# How the SSR, the PID and the thermocouple work together.



## bradleybhiking (Sep 14, 2017)

*How the SSR, the PID and the thermocouple work together. *

The PID provides DC (low voltage) and AC (high voltage) power to the SSR. When the PID senses an error in temp it sends DC voltage to the SSR low voltage side. When powered the SSR closes the “switch” on the low voltage side which in turn closes the “switch” on the high voltage side. When the high voltage “switch” is closed high voltage can flow to the load. The PID is monitoring the change in temperature through the thermocouple. When the thermocouple reading indicates the temperature change is in accordance with the selected temperature value setting (SV) the PID stops sending the low voltage to the SSR which opens the “switches” and stops the action. When the PID senses the temperature is in variance with the SV the process begins again.


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## old sarge (Sep 14, 2017)

Like a thermostat only more complicated?


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## tiros (Sep 15, 2017)

The explanation above is really a thermostat.

The PID performs a calculation, at some set interval. It looks at where it is at and where it wants to be. It than figures a power level to apply (%) for the next interval. The big difference is that time is also used in the calculation.  It adjusts its calculation each interval based on how the oven is responding. For instance, if not changing fast enough it will add more power.


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## bradleybhiking (Sep 15, 2017)

Yes no chance of over heat or under heat.


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## dward51 (Sep 15, 2017)

bradleybhiking said:


> *How the SSR, the PID and the thermocouple work together. *
> 
> The PID provides DC (low voltage) *and AC (high voltage) power to the SSR*. When the PID senses an error in temp it sends DC voltage to the SSR low voltage side. When powered the SSR closes the “switch” on the low voltage side which in turn closes the “switch” on the high voltage side. When the high voltage “switch” is closed high voltage can flow to the load. The PID is monitoring the change in temperature through the thermocouple. When the thermocouple reading indicates the temperature change is in accordance with the selected temperature value setting (SV) the PID stops sending the low voltage to the SSR which opens the “switches” and stops the action. When the PID senses the temperature is in variance with the SV the process begins again.


Actually the PID only provides a DC triggering signal to the low voltage side of the SSR.  The AC voltage is entirely on the load side of the SSR.  The PID is generally powered by AC line voltage, but will in most configurations, only output a low voltage DC triggering signal.  

Another tip when using SSR units is if you connect a DC control voltage to the input side and try to hook up a meter to measure continuity on the output/control side like you would with a dry contact relay, you get no continuity on the load side even though the triggering LED is lit.  This has caused some people to think their SSR was bad.  SSR's need an AC voltage on the load side and a feedback loop is created within the circuitry of the SSR to "switch" the load on or off.  In the end it functions like a plain old analog "switch" but there is no physical switch or moving parts in a SSR.  But for all appearances a SSR will function like a simple analog SPST switch with the PID triggering signal acting like the "finger" that flips the switch.


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## old sarge (Sep 15, 2017)

That's what I like about technology: taking the simple and complicating it for minute gains in efficiency and accuracy.


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## bradleybhiking (Sep 15, 2017)

Thank you dward51!!!  This brings complicated technology down to a level even I can understand with common language, well done.


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## redoctobyr (Sep 15, 2017)

old sarge said:


> That's what I like about technology: taking the simple and complicating it for minute gains in efficiency and accuracy.


I mean no disrespect, but that seems like an overly-broad statement, to me.

Simple: charcoal/wood, but you have to light it, and tend it, to manage temperature over time.

Some technology: electric with analog controller. Simple mechanism, but difficult to set temperature accurately, and comparatively wide temperature swings.

Medium technology: electric with basic digital controller. Complex to design and manufacture, but easy for the user, and easy to set accurately. Smaller temperature swings.

Most technology: electric with add-on PID digital controller. More complex to design and manufacture, more complex for the user to configure. After that, easy to set accurately, smallest temperature swings, best consistency.

The nice thing is that we have access to all of those solutions, and we can each pick what we feel is best for us, including our preferred level of performance.

I wouldn't criticize the person who selects charcoal, even if it's not my choice. Any digital controller is fairly complicated already, it's just that it's presented to the user as being all ready-to-go. And I think it's great that there are PID options available. Personally, I like to tinker, so if my MES controller failed, I'd probably go PID. I realize I'd be signing myself up for a little project, and I would actually kind of enjoy that aspect!


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## dward51 (Sep 15, 2017)

Nothing wrong with tinkering with PID setting manually if one was so inclined.  And for others who want it simple, most PID units do have the auto tune feature which is basically a one time, push button and let the computer figure out what works best for your smoking pit.  Once done, the auto tune setting are stored in the PID memory and unless you just want to do it again at some later date, you really are done.  Once auto tuned, just turn it on and set your desired pit temp and let it do it's job.

Here is some basic PID & SSR wiring diagrams that may be helpful (again, use the pin out diagram or schematic on the label that is on your PID as they can be different even with the same brand & model).  Some of these are 110 and others are 220v (most PID units will work on either):













Generic PID wiring.jpg



__ dward51
__ Jul 29, 2017


















230V WIRING.jpg



__ dward51
__ Oct 28, 2013






Note:  There is no "Switch" inside a SSR, but this is an illustration of how a SSR works, the triggering signal on the DC input side (left side of circuit below) causes the state of the SSR on the load side (the right side of the SSR below) to change it's state and pass the 115v AC power acting "like" a mechanical switch (which is why it is illustrated as a switch between terminal 1 & 2 in the drawing below).













PID image 7.jpg



__ dward51
__ Apr 6, 2013






Another illustration of how a SSR functions "like a switch" when the DC triggering input is applied to the triggering side of the SSR at terminals 3 & 4.













PID image 6.jpg



__ dward51
__ Apr 6, 2013


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## old sarge (Sep 15, 2017)

On the subject of auto-tune, and keeping in mind I have a Smokin-it 3D, the guidance on the Smokin-it site concerning the auto-tune  has often included putting in a dummy load simulating the meat.  The dummy could be a loaf pan of damp sand or just a brick or two.  This in addition to just keeping the smoker empty during the process. Different strokes for different folks.  I have not done an auto-tune believing the smoker was tuned when manufactured and it works just fine.  I just never understood all those possible values/ranges.  See pages 10 and 11 at the link:

http://auberins.com/images/Manual/WSD-1501GPH/WSD-1501GPH.pdf


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## dward51 (Sep 16, 2017)

Yes, using a "meat mass" will give a more precise auto tune cycle.   I used a disposable pan with about 1 to 1 1/2" of water (that later became a disposable drip pan) when I ran the auto tune cycle on my MES Custom SS.  Although not absolutely necessary, the more closely you can simulate the actual smoking conditions, the more accurate the tuning cycle will be.  But considering that most stick and charcoal burners can have temp fluctuations of 25-35* and no one bats an eye and thinks that their temp control is spot on, a 1 or 2 degree variance between a empty smoker and a load simulated smoker auto tune cycle probably will not matter.  Also there is some variance introduced by what type and amount of meat you are smoking.  Compare a 8 pound pork butt with it's thick mass that is slow to heat to 8 pounds of 1/4" sliced whole meat jerky.  The pit will perform differently with both, but in my experience it is NOT necessary to adjust the PID tuning once it is set.  Again a 1 or 2 degree variance or even a 2 degree overshoot matters not in smoking meat or cold smoking.   Now in the commercial world a 1 degree variance can destroy the chemical process being controlled.  That is what PID units were made for.  Their precision is way overkill for a smoker, but they make electric smoking so nice and easy.

Great point though on the tuning with simulated mass!


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