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Old 08-22-12, 05:07 PM   #1
AC_Hacker
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Default PID Temperature Controller

PID stands for proportional–integral–derivative and is a way to apply optimized feedback to reach a desired target, be it a temperature, volume, speed, or even an enemy naval vessel. It will maintain a temperature with great accuracy, too.

I found a really cheap one on ebay for about $25, free shipping, so I thought I'd investigate.


It took so long for it to arrive from China that I forgot that I had ordered it, but I still remembered that I wanted to get one and was about to order, when the package turned up in my mailbox.

The product and the enclosed document were all in Chinese, but there was the model number written on the controller: XMT7100

I looked through the Internet for a reasonable manual, found a lot of poorly written ones, but finally found a good one LOCATED HERE.

The controller needs voltage (110v to 220v) not only to work, but also to set it up for your application.

Here is a diagram in the manual which includes the 'pin out' (screw-out?):


I scrounged an old lamp cord & plug and connected to screws 1 & 2 (if you look at the screw area, there are numbers near each screw). I used small crimp-on spade connectors to avoid any nasty shocks.

Now the unit will light up and the menus can be investigated. When you push the SET button, the numbers will all turn to blinking zeros, the ">" button will let you step from number to number. The '^' button will let you increment the digit you are on, and the 'V' button will let you decrement the digit.

The codes to get you into the various menus are:
  • 0089
  • 0036
  • 0001

To get into a particular menu,
  1. push SET
  2. use > to go to the digit you want to change
  3. use ^ an V to set the digit
  4. repeat for other digits if reqiired
  5. press SET to enter the menu

Once in the menu, various choices are available to you, and each menu includes a choice called 'END' which allows you to exit a menu.

The diagram below, is in the manual and is supposed to explain all of this to you. Hopefully, between my description and the manual, you'll have no problem.


The controller will allow you to use an auto tune feature that will automatically go through a brief test cycle and set the optimum PID coefficients for best operation, based on the heat output of your heat source, and the thermal mass of your system. The auto tune function is initiated by pressing the '>' button for more than 3 seconds, at which time the small LED on the left side of the display begins rapidly blinking. When the test phase is complete, the LED returns to normal operation. So, if your heat source or the thermal mass of your system change appreciably you should run the auto tune feature again.

I sketched out a hasty wiring picture:


And wired it up to a small water heater to use as a Sous-vide egg cooking experiment. Again I used crimp-on insulated connectors:


After I ran the auto tune function, I put my egg in for one hour at 65 decrees C.

The controller will rapidly increase heat to the set point, not overshoot, and with brief pulses of electricity, hold the temperature dead on.

It will remember its settings when unplugged.

The controller has a built in relay (screws 4 & 5) that is designed to operate a visible or audible alarm if some set point is reached. That set point is programmable via the menu system. The set point can be established to be at the target temperature, at a higher temperature, or at a lower temperature. The relay, though designed as an over-temperature alarm, can be used for other interesting purposes, for instance some folks have used it to activate the water pump in an espresso machine, so other uses may come to mind.

The main output of the PID (screws 9 & 10) is a pulse of about 4 volts, low milliamps, of PID controlled variable-duration, that is designed to turn on and off a relay, or better yet, a solid state relay. In the early part of the warm-up cycle, the pulse is fairly long duration, as the controller approaches the target temperature, the pulse width begins to get shorter, and when the target temp is reached, very brief pulses precisely maintain the temperature.

If you get a PID controller, you'll likely want to get a solid state relay, too. I got a 25 A 220v SSR, from ebay for about 6 or 8 bucks, free postage. I bought four of them.

You'll also need a thermocouple. I already had some K type thermocouples laying about. My controller will also ten different types of thermocouples, so you'll need to carefully read the specs on the controller you choose. I'd say that a thermocouple with a three foot long cable would be better than the one that I have with a two foot cable.

BTW, of you plug the thermocouple leads in backwards, the temp readings will go down when they should be going up... just switch the leads.

Uses:
  • I got this controller to put in my espresso machine, because it's temperature controller went on vacation.
  • I can see that one would also be good to control a small annealing oven for treating steel.
  • a Sous_vide oven (they usually cost a bunch-o-bucks)
  • Also to control a kiln for ceramics.
  • Also an aquarium.
  • Also a refrigerator (my controller has cooling mode).
  • and many more...

It's a fist full of technology for $25.

Incidentally, the 65 degree egg was delicious.

-AC

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Old 08-22-12, 06:33 PM   #2
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Originally Posted by AC_Hacker View Post
... was about to order, when the package turned up in my mailbox.
Thats almost as good as Amazon yesterday shipping
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Old 08-23-12, 08:32 AM   #3
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These look very similar to devices produced by these guys but prices through other sources seem much cheaper. I'm not sure if they are made by the same manufacturer or not but they look pretty much the same.
Lightobject.com Sacramento (Elk Grove) DSP CO2 laser machine & Parts. Solar panel & Electronics Supplier all of Northern California
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Old 08-23-12, 10:00 AM   #4
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Originally Posted by MN Renovator View Post
These look very similar to devices produced by these guys but prices through other sources seem much cheaper.
I'm not so sure that Light Source actually produces anything... I think that their value is that they select and organize and warehouse and ship and you pay for that service.

