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Old 10-25-2011, 08:12 PM   #1
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Default Three PID - SSR powered elements on one 240V - 30A circuit..

I knew that would get some folks attention.... :-)



OK, what if we use logic based 'priority' to only send power to one element at a time... in my system, with a 50A breaker and cord, I did not see many times when 2 4500W elements were on at one time.. only when heating strike water or initial boil is any one element on 100%, my RIMS is maxed at 25%... I even think I could do consecutive/stacked brews and not have an issue other than the MLT taking a little longer.......

I believe that if you had HLT, MLT(RIMS), and BK, the priority would be MLT, then BK, then HLT...

MLT:
If we hooked the MLT PID signal direct to the MLT SSR.

BK:
Run MLT signal into a logic inverter and then that inverted signal and the BK PID signal int a logical AND, and send that to the BK SSR

HLT:
Invert the BK PID output and AND it to the inverted MLT PID signal, AND that signal with the HLT PID output to drive the MLT SSR..


PID outputs:
MLT 0 1 0 1 0 1 0 1
BK. 0 0 1 1 0 0 1 1
HLT 0 0 0 0 1 1 1 1

SSR inputs:
MLT 0 1 0 1 0 1 0 1
BK. 0 0 1 0 0 0 1 0
HLT 0 0 0 0 1 0 0 0

only one element powered at a time, and the PID functions will take care of *small* missed power pulses that the BK or HLT miss...


Obviously, circuit breaker to protect things from a failure that tried to power two elements and overload circuit....


I thought this would be really helpful for folks not wanting a huge investment in 50A power cords/plugs/receptacles/load centers... thoughts?


-mike
www.clearwaterbrewery.com

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Old 10-26-2011, 01:29 AM   #2
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A

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Old 10-26-2011, 11:43 AM   #3
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interesting concept... so would you use relay logic or is there some PID programing that i havent heard of?

Someone on here had a system designed to run all at once using the elements at 120V to reduce power consumption when they were all on... I'll see if I can find that thread.

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Old 10-26-2011, 12:51 PM   #4
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OK, I see what you did there. The "PID outputs table" is just a counting pattern from binary 0 to 7 (to cover all the possible outputs from the PIDs).

Then theres some logic between the outputs and the SSRs to ensure that only one turns on, and the result is shown in the "SSR inputs" table.

Seems like a perfect application for a 3-to-8 decoder. See this:
http://www.fairchildsemi.com/ds/MM/MM74HC138.pdf

It'd be easy to do in code with a microcontroller, but the part above would be the minimal solution.

Interesting post Mike.

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Old 10-26-2011, 04:58 PM   #5
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Yes,.. and the output table was an afterthought... I filled it out by counting binary... I figured it would help people see what I was doing...

I was actually going to use 7400 series TTL inverter and AND chips powered from my BCS-462 5V out, for real-time system, not relying on the BCS ladder logic if that was even possible...

I was hoping someone would point out a more elegant solution, I should have known it would be Andrew...

thanks for the 74138 idea, I will search through my 1980's chip stash ;-)

edit - darn... outputs are inverted and I would have to OR the 8 outputs down to 3... an hex inverter and quad AND chip it is..

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Old 10-26-2011, 05:08 PM   #6
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Quote:
Originally Posted by clearwaterbrewer View Post
edit - darn... outputs are inverted and I would have to OR the 8 outputs down to 3... an hex inverter and quad AND chip it is..

The inputs to the SSRs are optocouplers. Just tie the '138 logic output to the (-) side of the SSR input, and tie the SSR (+) input to the same voltage as the '138 supply. That will get the invert for you.

Pick the logic family that will get you the current you need. 'ACT or something.

[edit] or look for an open-drain outputs to tie to the (-)
[eidt] to combite outputs, isolate each inverted output with shottky diode - high output does nothing, low sinks current through diode
[edit] or maybe there is a non-inverting logic part ('238?).
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Old 10-26-2011, 05:13 PM   #7
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I found the other thread that was similar... but after reading these other comments it dosent sound like you are going the direction as this guy did. Both sound sweet and I like the direction you are going... I would love to stack brew days and this would allow it. Keep us posted on how it turns out!

PS: for reference this is how someone else did this http://www.homebrewtalk.com/f170/3-v...ms-30a-203926/

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Old 10-26-2011, 05:34 PM   #8
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One thing to look out for here is that the AC SSRs switch on zero-crossing, which means there is up to half-cycle delay ( 1/ 120 second) between the input changing and the output turning off. Probably not a problem, but something to keep in mind.

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Old 10-26-2011, 05:45 PM   #9
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Looks like it would work you just wouldnt get any pulses to the HLT SSR while you BK was at 100% getting up to a boil. Only matters if you are doing double batches.

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Old 10-26-2011, 06:49 PM   #10
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I distilled the idea into having the RIMS be actively controlled and the remaining duty applied to the BK and HLT. The question I have is how would you measure for HLT temp? You wouldn't want it running fully manual like the BK.

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