Charging Time - T1 = 0.693(Ra+Rb1)C
Discharge Time - T2 = 0.693(Rb2)C
Frequency - f = 1/(T1+T2)

If you're using the circuit above, Ra and Rb are both 1k ohm resistors.

Rb1 = 1k + one side of 1M pot depending on where it's position is
Rb2 = 1k + other side of 1M pot depending on where it's position is

Pot all the way to one side: Rb1 - 1k ohm and Rb2 - 1M ohm
Pot in the middle: Rb1 - 501k ohm and Rb2 - 501k ohm

If you use the 4.7uF cap you'll get a frequency around 0.31. If you use a 1uF cap you'll get a frequency around 1.44.

Hope that helps.
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Old 08-10-2010, 12:19 PM   #8
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I use a 2.2uF cap and it works great, around 1 second cycle time. I recall finding an error (or typo) with the calculations provided with the kit but I don't recall exactly what it was.

I've done quite a few batches using this system and I absolutely love it. Very "responsive" with my 5.5kW element, even with a full 13.5 gallons in the BK. I can go from rolling boil to gentle boil and back to rolling in a few seconds by adjusting the potentiometer. I put a big knob on there so it looks sweet too.
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Old 08-10-2010, 02:18 PM   #9
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Thanks to the both of you for your info. I'll be building on very soon.
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Old 11-08-2010, 10:39 PM   #10
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Send me some photo's and steps for dummy's:-) I just bought components and would like some kind of photo's of PWM controller:-)
Cheers


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