Interesting line of stuff. Probably don't have to wait for months to get what you want.

-AC
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Old 08-23-12, 10:41 AM   #5
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Nice explanation of the Auto-Tune. I like that it can learn to be smart enough to sneak up on the set-point,
and not just plow right past it..

A PID with NV settings might be just the thing for adding some aux heat to my new hotwater tank.

If my A7 ASHP turns our basement into a giant beer cooler, during in the cold winter months,
pumping in some extra power using the heating elements (@1200w) will be considered.

Controlling the SSR's DC power with a simple room thermostat would keep
power use down, when the basement wasn't real cold..
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Old 09-22-12, 12:39 PM   #6
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Default Dang, no SSR pins..

Seems like I got one without SSR control..


New Digital PID Temperature Control Controller Thermocouple AC 220V REX-C700 | eBay

The item description was confusing..


Trigger signal of controlled silicon: 5V/40μ S or 100mA rated current, 1A instantaneous current (self-triggering)
Output signal of solid state relay: 15±3V/30mA


I took that to mean it was SSR ready. Those SSR control pins aren't on mine.

So, I now have another digital temperature display..
I need a display for the hot water temp, and I can use this PID's relay to drive a Low temperature alarm.
(I'm going to use the smaller fish tank on left controller elsewhere).

Those two displays track very close together, but the PID's sensor is slower to change.
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Old 12-05-12, 11:36 AM   #7
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Thanks for the info of the PID. Can you recommend any other application of this device? And also can you post the diagram on how you use it in your espresso machine? Just want to try it and see how it really works.

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Old 12-06-12, 11:35 AM   #8
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Quote:
Originally Posted by Nellie44Militello View Post
Thanks for the info of the PID. Can you recommend any other application of this device? And also can you post the diagram on how you use it in your espresso machine? Just want to try it and see how it really works.
The information that comes with the device is not so useful. This device has been used by many people to control espresso machine temperatures. Just use Google, you'll find application notes quicker than I can retrive them from my archives.

But yes, you can use this thing to bring any temperature up to a set point (or down to a set point in cooling mode) and hold that temperature very accurately.

It is a no-brainer if you are using resistance heat. If you are using something like a heat pump, you will have to be intelligent about your approach to avoid short-cycling.

But the applications for this device are only limited by your imagination.

So there should be no problem...

Best,

-AC
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Old 12-08-12, 10:17 PM   #9
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[scratch]If this device loses power does it lose it's tuning and temperature settings?[/scratch]
Edit: Nevermind you said "It will remember its settings when unplugged." already.

Here's the scenario. My refrigerator in my low ambient kitchen(human comfort, what's that?) and the refrigerator needs to be set at a colder temperature than usual to actually run often enough to keep the freezer at a safe temperature(even though it's set to the coldest freezer setting) but sometimes the setting runs the freezer even colder than it needs to be(and the refrigerator nearly freezing the contents). Would I be able to use this wired into the compressor circuit and setting the refrigerator thermostat at the coldest I want it to be so that if the freezer didn't quite make it to the temperature.

My freezer is cooling to negative temperatures sometimes but also spiking 5 degrees above where I consider it safe in a 15 degree swing when during the summer the refrigerator runs more often at a similar ~15 minute duration.

Basically I've been nudging the refrigerator to a colder temperature if I see it's getting to warm and if it even gets too cold in the freezer I nudge it back up. I'm not really looking to freeze my refrigerator contents by keeping the freezer "always safe" but I don't want the refrigerator to be any colder than it needs to be. Seems like a good solution if this would sounds viable.

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Old 12-09-12, 02:47 PM   #10
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Quote:
Originally Posted by MN Renovator View Post
...My refrigerator in my low ambient kitchen(human comfort, what's that?) and the refrigerator needs to be set at a colder temperature than usual to actually run often enough to keep the freezer at a safe temperature(even though it's set to the coldest freezer setting) but sometimes the setting runs the freezer even colder than it needs to be(and the refrigerator nearly freezing the contents)...
I think you can buy a small freezer for about $130 (US). The running cost will be about $24/yr... that would be the easiest fix.

The PID controller is primarily intended to control devices such as precision process ovens, precision cooking devices, chicken brooding devices (if you want precision), etc. that are heated by electric resistance heat. The auto mode is all about deriving the heating power available to your heating device, and the effective thermal mass, but that doesn't mean that the PID can't be utilized for other purposes... Maybe for steering a ship, or automatically aiming a firearm, or controlling the direction of a self-propelled flying object. The PID controller can maintain a temperature closer than one degree. There are several parameters in the unit that can be manually tweaked (Auto mode is not mandatory). One of the parameters is the time between sensor-read intervals, which can give you some latitude if you want to use a vapor-compression machine for heating or cooling.

The PID controller can be purchased for less than $30, so why don't you buy one and do some testing and report back on your findings?

Kind of a 'learning by doing' endeavor...

Best,

-AC

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